Footing Size Calculator
Find the minimum footing size for a post, column, or wall based on the load it carries and your soil’s bearing capacity, plus the concrete needed to pour it.
Total load carried by this footing from the post or beam above.
Check your local building code or soil report — 1,500–3,000 psf is common for average soils.
Footing sizes round up to the nearest 2 in for buildable form dimensions. Confirm soil bearing capacity, frost depth, and reinforcement with your local building department or a structural engineer.
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Footing Size Calculator
Minimum footing size is found by dividing the load the footing carries by the soil’s allowable bearing pressure, which gives the required area. Enter your load, soil bearing pressure, and thickness above for a project-specific footing size and concrete volume.
How footing size is calculated
- Determine the total load the footing carries, from the post, column, or wall above.
- Confirm your soil’s allowable bearing pressure from a geotechnical report or your local code’s presumptive value table.
- Divide the load by the bearing pressure to find the required footing area.
- For a square footing, take the square root of the area to get the side length; for a continuous footing, that area is per linear foot of wall.
- Round the result up to a buildable form dimension — this calculator rounds to the nearest 2 in.
- Check the actual bearing pressure at the rounded size stays at or below the allowable value.
Worked example
A deck post carries an 8,000 lb load and rests on soil rated for 2,000 psf. Required area is 8,000 ÷ 2,000 = 4 ft², so the square footing side is √4 = 2 ft, or 24 in. At an 8-inch thickness, that footing needs about 0.10 cubic yards of concrete, and the actual bearing pressure at 24 × 24 in works out to exactly 2,000 psf — right at the soil’s limit, so rounding up on a borderline result is worth considering.
Typical presumptive soil bearing values
These are general presumptive bearing values commonly referenced in residential codes for preliminary sizing. Always confirm against your local code or a soil report before finalizing a design.
| Soil or rock class | Typical bearing capacity |
|---|---|
| Crystalline bedrock | 12,000 psf |
| Sedimentary rock | 6,000 psf |
| Sandy gravel / gravel | 3,000 psf |
| Sand, silty sand, clayey sand | 2,000 psf |
| Clay, sandy clay, silty clay | 1,500 psf |
| Soft clay / uncompacted fill | 1,000 psf or less (site testing required) |
Assumptions this estimate makes
- Soil is assumed level, undisturbed, and uniform to the footing’s bearing depth.
- Footing self-weight is not added to the load — typically minor at standard residential depths.
- Frost depth and local frost-line requirements are not included; check your local code for minimum footing depth.
- Footing dimensions round up to the nearest 2 in for practical, buildable form sizes.
Common mistakes to avoid
- Using a soil bearing value from a neighboring property instead of confirming your own site’s soil.
- Ignoring frost depth requirements and pouring a footing shallower than your local code allows.
- Rounding a footing size down instead of up when converting to actual form dimensions.
- Skipping a structural engineer’s review on footings carrying unusual, heavy, or eccentric loads.
Frequently asked questions
How do I calculate footing size from soil bearing capacity?
Divide the total load the footing carries by the allowable soil bearing pressure to get the required area, then take the square root for a square footing or divide by the wall length for a continuous footing.
What is a typical safe bearing capacity for soil?
Average soils are commonly rated between 1,500 and 3,000 pounds per square foot, though soft clay and uncompacted fill can be far lower and dense gravel or rock much higher.
Do I need a geotechnical report to size a footing?
For most residential structures, local building codes allow using presumptive soil bearing values from a table, but a geotechnical report is recommended for larger structures, poor soil, or when the presumptive values are in question.
How thick should a concrete footing be?
Residential footings are commonly 8 to 12 inches thick, though the required thickness depends on the load, footing width, and local code requirements for reinforcement and frost protection.
What’s the difference between a footing and a foundation?
A footing is the widened base poured below grade that spreads a structure’s load onto the soil, while the foundation is the wall or system built on top of the footing that supports the structure above.