Concrete calculator
The volume of a slab or footing and the number of bags of premixed concrete.
How it is calculated
The volume is length × width × thickness, with the thickness converted from centimeters to meters, or from inches to feet. To get the number of bags, that volume is divided by the yield printed on the bag and rounded up.
Bags = length × width × thickness ÷ yield per bag
Example
A 10 × 10 ft patio slab 4 in thick is 33.33 ft³, or 1.23 yd³. With bags that yield 0.6 ft³ each, that is 55.6 bags, so you need 56.
In metric, a 3 × 2 m slab 10 cm thick is 0.6 m³, or 600 L. With bags that yield 12 L each, you need 50.
Limitations
The calculation assumes a flat base and an even thickness. If the ground is uneven, measure the thickness at several points and use the largest. In imperial units the volume is shown in cubic yards, the unit ready-mix suppliers quote. No extra is added for spillage.