Footing size calculator for a column
This footing size calculator gives a first estimate of the plan size of an isolated (pad) footing under a column. Enter the column load, the safe bearing capacity of the soil and the column size, and it works out the area needed, rounds the sides up to a buildable size, checks the bearing pressure and shows Rankine’s minimum depth of foundation. It then gives the concrete volume and the cement, sand and aggregate for the mix you choose.
It is meant for planning, budgeting and checking a drawing — not for issuing a design. A footing also has to be checked for bending, one-way shear and punching shear, and its reinforcement detailed, by a qualified structural engineer.
How to use the Footing Size Calculator
- Choose metric (kN, kN/m², mm) or US units (lb, psf, inches).
- Enter the column load at service level — the unfactored dead plus live load the column carries — and the allowance for the weight of the footing and the soil over it (10 % is usual).
- Enter the safe bearing capacity from a soil test, or for a US house pick one of the IRC presumptive values.
- Choose square or rectangular, enter the column sides and the footing thickness, and the soil unit weight and friction angle for the depth check.
- Read the footing size, pressure, depth and concrete, and copy the summary.
Footing Size Calculator features
Square or rectangular
Rectangular footings keep equal projections from every column face, the usual economical shape.
Buildable sizes
Sides rounded up to 50 mm (2 in), with the actual bearing pressure checked against the capacity.
Depth of foundation
Rankine’s minimum depth beside the code minimum, and the larger of the two suggested.
Concrete and cement
Volume for one or several identical footings, with cement bags, sand and aggregate for the mix.
The footing size formula
Area required = (column load + self-weight) ÷ safe bearing capacity. For a square footing each side is the square root of the area. Example: a column carrying 600 kN, plus 10 % for the footing and backfill, is 660 kN; on soil with a safe bearing capacity of 200 kN/m² the area is 660 ÷ 200 = 3.3 m², so the side is √3.3 = 1.82 m, rounded up to 1.85 × 1.85 m. The pressure under it is 660 ÷ 3.42 = 193 kN/m², within 200. At 450 mm thick it holds 1.54 m³ of concrete — about 13 bags of cement in M20.
For a rectangular column the footing is made rectangular so that it projects equally on all sides: L − B = a − b. The bearing capacity is a working (allowable) value, so use the unfactored load, not the factored load used for limit-state design of the concrete.
How deep should a footing be?
Rankine’s formula gives a minimum depth for cohesionless soil: D = (q ÷ γ) × ((1 − sin φ) ÷ (1 + sin φ))². With q = 200 kN/m², γ = 18 kN/m³ and φ = 30° it is 11.1 × 0.111 = 1.23 m. It is a textbook check and is not given in IS 1904; clayey or expansive (black cotton) soil usually needs a deeper or special foundation.
IS 1904:1986 clause 7.2 requires foundations at least 50 cm below natural ground level, and IS 456:2000 clause 34.1.2 sets a minimum edge thickness of 150 mm for footings on soil. In the US, IRC section R403.1.4 asks for at least 12 in below undisturbed ground and below the local frost line. Check the latest edition of the code you build under.
Presumptive bearing values (US houses)
These values (as printed in the 2025 California Residential Code) apply where no soil test is required; weaker soils need a soils investigation. In India, IS 1904 clause 18.1 asks for the safe bearing capacity to be worked out per IS 6403 from soil tests — local experience may guide a preliminary estimate only.
| Class of material (IRC Table R401.4.1) | psf | kN/m² |
|---|---|---|
| Crystalline bedrock | 12,000 | 575 |
| Sedimentary and foliated rock | 4,000 | 192 |
| Sandy gravel or gravel (GW, GP) | 3,000 | 144 |
| Sand, silty sand, clayey sand, silty or clayey gravel | 2,000 | 96 |
| Clay, sandy clay, silty clay, clayey silt, silt | 1,500 | 72 |
Footing Size Calculator FAQ
How do I calculate the size of a footing?
Divide the column load plus about 10 % for self-weight by the safe bearing capacity to get the area, then take the square root for a square footing. 400 kN on 150 kN/m² soil needs 2.93 m², a 1.75 m square.
What is a typical footing size for a G+1 house?
Often 1.2 × 1.2 m to 1.8 × 1.8 m under each column, depending on the column loads and soil. Only your structural engineer’s drawing can confirm it.
What is the minimum depth of a foundation?
IS 1904 requires at least 0.5 m below natural ground; in practice footings are commonly 1.2–1.5 m deep, and deeper in expansive soil. In cold US climates the frost line governs.
What is the minimum thickness of a footing?
IS 456 clause 34.1.2 sets 150 mm at the edge for footings on soil. Real footings are usually 300–600 mm thick, set by shear design.
Which load should I use?
The service load (dead + live, unfactored) when comparing with a safe bearing capacity. Factored loads are for designing the concrete and steel.
Does it design the reinforcement?
No. For bar quantities once the design is known, use the steel quantity calculator or the bar bending schedule; for digging, the excavation calculator.