How the concrete calculator works
Concrete is sold by volume. Multiply length by width by thickness, converting the thickness from inches to feet, then divide by 27 to get cubic yards.
Cubic yards = Cubic feet ÷ 27
Bags = Cubic feet ÷ yield per bag (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³)
Add 5% to 10% for spillage, uneven ground and form flex. Standard walkway and patio slabs are 4 in thick; driveways are usually 4 to 6 in, depending on the loads. For around a cubic yard or more, ready-mix delivery is usually cheaper and easier than mixing dozens of bags by hand.
Worked example
A 10 × 10 ft patio slab 4 in thick is 33.3 cubic feet. With 10% waste you need 36.7 ft³, or 1.36 cubic yards. In bags that is 62 bags of 80 lb or 82 bags of 60 lb, which is why most people order ready-mix for a job this size.
Frequently asked questions
How many bags of concrete do I need for a 10 × 10 slab?
At 4 in thick, about 56 bags of 80 lb mix without waste, or 62 with 10% extra.
How many cubic feet are in a yard of concrete?
27.
How thick should a concrete patio be?
4 inches is standard for patios and walkways.
How much does an 80 lb bag of concrete make?
About 0.6 cubic feet.
Updated October 2026. Results are estimates for education, not financial or medical advice. All math runs in your browser; nothing you type is sent anywhere.