How Much Concrete Do I Need? Slab, Footing & Column Formulas
The formulas for concrete volume by shape, how to convert to bags or ready-mix yards, and when to order a truck instead of bagging it by hand.
Concrete is unforgiving to estimate: order too little and your pour sets before you can finish; order too much and you have paid for a wheelbarrow of waste. Here is how to get the number right.
Start with volume
Every estimate begins with volume - length × width × depth, in the same units. Working in feet gives cubic feet:
Volume (cu ft) = length (ft) × width (ft) × thickness (ft)
A 10 ft × 10 ft slab at 4 inches thick (0.33 ft): 10 × 10 × 0.33 = 33 cubic feet. Concrete is usually sold by the cubic yard, so divide by 27: 33 ÷ 27 ≈ 1.2 cubic yards.
Other shapes
- Footing - treat it as a long box: length × width × depth.
- Column or pier - π × radius² × height for a round column.
- Wall - length × height × thickness.
Bags or a truck?
Bagged mix yields roughly: an 80 lb bag ≈ 0.6 cu ft, a 60 lb bag ≈ 0.45 cu ft. So that 33 cu ft slab needs about 55 × 80 lb bags - which is a lot of mixing. As a rule of thumb, once you are past roughly a cubic yard (about 40+ bags), a ready-mix truck is cheaper, faster, and gives a more consistent pour. Small footings and post holes are bag territory; slabs and driveways usually are not.
Always add a waste factor
Add 5-10% for spillage, uneven subgrade, and over-excavation. Running short mid-pour is far more expensive than a little extra.
Skip the arithmetic
Rather than juggling shape formulas and bag yields, Pour does it for slabs, footings, columns, and walls - giving you cubic yards, bag counts at your chosen bag size, estimated cost, and a flag for when to order a truck. Imperial or metric, with a built-in waste buffer.