Concrete Footing Calculator
Work out the concrete for strip footings, pad footings and round or bell-bottom piers, with bags, trucks, digging, rebar, cost and how wide the footing should be.
Measure along the middle of the footing, all the way round.
Outside edges of the footing. We take off the corners so they are not counted twice.
Sonotube or auger hole size.
Tube sticking up out of the ground.
Leave blank for a plain cone.
Separate runs of the same size.
Leave the height blank for a footing on its own. The wall runs the full length of the footing.
Stepped footing (sloping ground)At each step the lower footing carries on under the higher one for the overlap. UK Approved Document A: overlap at least twice the step height, the footing thickness or 300 mm, whichever is most; step no taller than the footing is thick.
5–10% is usual; more for trenches with rough sides.
Full trucks carry about 8–10 cu yd (6–8 m³).
From ground level. Below your local frost line.
0 if you pour straight into a neat trench or hole.
0 if there is no gravel base.
Ask your building department.
Dug soil takes up more room: 20–30% is typical.
US ACI 318: 3 in (75 mm) when poured against earth.
Often 40 bar diameters. Check your plans.
Weight of the wall, floors, roof and snow per foot or metre. From your plans or engineer.
The weight one post or column brings down. From your plans or engineer.
Soil values are the presumptive values in US IRC Table R401.4.1, used when there is no soil test. A soil report or engineer's figure is better.
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How your footing concrete is worked out
Volume of the footing shape × how many, plus a little extra, then ÷ what one bag or truck holds.
- 1
Sizes in feet
100 ft16 in = 1.333 ft8 in = 0.667 ft
- 2
Find the volume
100 × 1.333 × 0.667= 88.89 cu ft
- 3
Add 10% extra
88.89 × 1.1= 97.78 cu ft= 3.62 cu yd
- 4
Bags or trucks
97.78 ÷ 0.6= 163 × 80 lb bagsor 1 truck
Read more: the formulas, a worked example, soil bearing values and minimum sizes
A footing is the wide concrete base that spreads the weight of a wall, post or column over the soil. The concrete you need is the volume of that shape, times how many there are, plus a little extra. Then you divide by what one bag makes or what one truck carries. For the hub on all concrete shapes (slabs, walls, stairs, curbs), use the concrete calculator.
The formula
strip footing = length × width × thickness
length from a rectangle outline = 2 × (outline length + outline width) − 4 × footing width
wall on footing = length × wall height × wall thickness
each step = width × step height × overlap
pad footing = length × width × thickness × number of pads
round pier = π × (diameter ÷ 2)² × (depth + height above ground)
bell = π × bell height ÷ 12 × (bell diameter² + bell diameter × shaft diameter + shaft diameter²)
to order = volume × (1 + extra % ÷ 100)
bags = volume to order ÷ what one bag makes, rounded up
width needed = load per length ÷ (soil bearing − footing weight per area)
cubic yards = cubic feet ÷ 27 · cubic metres = cubic feet × 0.0283
Worked example
A strip footing 100 ft long, 16 in wide and 8 in thick. 16 in is 16 ÷ 12 = 1.333 ft and 8 in is 0.667 ft, so the volume is 100 × 1.333 × 0.667 = 88.89 cu ft. Adding 10% extra gives 88.89 × 1.10 = 97.78 cu ft, which is 97.78 ÷ 27 = 3.62 cu yd (2.77 m³). An 80 lb bag makes about 0.60 cu ft, so you need 97.78 ÷ 0.60 = 162.96, rounded up to 163 bags, or one ready-mix truck of 10 cu yd. The footing itself is 2.52 m³ and weighs about 2.52 × 2,400 = 6,041 kg (about 13,320 lb).
Sizing it: a wall that brings 1,500 lb onto each foot of footing, on clay rated 1,500 psf (US IRC). An 8 in footing weighs about 149.8 × 0.667 = 99.9 psf, so the soil has 1,500 − 99.9 = 1,400.1 psf left. Width needed = 1,500 ÷ 1,400.1 = 1.07 ft (12.9 in), so the 16 in footing is enough. It uses (1,500 ÷ 1.333 + 99.9) ÷ 1,500 = 82% of the soil's capacity.
Soil bearing values (US IRC Table R401.4.1)
| Soil | psf | kPa |
|---|---|---|
| Crystalline bedrock | 12,000 | 575 |
| Sedimentary and foliated rock | 4,000 | 192 |
| Sandy gravel or gravel (GW, GP) | 3,000 | 144 |
| Sand, silty or clayey sand, silty or clayey gravel (SW, SP, SM, SC, GM, GC) | 2,000 | 96 |
| Clay, sandy or silty clay, clayey silt, silt, sandy silt (CL, ML, MH, CH) | 1,500 | 72 |
Presumptive load-bearing values from the US International Residential Code, used when no soil test is done. Your local code, a soil report or an engineer may set other values. Rules not yet checked against the code text for this page. The US IRC and UK Approved Document A figures here are summarised from the codes and have not yet been checked line by line against the official text (12 October 2026).
Minimum sizes and depth
- US IRC R403.1.1: concrete footings at least 12 in wide and 6 in thick. Where a wall sits on the footing, the footing sticks out at least 2 in each side, and no more than its own thickness.
- US IRC R403.1.4: the bottom of an outside footing at least 12 in below undisturbed ground, and below the frost line your building department sets.
- UK Approved Document A: strip footings at least 150 mm thick; the underside usually at least 450 mm below the ground, or 750 mm in shrinkable clay.
- US ACI 318: 3 in (75 mm) of concrete around the steel when concrete is poured against earth.
Bags or ready-mix?
Under about 1 cu yd (0.75 m³), bags are usually easier: 1 cu yd is 45 bags of 80 lb. Above 2 cu yd, ready-mix saves a lot of hard work, but small loads often carry a short-load fee. An 80 lb bag makes about 0.60 cu ft, a 60 lb bag 0.45 cu ft and a 40 lb bag 0.30 cu ft; kilogram bags make about 0.47 litres per kg. Check the label on the bag you buy.
Digging, gravel and rebar
The hole is the footing's outline (plus any working room) times the depth to its bottom plus the gravel. Dug soil swells by about 20–30%, so a trailer or skip fills faster than the hole size suggests. Rebar runs set in from the ends by the cover; a run longer than one bar needs joins, and each join adds one lap. This is an estimate for ordering materials, not a structural design: footings for a house need the sizes on your approved plans.
Common questions
How much concrete do I need for a footing?
Multiply length × width × thickness in the same unit. A strip footing 100 ft long, 16 in wide and 8 in thick is 100 × 1.333 × 0.667 = 88.89 cu ft. Add 10% extra to get 97.78 cu ft, then divide by 27: 3.62 cu yd. For round piers use π × (diameter ÷ 2)² × depth.
How many bags of concrete for a footing?
Divide the volume you order by what one bag makes and round up. An 80 lb bag makes about 0.60 cu ft, so 97.78 cu ft needs 163 bags of 80 lb, or 218 bags of 60 lb. Above about 1 to 2 cu yd, ready-mix is usually easier than mixing that many bags.
How wide should a concrete footing be?
Divide the load per foot of wall by what the soil can carry. A wall bringing 1,500 lb per foot onto clay rated at 1,500 psf needs about 1.07 ft (13 in) once the footing's own weight is counted. In the US, the IRC sets 12 in wide and 6 in thick as the least for a concrete footing.
How deep should a footing be?
Below the frost line, so the ground can't freeze and lift it. In the US, the IRC says at least 12 in below undisturbed ground, and deeper where your local frost depth is greater. In the UK, Approved Document A usually means at least 450 mm, or 750 mm in shrinkable clay. Your local building department has the figure.
How do I work out a bell-bottom pier?
Add the straight shaft and the cone-shaped bell. The bell is a frustum: π × height ÷ 12 × (D² + D × d + d²), where D is the bell and d the shaft diameter. A 12 in shaft 4 ft deep with a 24 in bell 12 in tall holds 0.75π + 7π ÷ 12 = 4.19 cu ft.
How much soil comes out of a footing trench?
The hole's volume, then about 25% more because dug soil fluffs up. The concrete and gravel take up some of the hole, so that much soil is left over once you backfill. For a 100 ft trench 16 in wide and 28 in deep, about 133 cu ft is spare, or about 167 cu ft loose.
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