Footing Size Calculator
This free footing size calculator sizes a continuous (strip/wall) footing the way a plans examiner checks it — from the IRC prescriptive width/thickness table, not a rule of thumb. Tell it how many stories the wall carries, whether the building has a slab-on-grade, crawl space, or basement, your soil's bearing value, and the ground snow or roof live load, and it returns the minimum width and thickness straight from IRC Table R403.1(1).
Soil bearing is the biggest lever in the table — the same wall can need a footing nearly twice as wide on soft clay as on gravel or rock. This tool lets you pick a standard IRC R401.4.1 default (clay, sand, gravel, rock) or enter your own soils-report value, and it linearly interpolates between the table's six soil columns exactly as footnote (a) permits — never extrapolating past 1,500 or 4,000 psf, where the code stops tabulating and an engineer is needed.
It also handles the table's building-width adjustment: the base table assumes a 32-foot building width, and footnotes (c) and (d) widen or narrow the footing for other widths. Enter a footing run length and it adds concrete volume in cubic yards and bags. Distinct from the Deck Footing Calculator (an entirely different table, R507.3.1, for isolated posts) — this sizes the continuous footing under a wall. Sizing only — no pricing, no signup.
Footing Size Calculator
Size a continuous strip footing under a load-bearing wall — width and thickness from story count, foundation type, snow/live load, and soil bearing, straight from IRC Table R403.1(1). Free, no signup.
Building
Story count, foundation type, and snow/live load drive the table row.
The table assumes a 32 ft building width. Wider or narrower adjusts the footing size automatically (footnotes c/d).
Soil
Soil bearing is the lever — better soil means a smaller footing.
Not sure of your soil? 1,500 psf (clay/silt) is the conservative code default without a soils test.
Footing length (optional)
Enter a length to get concrete volume for this run of footing.
Calculation Formulas
Minimum footing width and thickness are read directly from IRC Table R403.1(1), indexed by ground snow/roof live load, story count, foundation type (slab-on-grade, crawl space, or plus basement), and soil bearing. Values are transcribed from the code table, not calculated.
Example:
20 psf load, 1-story, slab-on-grade, 1,500 psf soil → 12-inch wide × 6-inch thick footing.
The same wall load on stronger soil needs less bearing width. The table's six soil columns show the payoff directly — moving from clay (1,500 psf) toward gravel/rock can nearly halve the footing width on a 3-story basement wall.
Example:
3-story basement, 20 psf: 22-inch wide on 1,500 psf soil vs. 12-inch wide on 3,000+ psf soil.
A soil-test value falling between the table's six columns (1,500/2,000/2,500/3,000/3,500/4,000 psf) may be linearly interpolated per footnote a, then rounded up to the nearest whole inch — a prescriptive minimum is never rounded down. Values below 1,500 or above 4,000 psf are not extrapolated; the table clamps at those boundaries.
Example:
A 2,750 psf soil test interpolates halfway between the 2,500 and 3,000 psf columns.
The table is built for a 32-foot building width perpendicular to the footing (footnote b). Wider buildings need more footing per footnotes c/d; narrower buildings may reduce it, floored at the table's own 12"×6" minimum. This tool rounds the adjustment conservatively — up when growing, down when shrinking — so it never under-sizes.
Example:
A 40-ft-wide building (8 ft over baseline, 2 steps of 4 ft) adds 4" to width and 2" to thickness.
Concrete for a run of continuous footing is its cross-sectional area times the length. Bag count divides that volume by the bag yield (0.45 ft³ for 60 lb, 0.60 ft³ for 80 lb), rounded up — though continuous footings are almost always poured with ready-mix.
Example:
A 16-inch wide × 6-inch thick footing, 40 ft long ≈ 26.7 ft³ ≈ 1 yd³.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Table load basis | 20/30/50/70 psf | R403.1(1) tabulates four ground-snow-or-roof-live-load levels; 20 psf roof live load and 25 psf ground snow load share the same table row. |
| Soil columns | 1,500 / 2,000 / 2,500 / 3,000 / 3,500 / 4,000 psf | Six soil-bearing columns, with linear interpolation permitted between them (footnote a). No extrapolation outside this range. |
| Default soil | 1,500 psf | Clay/silt is the conservative default where no soils investigation is done. Below 1,500 psf suspected → a soils investigation is required (R401.4.1). |
| Baseline building width | 32 ft | Footnote b — the table's loads assume a 32-ft building width perpendicular to the footing. Footnotes c/d adjust the size for other widths. |
| Baseline wall height | 9 ft (8 ft basement) | Footnote b — the table's dead/live loads assume this wall height with an interior bearing wall or beam at each floor. |
| Minimum thickness/width | 6" × 12" | The table's own floor — no footing this table produces is ever smaller than 6 inches thick or 12 inches wide. |
| Bag yield | 0.45 / 0.60 ft³ | A 60 lb bag yields ~0.45 ft³ of concrete; an 80 lb bag ~0.60 ft³. Used for the optional bag counts. |
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 Table R403.1(1) — Minimum Width and Thickness for Concrete Footings(IRC R403.1 / Table R403.1(1))
View StandardThe prescriptive table that sizes a continuous (strip/wall) footing under light-frame construction, by ground snow/roof live load, story count, foundation type, and soil bearing.
Key Requirements:
- •Footing size read by load × story × foundation type × soil bearing
- •Interpolation permitted between soil-bearing columns; extrapolation not permitted
- •Table assumes 32 ft building width, 9 ft wall height (8 ft basement) — footnotes c/d adjust otherwise
- •Applies to light-frame construction only
IRC Table R401.4.1 — Presumptive Load-Bearing Values(IRC R401.4.1)
View StandardPresumptive soil load-bearing values used where no geotechnical investigation is done — from 1,500 psf clay to 12,000 psf bedrock.
Key Requirements:
- •Clay/silt (CL, ML, MH, CH) — 1,500 psf
- •Sand / silty-clayey sand & gravel (SW, SP, SM, SC, GM, GC) — 2,000 psf
- •Sandy gravel / gravel (GW, GP) — 3,000 psf; sedimentary rock 4,000+; bedrock 12,000
- •Below 1,500 psf suspected → a soils investigation is required
IRC R403.1.4 / R403.1.4.1 — Footing Depth & Frost Protection(IRC R403.1.4, R403.1.4.1)
View StandardSets the minimum footing depth and requires footings to be protected from frost heave — applies alongside the width/thickness table.
Key Requirements:
- •Exterior footings at least 12 inches below the undisturbed ground surface
- •Extend below the frost line (Table R301.2), or per R403.3 / ASCE 32, or bear on rock
- •Footings shall not bear on frozen soil unless permanently frozen
ACI 332 — Residential Code Requirements for Structural Concrete(ACI 332)
View StandardThe concrete-industry companion standard the IRC's footing and foundation provisions align with for residential construction.
Key Requirements:
- •Minimum concrete compressive strength for footings
- •Reinforcement detailing where required by seismic design category
- •Placement and curing practices for foundation concrete
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
Bearing value sets the footing width
Presumptive bearing ranges from 1,500 psf (clay) to 4,000+ psf (rock). Where a geotechnical report gives a higher value, footings can shrink significantly; where soft or organic soils are found, the table doesn't apply and the site needs engineering.
Regional Examples:
Snow Load
20 to 70+ psf
The footing table steps up with ground snow/roof live load. Mountain and northern regions with heavy snow need wider footings for the same building than mild regions at 20 psf.
Regional Examples:
Frost Depth
Depth, not width, but still regional
This table sizes width and thickness only — depth is set separately (R403.1.4) by the local frost line, which ranges from essentially zero in the Deep South to 60+ inches in the far North.
Regional Examples:
Expansive & Poor Soils
When the table doesn't apply
Expansive clays, uncontrolled fill, high water tables, and slopes fall outside the presumptive-bearing table and often require a geotechnical report, deeper/wider footings, or an engineered foundation.
Regional Examples:
Seismic Design Category
Reinforcement, not just size
In higher seismic design categories (D0–D2), the IRC adds continuous footing and reinforcing requirements (R403.1.2/R403.1.3) beyond bare width and thickness — this tool doesn't cover that reinforcement.
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
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
- Pick your story count and foundation type (slab-on-grade, crawl space, or plus basement).
- Select your ground snow or roof live load.
- Enter your building width perpendicular to this footing — 32 ft is the table's baseline.
- Choose your soil bearing value, or enter a soils-report psf if you have one.
- Optionally enter the length of this footing run for a concrete volume estimate.
- Click Calculate for the minimum footing width and thickness, a soil-bearing comparison, and the code notes behind the number.
How the IRC Sizes a Continuous Footing
A continuous footing spreads a wall's load over enough soil that the ground doesn't settle unevenly under it — and the IRC sizes that prescriptively from four things: how many stories bear on the wall, whether there's a basement (which adds hydrostatic and lateral load), how much snow or live load the roof and floors carry, and how strong the soil is. More stories and a basement both widen the footing; better soil narrows it. The table is built around a 32-foot building width and a specific set of dead and live loads (footnote b) — outside those assumptions, or on a different framing plan, the actual required footing can differ, which is why this stays a screening tool rather than a substitute for a stamped foundation plan. Soil bearing is where most homeowners can meaningfully shrink a footing: the 1,500 psf clay default is conservative, and a geotechnical report showing better soil — sand, gravel, or rock — can narrow the footing significantly, sometimes by close to half on a multi-story basement wall. When the table's columns don't bracket your soil value or your building falls outside its assumptions, that's the signal to bring in a foundation engineer.
Frequently Asked Questions
What size footing do I need for a foundation wall?
It depends on story count, foundation type, soil bearing, and snow/live load. Per IRC Table R403.1(1), a 1-story slab-on-grade wall at 20 psf load needs just a 12-inch wide × 6-inch thick footing on any soil 1,500 psf or better. A 3-story wall with a basement at the same load needs 22 inches wide × 7 inches thick on 1,500 psf clay, dropping to 12×6 once soil bearing reaches 3,000 psf. Enter your numbers and this calculator returns the minimum width and thickness straight from the table.
How is this different from the Deck Footing Calculator?
They size two completely different footing types from two different IRC tables. This calculator sizes a CONTINUOUS (strip/wall) footing under a load-bearing wall or stem wall — IRC Table R403.1(1), by story count and soil bearing. The Deck Footing Calculator sizes an ISOLATED point footing under a single deck post — IRC Table R507.3.1, by tributary area. A deck post footing and a house foundation footing are never sized from the same table.
Does soil type really change the footing width?
Yes — it's the biggest lever you control. The IRC table has six soil-bearing columns from 1,500 to 4,000 psf, and stronger soil carries the same wall load over less width. On a 3-story basement wall, moving from clay (1,500 psf) to gravel or rock (3,000+ psf) can shrink the footing from 22 inches down to 12 inches — nearly half. If you don't know your soil, 1,500 psf is the conservative code default; a soils report showing better bearing can justify a narrower footing, and where soils below 1,500 psf are suspected, a soils investigation is required.
Does building width matter, not just story count?
Yes. IRC Table R403.1(1) is built around a 32-foot building width perpendicular to the footing — wider buildings funnel more roof and floor load to the wall below, so footnotes (c) and (d) widen the footing 2 inches and thicken it 1 inch for every 4 feet over 32 feet, and allow the reverse (down to the table's own 12-inch/6-inch floor) for narrower buildings. This calculator applies that adjustment automatically from the building width you enter.
What's the difference between this and the concrete-calculator's footing mode?
concrete-calculator's footing option takes a width and depth you already know and computes the concrete volume — it doesn't size the footing itself. This calculator does the sizing step first: it reads the minimum width and thickness straight from IRC Table R403.1(1) for your story count, soil, and load, then (optionally) computes the concrete volume for a given run of footing. Use this one to find the size; feed that size into concrete-calculator (or the volume this tool already gives you) for materials.
Can I interpolate between the table's soil-bearing values?
Yes — IRC footnote (a) explicitly permits linear interpolation between the table's soil columns (1,500/2,000/2,500/3,000/3,500/4,000 psf), and this calculator does that automatically when you enter a soils-report value that falls between two columns, rounding the result up to the nearest whole inch so it never under-sizes. Extrapolation outside that 1,500–4,000 psf range is not permitted by the code — below 1,500 psf a soils investigation is required, and above 4,000 psf (essentially rock) the table simply doesn't need to go higher.
Does this footing size include reinforcement?
No — this table (R403.1(1)) sizes plain, unreinforced concrete width and thickness only. In higher seismic design categories (D0–D2), the IRC separately requires continuous footings and specific rebar per R403.1.2 and R403.1.3, which this calculator doesn't compute. It also doesn't set footing depth — that's a separate rule (R403.1.4) driven by your local frost line, not by this table.