Concrete Footing Size Calculator
IRC footing width and thickness by stories, foundation, and soil.
What it does
Footings spread wall loads onto soil, and soil strength decides their width. Strong gravel carries a house on 12 inches while soft clay needs twice that, which is why the code tables width against soil capacity rather than guessing one size. Undersized footings settle and crack walls.
The tool above reads IRC Table R403.1(1) for light frame houses. Pick stories, foundation, and soil psf. It returns minimum width and thickness with interpolation between soil columns. Defaults of 2 story basement at 1500 open solved at 22 by 6 in.
The table packs real load paths into small numbers. A 2 story basement at 1500 psf needs 22 in while the same house at 2000 needs only 16, because width tracks load divided by capacity. One story slabs sit at the 12 in minimum almost everywhere.
Interpolation has a firm boundary: between columns yes, beyond 1500 or 4000 never. At 1750 the worked row reads 19 in, splitting 22 and 16 evenly. Below 1500 the official must order a soils investigation instead of math.
Use the table for common cases, then build to frost depth on undisturbed soil. Wider houses, heavier snow, piers, and fireplaces each carry their own adjustments beyond this page.
How to use
- Pick 1, 2, or 3 stories.
- Pick slab, crawl, or basement.
- Enter soil capacity 1500 to 4000 psf.
- Read width first, then thickness.
- Confirm frost depth and projection rules.
- Call an engineer outside the table.
Formula
- Stories
- 1, 2, or 3 stories of light frame.
- Foundation
- Slab on grade, crawl space, or basement.
- Soil
- Allowable soil pressure 1500 to 4000 psf.
- Width
- Interpolated table width in inches.
2024 IRC Table R403.1(1) minimum width and thickness for light frame construction at 20 psf roof live load, 20 psf subset: 1 story basement 18-14-12, 2 story basement 22-16-13-12, 3 story crawl 19-14-12, 3 story basement 25-19-15-13-12 across 1500 to 4000 psf. Linear interpolation of width between soil columns is permitted; extrapolation is not. Minimum 12 in wide by 6 in thick.
Worked example
Example: 2 story basement, 1500 psf
Two story house with basement on 1500 psf soil. Find minimum footing size.
- Pick 2 stories with basement on 1500 psf soil.
- Table R403.1 row 2 story plus basement at 1500 reads 22 x 6.
- No interpolation: 1500 sits on a column.
- Minimum width 22 in, thickness 6 in.
- Projection rules and frost depth still apply.
- Heavier snow or wider houses need the full table.
Reference table
| House | 1500 psf | 2000 psf | 2500 psf |
|---|---|---|---|
| 1 story basement | 18 x 6 | 14 x 6 | 12 x 6 |
| 2 story basement (worked) | 22 x 6 | 16 x 6 | 13 x 6 |
| 3 story crawl | 19 x 6 | 14 x 6 | 12 x 6 |
| 3 story basement | 25 x 8 | 19 x 6 | 15 x 6 |
Limits
- 20 psf snow subset; heavier snow differs.
- 32 ft house assumption; adjust wider.
- Continuous wall footings only.
- Undisturbed soil or engineered fill assumed.
Common mistakes
- Extrapolating below 1500 psf.
- Skipping frost depth.
- Pouring on topsoil or fill.
- Ignoring projection limits.
- Sizing piers from wall tables.
Try it yourself
Quick checks. Answers sit tucked under each question.
1 story basement at 2000 psf?
Row 1 story basement at 2000 reads 14 in by 6 in.
3 story crawl space at 1500 psf?
Row 3 story crawl at 1500 reads 19 in by 6 in.
2 story basement at 1750 psf?
Between 22 and 16: 22 + (16 - 22) x 0.5 = 19 in, thickness from the 1500 column is 6 in.
Questions
How does the table work?
Stories plus foundation type against soil capacity. A 2 story basement at 1500 psf needs 22 in; at 2000 it needs 16.
Can I interpolate?
Yes, between soil columns only, never beyond 1500 or 4000. At 1750 the 2 story basement reads 19 in.
What is the absolute minimum?
12 in wide by 6 in thick, on undisturbed soil or engineered fill below frost line.
How do I know my soil psf?
Presumptive values: gravel 3000, sand mixes 2000, clay less. Below 1500 needs a soils investigation.
What are the table assumptions?
The table assumes a 32 ft wide house and 20 psf snow. Wider houses add 2 in width per 4 ft; heavier snow needs full calculation.
Does it cover piers?
No. Piers, columns, fireplaces, and retaining walls follow their own tables and figures.
Sources
- ICC IRC Chapter 4: foundations, Table R403.1
- Washington State code: R403 footings minimum size (WAC)