Slope Calculator
This free slope calculator converts a ground or pipe slope between every form the trades use: percent grade for yard grading and driveways, inches per foot for drain pipe and patios, degrees for angle finders, and H:1V ratios for retaining-wall backslopes and embankments. Enter the slope in whichever form you have — including a raw rise-and-run measurement in feet and inches — and get all the others instantly.
Add your run length and the calculator returns the total fall: the drop you actually set with stakes and a string line. Pick an application and it checks your grade against the published requirement — IRC R401.3 foundation drainage (6 inches of fall in the first 10 feet), IPC 704.1 drain-pipe pitch by diameter, ADA walkway and ramp maximums, and the ICPI 2% patio recommendation — and tells you how much fall you are short.
Free and no signup, with the common construction slopes tabled next to your result. Once you have your grade, feed it into the French Drain Calculator for trench materials, the Retaining Wall Calculator for backslope loading, or the Paver Calculator for patio base planning.
Slope Calculator
Convert a slope between every form the trades use — rise over run, percent grade, degrees, inches per foot, and H:1V ratio — and turn a slope plus a run length into the total fall to stake out. Check your grade against code minimums like IRC R401.3 foundation drainage, IPC 704.1 drain pitch, and ADA ramp maximums. Free, no signup.
How do you know the slope?
Rise and run
Run length & code check (optional)
How slope actually works
Slope confuses people because every trade writes the same number differently — plumbing specs say inches per foot, ADA says percent, grading plans say H:1V. These engineering-style diagrams show why they're all one rise-over-run number, how to stake a slope outside with a string line, and which code minimums and maximums your grade has to clear.
The notation diagram is why the calculator accepts five input modes and always returns the same four answers. A 6″ fall over 10 ft — the IRC R401.3 foundation-grading minimum — is 5%, 2.86°, ⅝″ per foot, and 20H:1V at the same time. It also flags the classic mix-up: a 100% grade is 45°, not 90°.
The string-line diagram turns the calculator’s total fall output into a field procedure. Fall = slope × run, so a 1% french drain over 40 ft drops 4.8″ — stake both ends, level the string with a line level, and measure down at both stakes until the outlet reads that much more.
The code ladder is what the calculator’s application presets check against. Drainage slopes are minimums — flatter and water stands — while accessible routes and driveways are maximums. Knowing which direction your slope is regulated in is half the answer.
Calculation Formulas
Rise and run measured in the same unit. Percent grade is the rise per 100 units of horizontal run — the form civil, landscape, and driveway specs use.
Example:
A 6 in drop over 10 ft: (0.5 ft ÷ 10 ft) × 100 = 5% — exactly the IRC R401.3 foundation grading minimum.
The inverse tangent of the slope. Percent and degrees are different scales that only track each other at shallow slopes.
Example:
A 100% grade (equal rise and run) is atan(1) = 45°, not 90°.
The rise in inches for each foot of horizontal run — the unit plumbing pitch and flatwork drainage specs are written in.
Example:
A 2.08% grade = 12 × 0.0208 = 0.25 — the ¼ in/ft pitch IPC 704.1 requires for small drain pipe.
Horizontal units per one vertical unit — the convention geotechnical and retaining-wall specs use for backslopes and embankments.
Example:
An 18° backslope has run ÷ rise = 1 ÷ tan(18°) ≈ 3, written 3H:1V.
The drop you actually stake and string: slope times horizontal distance. This is the number you set with a line level at the far stake.
Example:
A 1% french-drain grade over a 40 ft trench: 0.01 × 40 × 12 = 4.8 in of fall (~4 13/16 in).
The inverse question — how much drop a run needs to meet a code minimum. Used by the compliance check when a run length is entered.
Example:
To grade 10 ft away from a foundation at 5%: 0.05 × 10 × 12 = 6 in — the IRC R401.3 wording verbatim.
Converts plumbing-style pitch to percent. The common fractions: 1/16 in/ft ≈ 0.52%, ⅛ in/ft ≈ 1.04%, ¼ in/ft ≈ 2.08%.
Example:
⅛ in/ft = 0.125 ÷ 12 × 100 = 1.04% — why "1% minimum" and "⅛ inch per foot" are used interchangeably for french drains.
One vertical unit over H horizontal units. Steeper ratios have smaller H values.
Example:
A 1:12 ADA ramp (12H:1V) = 100 ÷ 12 = 8.33% — the ADA 405.2 maximum.
At grading slopes the hypotenuse and the horizontal run are nearly identical — at 5% the difference is 0.12%. The calculator treats tape-measured ground distance as run, which is standard field practice below ~10%.
Example:
A 40 ft run at 2%: slope length = √(0.8² + 40²) = 40.008 ft — a 0.1 in difference, far below tape accuracy.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Inches per foot | 12 | Length conversion used in every in/ft formula (NIST Handbook 44). |
| Foundation grading minimum | 5% (6 in in 10 ft) | IRC R401.3 — ground must fall at least 6 inches within the first 10 feet of the foundation. |
| Impervious surface minimum | 2% | IRC R401.3 exception — patios, walks, and driveways within 10 ft of the foundation slope at least 2% away where the 6-in fall is impractical. |
| Drain pipe pitch, ≤ 2½ in | ¼ in/ft (≈ 2.08%) | IPC Table 704.1 minimum slope for building drains and sewers 2½ inches and smaller. |
| Drain pipe pitch, 3–6 in | ⅛ in/ft (≈ 1.04%) | IPC Table 704.1 minimum slope for 3-to-6-inch pipe — the basis of the french-drain "1% rule." |
| Drain pipe pitch, ≥ 8 in | 1/16 in/ft (≈ 0.52%) | IPC Table 704.1 minimum slope for 8-inch and larger pipe. |
| Accessible walkway maximum | 1:20 (5%) | ADA 403.3 — a walking surface steeper than 1:20 must be built as a ramp. |
| Accessible ramp maximum | 1:12 (8.33%) | ADA 405.2 — maximum running slope for new-construction ramps. |
| Paver patio drainage | 1.5–2% | ICPI Tech Spec 2 recommended surface slope away from structures for interlocking concrete pavement. |
| 45° reference | 100% grade | Equal rise and run — the anchor point showing percent and degrees are different scales. |
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.
International Residential Code — Surface Drainage(IRC R401.3)
View StandardRequires the ground to fall away from foundations a minimum of 6 inches within the first 10 feet (5%). Where lot lines, walls, or slopes prohibit it, drains or swales are required, and impervious surfaces within 10 feet may slope a minimum of 2% away.
Key Requirements:
- •Minimum 6 in of fall within the first 10 ft of the foundation (5%)
- •Impervious surfaces within 10 ft: minimum 2% slope away from the building
- •Swales or drains required where the grade cannot be achieved
International Plumbing Code — Slope of Horizontal Drainage Piping(IPC Table 704.1)
View StandardSets the minimum pitch of horizontal drains and building sewers by pipe diameter: ¼ in/ft for 2½-inch and smaller, ⅛ in/ft for 3-to-6-inch, and 1/16 in/ft for 8-inch and larger pipe.
Key Requirements:
- •2½ in and smaller: ¼ in per foot (≈ 2.08%)
- •3 in to 6 in: ⅛ in per foot (≈ 1.04%)
- •8 in and larger: 1/16 in per foot (≈ 0.52%)
ADA Standards — Walking Surfaces(ADA 403.3)
View StandardThe running slope of an accessible walking surface may not exceed 1:20 (5%); cross slope may not exceed 1:48 (about 2%). Anything steeper than 1:20 must comply with the ramp provisions.
Key Requirements:
- •Running slope ≤ 1:20 (5%)
- •Cross slope ≤ 1:48 (≈ 2.08%)
ADA Standards — Ramps(ADA 405.2)
View StandardRamp runs in new construction may not exceed a running slope of 1:12 (8.33%), with a maximum 30-inch rise per run between landings.
Key Requirements:
- •Running slope ≤ 1:12 (8.33%)
- •Maximum 30 in rise per ramp run; landings between runs
- •Cross slope ≤ 1:48
ICPI Tech Spec 2 — Construction of Interlocking Concrete Pavements(ICPI Tech Spec 2)
View StandardRecommends a minimum 1.5% surface slope (2% preferred) on paver patios, walks, and driveways, pitched away from buildings, for surface drainage of interlocking concrete pavement.
Key Requirements:
- •Minimum 1.5% surface slope; 2% preferred
- •Pitch away from structures and toward drainage collection
NIST Handbook 44 — Units of Length(NIST HB 44)
View StandardDefines the foot–inch relationship (1 ft = 12 in) underlying every inches-per-foot conversion in this calculator.
Key Requirements:
- •1 foot = 12 inches exactly
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.
Municipal driveway grade caps
No national code — cities set their own maximums
There is no IRC maximum driveway slope; municipalities regulate it through zoning and public-works standards, most commonly capping residential driveways between 10% and 15%, with transition-zone rules at the street.
Regional Examples:
Stormwater and impervious-surface rules
Where your 2% patio slope is allowed to drain
Many jurisdictions regulate where graded surfaces may discharge — not just the slope itself. Directing runoff onto neighboring lots or into the sanitary sewer is widely prohibited, and some regions require on-lot infiltration.
Regional Examples:
Expansive-soil regions grade steeper
Clay soils want more positive drainage than IRC minimums
On expansive clays (Texas, Colorado Front Range, parts of California), geotechnical reports and local amendments commonly call for steeper positive drainage than the IRC 5% — often 10% in the first 10 feet for landscaped areas — because ponding against the foundation drives soil movement.
Regional Examples:
Cold climates: frost, ice, and walkway slope
Shallower can be safer where surfaces ice over
In freeze-thaw climates, exterior walks and ramps at the accessibility maximums become hazardous when iced. Northern jurisdictions and universal-design guidance often target shallower ramps (1:16 to 1:20) even though 1:12 is legal.
Regional Examples:
Plumbing code family differs by state
IPC vs UPC pitch tables
States adopt either the International Plumbing Code or the Uniform Plumbing Code. The commonly used pitches are the same (¼ in/ft small pipe, ⅛ in/ft larger), but UPC jurisdictions apply ¼ in/ft to pipe 2 in and smaller and permit ⅛ in/ft on 3-in-and-larger only where approved.
Regional Examples:
Hillside and grading-permit thresholds
When moving dirt itself needs a permit
Jurisdictions with hillside ordinances trigger engineering review by slope: cut/fill on natural grades steeper than a threshold (often 3H:1V or 15–20%) requires a grading permit, soils report, or engineered plan under IBC Appendix J or local equivalents.
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
Related Calculators
French Drain Calculator
Free French drain calculator estimates gravel (yd³ & tons), pipe length, and filter fabric. Get the right trench size, slope, and #57 stone amount.
Retaining Wall Calculator
Estimate blocks, caps, base & drainage stone, geogrid, rebar, timber, or gabion baskets for any retaining wall. Per NCMA & IRC. Free, no signup.
Paver & Patio Calculator
Calculate pavers, base aggregate, bedding sand, polymeric sand, and edging for sand-set, mortar-set, pedestal & PICP installs. ICPI Tech Spec.
How to Use This Calculator
- Pick how you know the slope: a measured rise and run, a percent grade, inches per foot, degrees, or an H:1V ratio.
- Rise-and-run mode: enter the drop and the horizontal distance in feet and inches — level a string from the high point and measure down to grade.
- Single-value modes: enter the percent, in/ft (⅛ = 0.125, ¼ = 0.25), degrees, or the H in the H:1V ratio.
- Optionally enter your run length in feet to get the total fall in inches — the number you stake and string.
- Optionally pick an application — foundation grading, french drain, drain pipe, patio, walkway, ramp, or driveway — to check the slope against its code minimum or maximum.
- Click Calculate to see the grade as percent, degrees, inches per foot, and ratio, the total fall, the code check, and a reference table of common construction slopes.
Percent, Degrees, Inches per Foot, and H:1V — Why Four Units?
Each trade writes slope in its own unit, and they are NOT interchangeable scales. Percent grade (rise ÷ run × 100) is how civil and landscape drawings call out grading — IRC R401.3 wants 5% away from the foundation. Inches per foot is the plumbing and flatwork unit — IPC 704.1 pitches small drain pipe at ¼ inch per foot, which is 2.08%, and the french-drain "1% rule" is the ⅛ inch per foot pitch of larger pipe. Degrees come off an angle finder or digital level and follow the tangent, not a straight proportion — a 100% grade is 45°, not 90°. H:1V ratios are geotechnical shorthand for embankments and retaining-wall backslopes, where 3H:1V means 3 feet of run per foot of rise (about 18°). This calculator holds one slope and speaks all four, so a "2% patio," a "¼-inch-per-foot pipe," and a "1.15° angle" stop being three different-sounding numbers for the same grade. Roof slopes use a fifth unit — x:12 pitch — covered by the Roof Pitch Calculator.
Frequently Asked Questions
How do you calculate slope percentage?
Divide the rise (the vertical drop) by the run (the horizontal distance) and multiply by 100 — rise and run in the same unit. A 6-inch drop over 10 feet is 0.5 ÷ 10 × 100 = 5%, which is exactly the IRC R401.3 minimum for grading away from a foundation. Measured in mixed units, convert first: 8 inches over 25 feet is 0.667 ÷ 25 × 100 = 2.7%. This calculator takes the rise and run in feet and inches and does the conversion for you.
What slope do I need for drainage?
It depends on what's draining. Ground next to a foundation: 6 inches of fall in the first 10 feet (5%) per IRC R401.3. Patios, walks, and driveways within 10 feet of the house: minimum 2% away, per the same section and ICPI Tech Spec 2. French drains and 3–6 inch drain pipe: ⅛ inch per foot (about 1%) per IPC Table 704.1; pipe 2½ inches and smaller wants ¼ inch per foot (about 2%). Lawns and swales keep water moving at 1–2%. The calculator's application check compares your grade to each of these and tells you how much fall you're short.
Is a 2% slope the same as 2 degrees?
No — percent and degrees are different scales. Percent grade is rise ÷ run × 100; degrees is the inverse tangent of rise ÷ run. A 2% grade is only 1.15°, and a 45° angle is a 100% grade, not 45%. They track each other loosely at shallow slopes (each degree is roughly 1.75% near zero) but diverge fast as slopes steepen. Specs mix both units — grading plans use percent, angle finders read degrees — which is exactly why this calculator shows the same slope in both.
How much fall does my french drain need over its full run?
Total fall = slope × run length. At the standard 1% minimum (⅛ inch per foot), a 40-foot trench needs 0.01 × 40 × 12 = 4.8 inches — call it 4 13/16 inches — of drop from inlet to outlet. A 60-foot run needs about 7¼ inches. Enter your run length in the calculator and it returns the exact fall to stake out, plus a string-line procedure: stake both ends, level the string, then lower the outlet end by the total fall.
What does 3H:1V mean, and how steep is it?
It's a grading ratio: 3 horizontal units of run for every 1 vertical unit of rise — about 18° or a 33% grade. Geotechnical and retaining-wall specs write slopes this way; the Retaining Wall Calculator's backslope options (3H:1V, 2H:1V, 1.5H:1V) use it because the slope above a wall increases the earth pressure on it. Measure your bank's rise and run, enter them here, and read off the H:1V ratio to pick the right backslope option. The smaller the H, the steeper the slope — 1H:1V is 45° and generally engineer territory.
What is the maximum slope for a driveway or a ramp?
Driveways have no IRC maximum — municipalities cap them, most commonly at 12–15%, with 10% or less considered comfortable, so verify locally. Accessible ramps are federal: ADA 405.2 caps new-construction ramps at 1:12 (8.33%) with a maximum 30-inch rise between landings, and ADA 403.3 caps walking surfaces at 1:20 (5%) — steeper than that and it must be built as a ramp. The calculator's driveway, ramp, and walkway checks apply these limits to your measured grade.
How do I measure the slope of my yard without special tools?
A string, a line level, a tape, and two stakes. Drive a stake at the high point and one downhill at a known distance (10 feet is convenient because the percent math is easy). Tie the string at grade on the high stake, level it with the line level, and measure from the string down to grade at the low stake. That drop over that distance is your rise and run — 7 inches down at 10 feet is a 5.8% grade. For short runs, a 4-foot level with a known-thickness block under one end does the same job.