How to use the Concrete Calculator
Enter the length, width, thickness and waste allowance and the result appears straight away. Everything runs in your browser, so nothing you type is uploaded.
From cubic feet to cubic yards
Ready-mix concrete is sold by the cubic yard, and one cubic yard of concrete is 27 cubic feet (3 ft × 3 ft × 3 ft). Work out the volume in cubic feet, then divide by 27. A 12 × 12 ft slab at 4 inches thick is 12 × 12 × 0.333 = 48 cubic feet, or about 1.78 cubic yards before waste.
Square footings, posts and concrete cost
For square footings, multiply the width by the width by the depth, all in feet, and multiply by the number of footings. Bags make sense for small jobs like fence posts or a shed base; for anything over about one cubic yard, ready-mix is usually cheaper and much less work. Concrete cost varies a lot by region and delivery distance, so get a quote per cubic yard from local suppliers and ask about short load fees for small orders.
The formula
US: cubic feet = length × width × thickness ÷ 12, ÷ 27 for cubic yards. Metric: m³ = length × width × thickness (cm) ÷ 100. Waste is added on top
Examples
- 10 ft by 10 ft slab, 4 in thick, 10% waste, 80 lb bags
Concrete needed: 1.36 cubic yards (36.67 cu ft, 1.04 m³) Bags of 80 lb mix: 62
- Larger 20 ft by 12 ft patio, 4 in thick, 10% waste
Concrete needed: 3.26 cubic yards (88 cu ft, 2.49 m³) Bags of 80 lb mix: 147
- Garage floor 24 ft by 24 ft, 6 in thick, 5% waste
Concrete needed: 11.2 cubic yards (302.4 cu ft, 8.56 m³) Bags of 80 lb mix: 505
Limitations and assumptions
Bag counts are US-only (80, 60 and 40 lb mixes) and approximate; metric mode gives cubic metres. Bag yields vary. Order ready-mix by the cubic yard for large pours.
Frequently asked questions
How do I calculate concrete for a slab?
Multiply length by width by thickness (all in feet), then divide by 27 to get cubic yards. Add 5 to 10 percent for waste.
How many 80 lb bags make a cubic yard?
Roughly 45 bags of 80 lb mix give a cubic yard, each bag yielding about 0.6 cubic feet. The exact yield is printed on the bag.
When should I order ready-mix instead of bags?
Beyond about 1 cubic yard, mixing bags by hand becomes slow and inconsistent. Ready-mix trucks are usually ordered in whole or half yards.
How thick should a concrete slab be?
Four inches is common for patios and walkways, with 5 to 6 inches for driveways and garage floors. Check local building rules and ground conditions.