IRC R401.4.1. Default to 1,500 with no soils report.
Footing 1
Extra run inside.
Advanced options
0 = use the code minimum.
IRC minimum 6 in, but the projection rule may force more.
From your building department.
Over-dig and irregular trench bottoms.
This footing calculator sizes continuous strip and pad footings from IRC Table R403.1(1), then checks the projection rule — the constraint that quietly makes a lot of published footing dimensions illegal.
The short answer
Footing width comes from the storeys supported, construction type and soil bearing. A two-storey light-frame house on 1,500 psf soil needs 15 inches wide; on 2,000 psf it drops to 12.
But width isn’t free of thickness. The projection past the wall must be at least 2 inches and no more than the footing is thick — which rules out a 6 inch footing at several of the table’s own widths.
On this page
How to use this footing calculator
Set the storeys from the header toggle, then the soil, wall thickness and construction type. Three layouts:
- Building perimeter — length and width, plus any interior bearing walls
- Straight run — a single length of footing
- Isolated pads — column and post footings, sized directly
The calculator returns the code minimum width, checks the projection against your footing thickness, and gives concrete volume and rebar. If the combination isn’t legal it says so and tells you the minimum thickness that fixes it.
IRC Table R403.1(1) — minimum footing width
Widths in inches, by construction type, storeys supported and soil bearing capacity.
| Construction | Storeys | 1,500 psf | 2,000 psf | 2,500 psf | 3,000 psf |
|---|---|---|---|---|---|
| Light frame | 1 | 12 | 12 | 12 | 12 |
| 2 | 15 | 12 | 12 | 12 | |
| 3 | 23 | 17 | 14 | 12 | |
| Light frame + brick veneer | 1 | 12 | 12 | 12 | 12 |
| 2 | 21 | 16 | 12 | 12 | |
| 3 | 32 | 24 | 19 | 16 | |
| 8 in solid masonry | 1 | 16 | 12 | 12 | 12 |
| 2 | 29 | 21 | 17 | 14 | |
| 3 | 42 | 32 | 25 | 21 |
Two things worth noting. The table is only valid at 1,500 psf and above — softer ground needs an engineer, not a table. And these are prescriptive minimums for conventional construction, based on assumed roof and floor tributary widths. Anything unusual falls outside them.
The projection rule
This is the part most footing calculators skip entirely, and it can invalidate an otherwise correct answer.
P = (Footing width − Wall thickness) ÷ 2
P must be at least 2 in — so the wall has room to sit
P must not exceed T, the footing thickness
Rearranged: Width ≤ Wall + 2 × Thickness
P is the projection — how far the footing sticks out past the wall on each side. The upper limit exists because a wide, thin footing fails in diagonal tension: the projecting lip cracks off under load, and the bearing area you paid for stops working.
The practical consequence is that footing thickness caps footing width:
| Thickness | Max width, 6 in wall | 8 in wall | 10 in wall | 12 in wall |
|---|---|---|---|---|
| 6 in | 18 in | 20 in | 22 in | 24 in |
| 8 in | 22 in | 24 in | 26 in | 28 in |
| 10 in | 26 in | 28 in | 30 in | 32 in |
| 12 in | 30 in | 32 in | 34 in | 36 in |
Where it bites
The IRC sets 6 inches as the minimum footing thickness, and people reasonably read that as the default. But run every width in Table R403.1(1) against an 8 inch wall and check the projection:
Widths of 21, 23, 24, 25, 29, 32 and 42 inches all fail at 6 inches thick. A 21 in footing under an 8 in wall projects 6.5 in each side — more than the 6 in thickness, so it violates R403.1.1.
These aren’t exotic cases. A two-storey house with brick veneer on 1,500 psf soil needs 21 inches, which is about as ordinary as residential construction gets. It needs a footing at least 7 inches thick, and 8 is the sensible number to pour.
| Width | Projection, 8 in wall | Minimum thickness |
|---|---|---|
| 12 in | 2.0 in | 6 in |
| 15 in | 3.5 in | 6 in |
| 16 in | 4.0 in | 6 in |
| 17 in | 4.5 in | 6 in |
| 21 in | 6.5 in | 7 in |
| 23 in | 7.5 in | 8 in |
| 29 in | 10.5 in | 11 in |
| 32 in | 12.0 in | 12 in |
Note also the lower bound. A 12 in footing on a 12 in wall projects nothing at all, and a 12 in footing on a 10 in wall projects only 1 inch — below the 2 in minimum. Many builders deliberately use 4 inches of projection so that small setting-out errors still leave the wall properly supported.
Soil bearing capacity
| Soil | Presumptive value |
|---|---|
| Clay, sandy clay, silty clay | 1,500 psf |
| Sand, silty sand, clayey gravel | 2,000 psf |
| Sandy gravel, gravel | 3,000 psf |
| Sedimentary rock | 4,000 psf |
| Crystalline bedrock | 12,000 psf |
1,500 psf is what you must assume without a report. If your ground is actually sand or gravel, moving up to 2,000 psf cuts footing width by 20% for light frame, 24% with brick veneer and 28% for masonry on a two-storey house.
On a large foundation that’s several cubic yards of concrete plus the excavation, which can exceed the few hundred dollars a test costs. Worth pricing before you dig.
One caution: soil penetrometers sold for this purpose are not accepted for structural determination. If you need a bearing value above the presumptive one, it has to come from a geotechnical report.
Depth is set by frost, not by load
Width carries the load. Depth stops the ground lifting the whole thing.
- Below the local frost line — typically 12–24 in in southern states, 42–54 in across the mid-tier, and 60 in or more in the far north. Your building department publishes the figure.
- At least 12 inches below finished grade even where there’s no frost, for bearing and lateral stability.
- On undisturbed soil, never on backfill. If you over-dig, fill with compacted granular material rather than the spoil you took out.
Frost-protected shallow foundations are an alternative under IRC R403.3, but they’re only permitted for heated buildings — not detached garages or sheds unless designed specifically as unheated FPSF.
Concrete quantity
ft³ per linear foot = (Width ÷ 12) × (Thickness ÷ 12)
Cubic yards = ft³ per lf × Total length ÷ 27
Concrete weighs about 4,050 lb per cubic yard
| Footing | ft³ per lf | 140 lf perimeter |
|---|---|---|
| 12 × 6 in | 0.500 | 2.59 yd³ |
| 15 × 8 in | 0.833 | 4.32 yd³ |
| 16 × 8 in | 0.889 | 4.61 yd³ |
| 20 × 8 in | 1.111 | 5.76 yd³ |
| 21 × 8 in | 1.167 | 6.05 yd³ |
| 24 × 10 in | 1.667 | 8.64 yd³ |
Worked example — 40 × 30 ft house, 2 storeys, light frame, 1,500 psf
Footings need a genuine waste allowance. A trench bottom is never perfectly flat, the sides slump, and concrete finds every low spot. Ten per cent is a sensible minimum; on rough ground 15% is realistic.
Rebar
The IRC does not require reinforcement in residential strip footings in most conditions, but two continuous #4 bars is near-universal practice and cheap insurance against differential settlement.
| Detail | Practice |
|---|---|
| Standard | 2 × #4 continuous, in the lower third of the footing |
| Wide footings | 3 bars, or #5 in place of #4 |
| Cover | 3 in from earth — bars sit on chairs, not in the dirt |
| Laps | 40 bar diameters — 20 in for #4, 25 in for #5 |
| Corners | Bend bars round corners or add L-shaped corner bars |
A 140 lf perimeter with 2 × #4 needs about 308 lf of bar allowing for laps, which is 206 lb, or sixteen 20 ft lengths.
Bars pushed into wet concrete after the pour do essentially nothing — they end up at the wrong height and without bond. Set them on chairs before you pour.
Pad footings
Isolated pads carry columns and posts rather than walls, and they’re sized differently — from the actual tributary load divided by soil bearing, not from the width table.
Common residential sizes are 24 × 24 × 12 in for a typical basement column and 30 × 30 × 12 in for a heavily loaded one. Four pads at 24 × 24 × 12 in come to 16 ft³, about 0.6 cubic yards.
For deck posts specifically, use the deck footing calculator, which works from tributary area and handles the IRC R507 deck provisions.
Pouring in a trench
Small residential footings are often poured directly against the trench walls with no forms — “trench pour” or “dirt forming”. It works, with caveats:
- The soil must hold a clean edge. Sand and loose fill slump; clay usually stands.
- You lose dimensional control. An over-dug trench takes more concrete than calculated, which is what the waste allowance is for.
- Keep the bottom clean. Loose spoil under the footing is a soft layer exactly where you need bearing.
- Set a level top. Stakes or a screed line — the wall above depends on a flat, level surface.
Formed footings cost more in time and material but give exact dimensions and a clean top. On anything wide, deep or on questionable ground, form it.
Footing mistakes to avoid
- Six inch thickness by default. Check the projection — many table widths need more.
- Ignoring the 2 in minimum projection. A 12 in footing under a 10 in wall doesn’t meet it.
- Using the table below 1,500 psf. It isn’t valid there; you need an engineer.
- Assuming a better soil value without a report. Presumptive means presumed, and inspectors know it.
- Bearing on backfill. Over-dig gets compacted granular fill, not returned spoil.
- Shallow of the frost line. The footing lifts and the wall above cracks.
- Pushing rebar in after the pour. Wrong height, no bond, no benefit.
- No waste allowance. Trench bottoms are never flat and concrete finds every hollow.
Frequently asked questions
How wide should a footing be?
From IRC Table R403.1(1), based on storeys, construction type and soil. A two-storey light-frame house on 1,500 psf soil needs 15 inches; with brick veneer it’s 21 inches; in solid masonry, 29 inches. The absolute minimum in any case is 12 inches.
How thick should a footing be?
Six inches is the code minimum, but it often isn’t enough. The projection past the wall must not exceed the footing thickness, so a 21 inch footing on an 8 inch wall projects 6.5 inches and needs at least 7 inches of thickness. Check the projection before defaulting to 6.
What is the footing projection rule?
IRC R403.1.1 requires the projection — half the difference between footing width and wall thickness — to be at least 2 inches and no more than the footing thickness. The upper limit stops a wide thin footing cracking off its own lip in diagonal tension.
How much concrete for a footing?
Multiply width by thickness in feet for the cubic feet per linear foot, then by total length and divide by 27. A 15 × 8 in footing is 0.833 ft³ per lf, so a 140 lf perimeter takes 4.32 cubic yards before waste, or about 4.75 with 10% added.
How deep do footings need to be?
Below the local frost line, which ranges from 12 inches in the far south to 60 inches or more in northern states, and at least 12 inches below finished grade regardless. Always on undisturbed soil — never on backfill.
What soil bearing capacity should I use?
1,500 psf unless you have a geotechnical report saying otherwise — that’s the presumptive value the code requires you to assume. Clay is 1,500, sand 2,000, gravel 3,000. Handheld penetrometers aren’t accepted for structural determination.
Is a soils test worth it?
Often, on a larger foundation. Moving from the presumed 1,500 psf to a measured 2,000 psf cuts footing width by 20% for light frame and 28% for masonry on a two-storey house. That saving in concrete and excavation can exceed the cost of the test.
Do footings need rebar?
The IRC doesn’t require it in most residential strip footings, but two continuous #4 bars is standard practice and cheap protection against differential settlement. Set them on chairs with 3 inches of cover before the pour — bars pushed into wet concrete do essentially nothing.
Can I pour footings without forms?
Yes, against the trench walls, provided the soil holds a clean edge — clay usually does, sand and loose fill don’t. You lose dimensional control and use more concrete, which is what the waste allowance covers. Form anything wide, deep or on questionable ground.
What size footing for a two-storey house?
15 inches wide for light frame on 1,500 psf soil, dropping to 12 inches at 2,000 psf. Brick veneer needs 21 inches at 1,500 psf and solid masonry 29 inches. Thickness follows from the projection check, typically 8 inches for the wider cases.
How is a pad footing different?
Pads carry columns rather than walls, so they’re sized from the actual tributary load divided by soil bearing rather than from the width table. Common residential sizes are 24 × 24 × 12 inches for a basement column. For deck posts, use the deck footing calculator.
Can I use this footing calculator in metric?
Dimensions are in feet and inches, since the IRC tables are dimensioned that way. Multiply inches by 25.4 for millimetres, cubic yards by 0.765 for cubic metres, and psf by 0.0479 for kPa.
Standards and sources
- IRC Table R403.1(1) — minimum width of concrete and masonry footings
- IRC R403.1.1 — minimum size, including the 12 in width, 6 in thickness and 2 in to T projection limits
- IRC Table R401.4.1 — presumptive load-bearing values of foundation materials
- IRC R403.1.4 — minimum depth and frost protection
- Projection compliance checked against every width appearing in Table R403.1(1)
- Local amendments are common and override these values — confirm with your building department
Related calculators
How this footing calculator works out its numbers
Width comes from IRC Table R403.1(1), indexed by construction type, storeys supported and soil bearing value, with a 12 inch floor applied since that’s the code’s absolute minimum. You can override the width, and the calculator flags it if your figure falls below the table.
The projection check computes P as half the difference between footing width and wall thickness, then tests it against both limits in R403.1.1 — at least 2 inches, and no more than the footing thickness. Where the thickness is inadequate, the calculator gives the minimum that satisfies the rule rather than simply reporting a failure.
Concrete is width times thickness times length, plus waste, converted at 27 ft³ per cubic yard and 4,050 lb per yard. Rebar allows 10% for laps and reports both weight and the number of 20 ft bars.
This is a prescriptive check, not a structural design. Table R403.1(1) covers conventional light-frame residential construction on soil of 1,500 psf or better, with assumed roof and floor tributary widths. Unusual spans, point loads, expansive or weak soils, and anything outside conventional construction need an engineer. Local amendments to the IRC are common, footings are an inspected item, and the trench is normally checked open before the pour.