Concrete Calculator
Work out how much concrete you need for a slab, patio, footing or post hole, in cubic yards or m³, and how many 40, 60 or 80 lb bags to buy. Free.
- Cubic feet
- 36.67 ft³
- Cubic metres
- 1.04 m³
- 80 lb bags
- 62
- 60 lb bags
- 82
- 40 lb bags
- 123
Ready-mix trucks sell by the cubic yard (or m³); bagged premix suits jobs under about 1 yd³. Bag counts use typical premix yields.
Concrete for slabs and posts
Concrete is sold by volume: cubic yards in the US, cubic metres elsewhere. For a slab, multiply length by width by thickness (in feet: a 4 inch slab is 0.333 ft thick); for a round column or post hole, use π × radius² × height. The calculator adds a waste allowance, 5–10% is typical for spillage, uneven ground and over-excavation, and converts the volume into bags of premix. Small jobs under about a cubic yard are usually done with bags; larger pours are cheaper by ready-mix truck.
How to use it
- 1
Choose the shape
A slab, patio or floor, or a round column or post hole.
- 2
Enter the dimensions
Length, width and thickness, or height and diameter.
- 3
Read the volume and bags
Includes your waste allowance.
Frequently asked questions
How many bags of concrete are in a yard?
About 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb premix make one cubic yard.
How thick should a concrete slab be?
4 inches (10 cm) for patios and walkways, 5–6 inches (13–15 cm) for driveways and garage floors.
How much concrete for a fence post?
A typical post hole 10 in wide and 2 ft deep takes about one 60 lb bag.
Is anything sent to a server?
No. Everything is calculated in your browser, and nothing you type is stored or sent anywhere.