How it works
Pick the kind of pour, enter the dimensions and the calculator returns the volume in cubic yards (how ready-mix trucks sell concrete), plus how many 60 lb or 80 lb bags you would need to mix it yourself.
Slab thickness is in inches because that is how it is usually specified: 4 in for sidewalks and patios, 5 to 6 in for driveways that carry cars. Footings take a length in feet and a width and depth in inches. Round columns and post holes use the diameter of the tube or hole.
The default adds 10% for spillage, uneven subgrade and over-excavation. For pours over about 1 cubic yard (roughly 45 bags of 80 lb), ready-mix delivery is usually cheaper and much less work than bags.
Formula
Volume (cu ft) = length × width × thickness (all in feet); round = π × (diameter ÷ 2)² × height. Cubic yards = cu ft ÷ 27. Bags = cu ft ÷ 0.60 (80 lb) or ÷ 0.45 (60 lb).
Questions people ask
How many 80 lb bags of concrete make a cubic yard?
An 80 lb bag yields about 0.60 cubic feet, so a cubic yard (27 cu ft) takes 45 bags. With 60 lb bags (0.45 cu ft each) it takes 60 bags.
How thick should a concrete slab be?
Four inches is standard for patios, walks and shed floors. Driveways and garage floors are usually 5 to 6 inches, and heavier loads call for thicker slabs and reinforcement.
How much extra concrete should I order?
Plan on 5% to 10% extra. Subgrade is rarely perfectly flat, forms bow slightly and some concrete is always lost in the chute or wheelbarrow. Running short mid-pour causes a cold joint.
How much concrete does a post hole need?
A 10 in diameter hole 3 ft deep holds about 1.6 cu ft before the post goes in, or roughly 3 bags of 80 lb mix. Subtract the post volume for a tighter estimate, or keep it as your waste margin.