How to use the Concrete Calculator
- Choose a slab or post holes, and feet or metres.
- Enter the size and thickness, or the hole diameter and depth, and how many you need.
- Keep about 10% extra for waste and uneven ground.
- Read the volume and the number of bags for each bag size.
How it works
Concrete is ordered by volume. For a slab, multiply length by width by thickness. Because slab thickness is usually measured in inches, divide it by 12 to turn it into feet. One cubic yard is 27 cubic feet, and ready-mix concrete in the US is sold by the cubic yard. In Canada, Australia, the UK and the Netherlands it is sold by the cubic metre.
For post holes the calculator uses the volume of a cylinder. It ignores the post itself, which gives a little extra. Typical thickness is 4 inches (10 cm) for patios and paths and 5 to 6 inches (12–15 cm) for driveways. For jobs above about one cubic yard, ready-mix delivery is usually easier than mixing bags by hand.
Formula
Slab (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12 Cubic yards = cubic feet ÷ 27 Post hole = π × (diameter ÷ 2)² × depth Bags = volume ÷ bag yield, rounded up
Example
A 10 × 10 ft patio, 4 inches thick, needs 33.33 cubic feet, or 1.23 cubic yards. With 10% extra that is 1.36 cubic yards, or 62 bags of 80 lb premix.
Eight fence post holes, each 10 inches wide and 36 inches deep, need 14.4 cubic feet with 10% extra, which is 24 bags of 80 lb concrete.
Bag yields
| Bag | Makes about | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
| 20 kg | 0.01 m³ (10 litres) | 100 per m³ |
Yields differ slightly between brands, so check the bag.
Frequently asked questions
How many 80 lb bags of concrete are in a yard?
About 45. Each 80 lb bag makes roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet.
How thick should a concrete slab be?
About 4 inches (10 cm) for patios, paths and shed bases, and 5 to 6 inches (12–15 cm) for driveways that carry cars. Heavier loads need more, and local building codes may set a minimum.
How much does a cubic yard of concrete weigh?
Roughly 4,000 lb, or about 2 US tons. A cubic metre weighs about 2,400 kg.
How do I convert cubic metres to cubic yards?
Multiply by 1.308. One cubic metre is about 1.31 cubic yards.
Last reviewed October 21, 2026. Spotted an error? Tell us.