French Drain Calculator
How much gravel do you need for a French drain? This free French drain calculator gives DIY homeowners and drainage pros the full take-off from one form: #57 washed stone in cubic yards and tons, perforated-pipe linear feet and stick count, filter-fabric square footage with a roll-width recommendation, the trench excavation and loose-spoil volume, and the total fall — with a built-in check against the 1% (1/8-inch-per-foot) gravity-drainage minimum.
It is built on engineering consensus: a 12-inch-wide trench, 3/4-inch clean #57 stone, a 4-inch perforated pipe with the holes facing down, a non-woven geotextile burrito wrap, and the IRC R405.1 minimums of 2 inches of bedding below the pipe and 6 inches of cover above it. Actual pipe outside diameters are modeled so the gravel volume correctly subtracts the pipe displacement, and an optional Manning's-equation panel estimates drainage capacity in gallons per minute.
This is a material and slope estimator, not a hydraulic or structural design. Trench safety (OSHA requires a protective system at 5 feet), discharge permits, and foundation-drain design are site- and code-specific. Call 811 before you dig. No pricing, no signup — just the quantities and the slope check you take to the job.
French Drain Calculator
Estimate everything a French drain needs — gravel in cubic yards and tons, perforated-pipe linear feet, filter-fabric square footage with a roll recommendation, excavation and spoil volume, and a slope pass/fail check — from your trench length, width, and depth. Built on IRC R405.1 bedding-and-cover minimums, ASTM C33 #57 stone, AASHTO M288 geotextile, and standard pipe outside diameters. Free, no signup.
Trench dimensions
Mapping a wet area? Work out the square footage first →
Not sure what goes in the trench? See the bedding, pipe & cover section
Pipe & gravel
IRC R405.1: pipe rests on ≥2" of washed stone and is covered with ≥6" of the same material on a foundation drain.
Why holes-down & ≥2″ bed / ≥6″ cover? See the trench section
Filter fabric & slope
1% = 1/8" per foot = 1 ft of drop per 100 ft. Target 1–2%; keep under ~5% to avoid scouring the gravel.
Only know the drop in inches? Convert it with the Slope Calculator →
Burrito wrap, sock, or none? See the filter-fabric comparison
Options
How a French drain actually goes together
A French drain is mostly washed stone, a holes-down pipe, and a fabric wrap — pitched to drain by gravity. These engineering-style diagrams cover what fills the trench, how much it has to slope, and which filter-fabric option keeps it free-flowing; use the “see the diagram” links beside the inputs above to jump to the figure you need.
The trench section is why the estimate stacks stone bedding, pipe, cover stone, and fabric rather than just a pipe. The perforated pipe is laid holes-down on washed stone, covered in more stone, and the whole column is wrapped in filter fabric and run past the footing. Holes face down so water enters as the table rises — the geometry is the point, not just the parts.
The slope comparison explains why the calculator holds a minimum continuous pitch. A French drain runs on gravity alone, so the pipe must fall continuously to a lower outlet; too flat and it stagnates and silts, too steep and fast flow scours the gravel. The slope is what makes the buried stone and pipe actually move water.
The fabric-wrap comparison is why the calculator adds overage to the filter fabric. A full burrito wrap protects the whole stone column and is the professional default, and it needs extra material for the side laps and top fold. A sock on the pipe alone clogs faster and no fabric silts up, so the wrap choice changes both longevity and the fabric quantity.
Calculation Formulas
Width and depth are entered in inches and converted to feet before multiplying by the run length. A trapezoidal section uses the mean of the top and bottom widths. This is the excavation volume before the pipe is subtracted.
Example:
50 ft × 12" × 18" = 50 × 1.0 × 1.5 = 75 ft³ = 2.78 yd³.
The perforated pipe occupies space the gravel does not fill, so its volume is subtracted from the trench. Using the ACTUAL outside diameter matters — a 4" PVC SDR-35 is 4.215" OD, not 4.0".
Example:
4" PVC SDR-35 (OD 4.215" = 0.351 ft) over 50 ft: π × 0.1756² × 50 ≈ 4.84 ft³.
Net stone is the trench minus the pipe. Cubic yards is the bulk-delivery unit; a waste/overage factor (default 10%) is applied before you order.
Example:
75 − 4.84 = 70.16 ft³ = 2.60 yd³; +10% waste → 2.86 → order 3 yd³.
Quarries sell by weight, so the volume is converted with the aggregate's density. #57 washed stone is ~105 lb/ft³ (~1.4 tons/yd³). Moisture and compaction can add 10–25%.
Example:
70.16 ft³ × 105 lb/ft³ = 7,367 lb = 3.68 tons.
The wrapped cross-section is the trench bottom plus both side walls (two depths + one width). The 1.20 factor covers the side laps and the top fold/overlap (~20%).
Example:
100 ft trench, 12" × 18": (2 × 1.5 + 1.0) × 100 × 1.20 = 480 ft².
A French drain works only by gravity. The minimum slope is 1% (1/8" per foot, ≈ 1 ft of drop per 100 ft); 1–2% is the practical target. The calculator flags anything under 1% or over ~5% (scour risk).
Example:
50 ft at 1% → 0.5 ft = 6" of total drop; 1/8" of fall per linear foot.
Soil expands ~30–40% when excavated (the swell factor). The loose-spoil volume helps you plan haul-off or stockpiling. Saturated clay can approach 3,000 lb per cubic yard.
Example:
75 ft³ trench × 1.35 = 101.25 ft³ = 3.75 yd³ of loose spoil.
An ESTIMATE of how much the pipe can carry, full bore, by Manning's equation. A = πD²/4 and R = D/4 for a round pipe. The roughness n swings the answer ~50% (smooth PVC ≈ 0.010 vs single-wall corrugated ≈ 0.022), so it is shown with assumptions stated, not as a design guarantee.
Example:
4" pipe at 1%, n = 0.010 (PVC): roughly 55 gpm half-full / 110 gpm full — a planning figure only.
The slope (hypotenuse) correction is sub-inch at residential grades, so the run rounds to the nearest foot. Each cleanout/fitting adds ~2 ft. Rigid PVC/HDPE comes in 10-ft sticks; corrugated comes in coils.
Example:
50 ft run at 1% with one cleanout: ≈ 50 + 2 = 52 LF → 6 ten-foot sticks.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Minimum Stone Bedding (below pipe) | 2 in (51 mm) | IRC R405.1 requires the perforated pipe to rest on at least 2" of washed gravel or crushed rock. The calculator warns below this. |
| Minimum Stone Cover (above pipe) | 6 in (152 mm) | IRC R405.1 requires at least 6" of the same washed stone over a foundation drain. The calculator warns below this. |
| Minimum Slope | 1% = 1/8" per foot | The gravity-drainage minimum. 1–2% is the practical target; foundation outfalls often use 1/4" per foot (≈2%). Keep under ~5% to avoid scour. |
| #57 Washed Stone Density | ~105 lb/ft³ (~1.4 tons/yd³) | Open-graded ¾"–1" angular crushed stone (ASTM C33 / AASHTO M43). Editable; moisture and compaction add 10–25%. |
| 4" PVC SDR-35 Outside Diameter | 4.215 in | Charlotte Pipe submittal value. Schedule 40 4" = 4.500"; 6" SDR-35 = 6.275". Corrugated HDPE ODs are loosely toleranced (single-wall ≈4.6"). |
| Filter Fabric Overlap Factor | 1.20 (≈20%) | Covers side laps and the top fold of the burrito wrap. The minimum top-seam overlap is 12" (range 6–18"). |
| Soil Swell Factor | 1.35 (≈35%) | Loose excavated soil occupies ~30–40% more volume than in place. Used for the spoil estimate. |
| Geotextile Weight (Residential) | 4–6 oz/yd² non-woven | AASHTO M288 'Subsurface Drainage' Class 2 non-woven. Heavier 8–12 oz/yd² for clay-heavy, high-sediment soils. |
| OSHA Trench Depth Trigger | 5 ft (protective system) · 20 ft (PE design) | 29 CFR 1926.652 requires sloping, shoring, or a trench box at 5 ft and an engineer's design at 20 ft. Surfaced as a non-blocking safety warning. |
| Manning's Roughness (n) | PVC 0.010 · N-12 HDPE 0.012 · single-wall 0.022 | Interior-wall roughness for the optional capacity estimate. Because Q ∝ 1/n, this choice swings the result by ~50%. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
IRC 2021 §R405.1 — Foundation Drainage(IRC R405.1)
View StandardThe residential code section governing foundation/footing drains. It sets the hard numbers this calculator uses for bedding, cover, footing offset, and the filter-membrane requirement. General yard drains aren't always code-regulated, but R405.1 is the best-sourced default and is widely adopted as best practice.
Key Requirements:
- •Perforated pipe rests on a minimum of 2" of washed gravel or crushed rock
- •Pipe is covered with not less than 6" of the same material
- •Drain extends not less than 1 ft (305 mm) beyond the outside edge of the footing
- •Drain is installed at or below the top of the footing / bottom of the slab
- •Gravel/stone is covered with an approved filter membrane
ASTM C33 / AASHTO M43 — Open-Graded Aggregate (#57 Stone)(ASTM C33 / AASHTO M43)
View StandardThe grading standards that define #57 and #67 washed stone — the open-graded, angular, no-fines aggregate that gives a French drain its void space and flow. ASTM D448 lists the size numbers; C33 / M43 set the gradation.
Key Requirements:
- •#57 stone: nominal ¾"–1", open-graded and washed (no fines)
- •#67 stone: nominal ¼"–¾" — an acceptable finer alternative
- •Angular crushed stone preferred for void space and interlock
- •Avoid pea gravel, crusher run, and screenings — fines compact and silt up
- •High void ratio is what carries water to the pipe
AASHTO M288 — Geotextile (Subsurface Drainage)(AASHTO M288)
View StandardThe specification for drainage geotextiles, including apparent opening size (AOS) and permittivity matched to the in-situ soil. A non-woven needle-punched fabric in the burrito wrap is the residential default.
Key Requirements:
- •Class 2 non-woven for typical subsurface drainage; non-woven preferred for filtration/flow
- •Residential weight 4–6 oz/yd²; 8–12 oz/yd² for clay-heavy, high-sediment soils
- •AOS (apparent opening size) and permittivity matched to soil gradation
- •Woven offers higher strength but lower flow — use only where strength governs
- •Sock pipe suffices only in clean, sandy soils; burrito wrap outperforms it in fines
Pipe Standards — PVC SDR-35 / Schedule 40 / Corrugated HDPE(ASTM D3034 · D1785 · AASHTO M252/M294)
View StandardThe material specs behind the pipe-OD table. Outside diameters differ materially between products, which is why the calculator models displacement by actual OD. Each pipe is a real trade-off, not a single right answer.
Key Requirements:
- •PVC SDR-35 (ASTM D3034/D2729): rigid, smooth bore, holds slope, snakes clean — 4" OD 4.215"
- •Schedule 40 PVC (ASTM D1785): thickest wall, highest crush rating — 4" OD 4.500"; best under driveways
- •Corrugated HDPE (AASHTO M252 3–10", M294 12–60"; ASTM F405/F667): cheap, flexible, freeze-resilient
- •Use perforated pipe on the collection run; solid pipe for the non-perforated outfall
- •Holes face DOWN (4 & 8 o'clock) so rising groundwater enters immediately
OSHA 29 CFR 1926 Subpart P — Excavation & Trench Safety(29 CFR 1926.652)
View StandardFederal trench-safety rules. Trench collapse is a leading cause of construction fatalities, so depth is a safety variable, not just a dimension. The calculator surfaces the 5 ft and 20 ft triggers as non-blocking warnings — it cannot assess soil type or site conditions.
Key Requirements:
- •A protective system (sloping, shoring, or a trench box) is required at 5 ft (60") and deeper
- •Trenches 20 ft and deeper require a registered professional engineer's design
- •A competent person must inspect the excavation for soil conditions on site
- •Spoil and equipment kept at least 2 ft from the trench edge
- •Call 811 for utility locates before any excavation
Manning's Equation — Pipe Flow Capacity (Estimate)(Manning's n)
View StandardUsed for the optional drainage-capacity output. The result depends heavily on the roughness coefficient n and the fill fraction, so it is presented as an estimate with assumptions stated — not a substitute for an engineer's hydraulic design.
Key Requirements:
- •Q = (1.486/n) × A × R^(2/3) × S^(1/2) in US units; gpm = cfs × 448.8
- •Roughness n: smooth PVC ≈ 0.010, dual-wall N-12 HDPE 0.012, single-wall corrugated ≈ 0.022
- •Capacity rises with diameter to roughly the 8/3 power — a 6" carries far more than a 4"
- •Half-full and full-bore figures bracket the planning range
- •Not a stamped design — n and fill choice swing the result ±50%
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Soil Type Drives Fabric & Gravel Choice
Clay and silt clog drains fast — fabric weight and wrap method matter most there
The finer the soil, the more aggressively it migrates into the stone and pipe. Heavy clay and silt call for a full burrito wrap and a heavier non-woven fabric; clean sand is forgiving enough that a sock may suffice. This is the single biggest driver of whether a French drain lasts decades or clogs in a few years.
Regional Examples:
Frost Depth & Foundation vs. Yard Drains
A footing drain in a cold climate runs far deeper than a yard drain
Yard drains run 18–24" deep almost everywhere. Foundation/footing drains, by contrast, follow the footing — which sits below the local frost line — so the same drain is a 2 ft trench in the warm South and a 4–6 ft trench in the deep-frost North. Depth changes excavation, safety, and the OSHA threshold.
Regional Examples:
Discharge & Outlet Rules Are Local
Where the water goes is often more regulated than the drain itself
A French drain has to discharge somewhere lower than the line. Whether you can tie into the municipal storm sewer, must use a dry well, or have to daylight on your own property varies widely — and dumping onto a neighbor's lot is both bad practice and often illegal. Permits frequently attach to the discharge, not the trench.
Regional Examples:
When a French Drain Isn't the Right Fix
Regrading, swales, and downspout extensions sometimes beat a buried drain
A French drain manages subsurface and intercepted water, but a lot of 'wet yard' problems are really surface-grading problems. Before trenching, check whether simply regrading away from the house, cutting a vegetated swale, or extending downspouts solves it for far less effort.
Regional Examples:
Pipe Material by Climate & Loading
Freeze cycles and traffic loads change the right pipe — and it's a real trade-off
No single pipe wins everywhere. Corrugated HDPE flexes with freeze-thaw and is cheap and forgiving; rigid PVC holds slope precisely and snakes clean but can crack under hard freeze movement; Schedule 40 takes traffic loads under a driveway. The calculator models each one's OD so the gravel volume stays correct whichever you pick.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Share this calculator with your customers
Run a contractor site, supply store, or blog? Drop this free french drain calculator straight onto your own page so your visitors can estimate materials without leaving. No signup, works on any website — just copy and paste.
Want to Learn More?
How to build a French drain — #57 stone, 4″ perforated pipe holes-down, the burrito wrap, the 1% slope minimum, and how much gravel per foot.
Read the French Drain Guide
Heavy material — watch the weight limit
Concrete, brick, and masonry hit tonnage caps fast. Most dumpsters cap heavy material at 10 tons, and overage fees stack quickly. See the disposal guide before you load.
Read the heavy-debris guide →
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How to Use This Calculator
- Enter the trench dimensions — drain length in feet, trench width and depth in inches. 12 inches wide × 18–24 inches deep is the residential standard for yard drainage.
- Choose the pipe type and diameter (4-inch perforated is the default) and the gravel — #57 washed stone is recommended. The pipe outside diameter is modeled so the gravel volume subtracts the displacement.
- Set the stone bedding below the pipe (2 inches minimum) and cover above it (6 inches minimum per IRC R405.1), then pick the filter-fabric strategy: burrito wrap, sock, or none.
- Enter the slope (1% minimum), any cleanouts, and a waste factor (default 10%). Optionally turn on the drainage-capacity estimate.
- Read your results: gravel in cubic yards and tons, pipe linear feet and sticks, fabric square footage and roll count, excavation and spoil, and the slope pass/fail. Copy or print the take-off and review the cross-section diagram.
Why Gravel Volume Subtracts the Pipe
The gravel fills the trench around the perforated pipe, so the stone you order is the trench volume minus the pipe's displaced volume — and that displacement depends on the pipe's ACTUAL outside diameter (a 4-inch PVC SDR-35 is 4.215 inches, not 4.0). Filter fabric for a burrito wrap is (2 × depth + width) × length × 1.20, where the 1.20 covers side laps and the top fold. The 1% slope minimum is the gravity-drainage performance basis; 1–2% is the practical target. These follow IRC R405.1, ASTM C33 / AASHTO M43 (#57 stone), and AASHTO M288 (geotextile).
Frequently Asked Questions
How much gravel do I need for a French drain?
Multiply the trench length × width × depth (converting inches to feet), subtract the pipe's displacement, then divide by 27 for cubic yards. A 50 ft trench that is 12 inches wide and 18 inches deep is 75 cubic feet of trench; a 4-inch PVC pipe displaces about 4.8 cubic feet, leaving roughly 70 cubic feet — about 2.6 cubic yards or 3.7 tons of #57 stone, plus a 10% waste factor (order 3 yards). As a rule of thumb, a 12-inch-wide × 18-inch-deep trench needs about 1.4 cubic feet of stone per linear foot. Enter your dimensions above and the calculator does the displacement and the tons conversion for you. The trench cross-section diagram shows the stone, pipe, and fabric layers.
What gravel is best for a French drain, and can I use pea gravel?
Use 3/4-inch clean washed #57 stone — open-graded, angular crushed stone with high void space and no fines (ASTM C33 / AASHTO M43). The open voids carry water to the pipe and the angular shape locks together without compacting. Do not use pea gravel, crusher run, or screenings: rounded and fine material compacts and silts up, choking the drain within a few years. #67 stone (1/4–3/4 inch) is an acceptable finer alternative. The calculator includes the correct density for each so your tons come out right, and it warns if you pick a non-recommended aggregate.
What slope does a French drain need?
At least 1%, which is 1/8 inch of drop per foot, or about 1 foot of fall per 100 feet — this is the gravity-drainage minimum. The practical target is 1–2%, and foundation outfalls are often run at 1/4 inch per foot (about 2%). A French drain works only by gravity, so flatter than 1% lets water stagnate (the single most common design mistake), while steeper than about 5% can scour the gravel and strand sediment. The calculator shows your total fall in inches and gives a clear pass/fail against the 1% minimum. The slope diagram shows the 1 percent minimum fall.
How deep and wide should a French drain be?
Yard drains are typically 18–24 inches deep and 6–12 inches wide, with 12 inches being the practical standard. Foundation/footing drains go deeper — down to the footing, often 2–6 feet, and below the local frost line. For width, leave at least 3 inches of stone on each side of the pipe, so a 4-inch pipe wants a 10–12-inch-wide trench. Per IRC R405.1, keep at least 2 inches of washed stone below the pipe and 6 inches of cover above it. Note that any trench 5 feet or deeper requires an OSHA protective system — the calculator flags this.
Do French drains need filter fabric, and which way does it go?
Yes. IRC R405.1 requires an approved filter membrane on foundation drains, and for any drain in clay or silt the fabric is what keeps soil fines from migrating into the stone and clogging it. The professional standard is the 'burrito wrap': line the whole trench with non-woven geotextile (4–6 oz/yd² for residential, per AASHTO M288), bed the stone and pipe inside, then fold the fabric over the top. A pre-wrapped sock pipe is convenient but clogs faster in fine soils. The calculator estimates fabric as (2 × depth + width) × length × 1.20 and recommends a roll width. The fabric wrap diagram shows the burrito-style wrap.
Which way should the pipe holes face — up or down?
Down — at roughly the 4 and 8 o'clock positions. Holes-down lets rising groundwater enter the pipe immediately at the bottom of the trench, which is how a French drain relieves a high water table. If the perforations face up, water has to flood the entire trench to the crown of the pipe before any enters, which defeats the purpose. This is a point of genuine field debate, but the standards-aligned majority practice is holes-down, and it is the orientation shown in the calculator's cross-section diagram.
What pipe should I use — corrugated HDPE or PVC?
Both work; it's a real trade-off. Corrugated HDPE (AASHTO M252/M294) is cheap, flexible, and freeze-resilient, which makes it the common pick in hard-freeze climates. PVC SDR-35 (ASTM D3034) is rigid with a smooth bore that holds slope precisely over long runs and snakes clean. Schedule 40 PVC (ASTM D1785) has the thickest wall and highest crush rating for use under driveways and patios. Use perforated pipe on the collection run and solid pipe for the non-perforated outfall. The calculator models each one's actual outside diameter so your gravel volume stays correct whichever you choose.
Does this calculator include French drain cost or pricing?
No. Like every calculator on the site, it is pricing-free — it gives you quantities (gravel in yards and tons, pipe linear feet, fabric square footage, excavation and spoil) and a slope check, not dollar figures. For French drains, materials are usually the smaller share of the job; labor and excavation dominate, and both swing widely with length, depth, soil type (clay and rock are harder to dig), accessibility, and whether it's an interior or exterior drain. Take the weight and linear-foot totals to a supplier for a current quote, and get labor bids locally.
Where should a French drain discharge, and can it run uphill?
It must discharge somewhere lower than the line — to daylight on a sloped lot, a dry well, a pop-up emitter, a sump pump, or (where permitted) a storm-sewer tie-in. It can never run uphill, because the system relies entirely on gravity; if you can't get fall, deepen the outlet end. Never discharge onto a neighbor's property — that's both bad practice and often illegal, and many jurisdictions require a permit for a storm-sewer connection. Confirm the outlet path and any permits with your building department before you dig, and call 811 first.