Concrete Calculator
Work out how much concrete you need for a slab, footing, wall, column, tube or steps – in cubic yards, cubic feet and bags of premix.
How to calculate concrete
Convert everything to feet first: a 4 in slab is 4 ÷ 12 = 0.333 ft thick. Example: a 10 × 10 ft patio, 4 in thick needs 33.3 ft³ = 1.23 yd³. With 5% extra for uneven ground and spillage that is 1.30 yd³ – about 59 bags of 80 lb premix, or a 1.5 yd³ ready-mix order.
Other shapes
- Columns and post holes: π × radius² × height. A 12 in hole 4 ft deep takes 3.14 ft³ (0.12 yd³).
- Tubes (Sonotubes): the area between the outer and inner diameter times the height.
- Steps: each step is a block as tall as all the risers below it, so 3 steps need 1 + 2 + 3 = 6 step blocks, plus the top platform.
Bags or ready-mix?
An 80 lb bag of premix makes about 0.6 ft³, a 60 lb bag 0.45 ft³ and a 40 lb bag 0.3 ft³. Bags make sense for post holes and small pads; above about one cubic yard, ready-mix delivered by truck is usually cheaper and gives a more even pour. Plants deliver in quarter-yard steps.
Typical thickness
- Walkways and patios: 4 in
- Driveways: 4–6 in
- Garage floors and slabs carrying heavy loads: 6 in or more, with reinforcement
Footings and structural slabs should follow your local building code and, where needed, an engineer’s design.
Frequently asked questions
How many bags of concrete are in a yard?
A cubic yard is 27 ft³: about 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb.
How much concrete do I need for a 10 × 10 slab?
At 4 in thick, 1.23 yd³ – about 1.3 yd³ with 5% waste.
How much extra concrete should I order?
Add 5–10% for uneven subgrade, form bulging and spillage.