How to use the Isolated Footing Size Calculator
- 1. Enter load and soilService column load, self-weight allowance and safe bearing capacity.
- 2. Enter column and materialsColumn size, concrete grade, steel grade, cover and bar size.
- 3. Get the footingPlan size, net upward pressure, moment and the depth needed for each check.
How an isolated footing is sized
Area = column load × (1 + self-weight %) ÷ safe bearing capacity. A square footing has side √A rounded up. A rectangular footing with equal projections solves (b + 2x)(D + 2x) = A.
For design the soil pressure uses the factored load without self-weight: qu = 1.5P ÷ area. The cantilever moment at the column face is Mu = qu × width × x²/2, and the depth for bending is d = √(Mu ÷ (0.138 fck b)) for Fe 415. Shear often governs footings: one-way shear is checked at d from the column face against τc (IS 456 Table 19), and punching shear at d/2 against ks × 0.25√fck.
Worked example and site tips
Column 400 × 400 mm, load 1000 kN, SBC 200 kN/m², 10% self-weight: area = 1100/200 = 5.5 m², side 2.345 → 2.35 m. qu = 1500/5.52 = 271.6 kN/m². Projection 0.975 m, Mu = 303.4 kNm, d for bending = 216 mm. With M20 and 0.25% steel, one-way shear needs about 420 mm and punching about 382 mm, so shear governs and overall depth is about 500 mm.
- The SBC must come from a soil report. Do not guess it.
- The tool assumes a concentric load. Eccentric loads and moments need a full design.
- Footings in IS 456 need at least 50 mm cover.
Frequently asked questions
How is the size of a footing calculated?
Divide the column load plus self-weight (often 10%) by the safe bearing capacity of the soil. That gives the area; take the square root for a square footing and round up.
Why is self-weight not used in the design pressure?
The footing's own weight and the soil on it are balanced directly by soil pressure under them, so they do not bend the footing. Bending and shear use only the column load.
Which check usually decides footing depth?
For typical loads, one-way or punching shear needs a larger depth than bending. That is why the tool checks all three and takes the largest.
Is this calculator free?
Yes. You fill a short form once to see your result, and other Uncodemy tools then open without the form. Everything is calculated in your browser and nothing you type is uploaded.