Concrete Calculator
Enter your slab dimensions to find out how much concrete you need — in cubic yards for ready-mix, or in bags for DIY.
Runs entirely in your browser — your numbers never leave this page.
How it works
Concrete is sold two ways: by the cubic yard (ready-mix truck) or by the bag (DIY from the hardware store). This calculator does the volume math for you:
Volume (cu ft) = Length (ft) × Width (ft) × Thickness (ft)
Cubic yards = Volume ÷ 27
- 80-lb bag yields about 0.6 cubic feet — it takes roughly 45 bags to make one cubic yard.
- 60-lb bag yields about 0.45 cubic feet — roughly 60 bags per cubic yard.
- Rule of thumb: under ~2 cubic yards, bags are practical. Above that, order ready-mix — it's cheaper and far less labor.
- Thickness guide: 4 in for walkways/patios, 6 in for driveways and garage floors.
- Always add ~10% extra — forms leak, subgrades are uneven, and running short mid-pour is a disaster.
Frequently asked questions
How many 80-lb bags of concrete are in a cubic yard?
About 45. One 80-lb bag yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet (27 ÷ 0.6 = 45).
Should I buy bags or order ready-mix?
For small jobs under about 2 cubic yards, bags are fine. For anything bigger, a ready-mix truck is usually cheaper per yard and saves enormous labor.
How thick should a concrete driveway be?
6 inches is the standard for residential driveways that carry cars. Walkways and patios are fine at 4 inches.
Why add 10% extra?
Forms are never perfectly true, subgrades vary, and some concrete always sticks to the mixer or wheelbarrow. Running short mid-pour creates a weak cold joint.
How do I measure an irregular shape?
Break it into rectangles, calculate each one, and add them up. For circles, use π × radius² × thickness.
80-lb vs 60-lb bags — which should I buy?
80-lb bags give you more concrete per bag and fewer bags to haul, but each one is heavy. 60-lb bags are easier on your back for small repairs.
How much area does one cubic yard of concrete cover?
One cubic yard covers about 81 square feet at 4 inches thick, or about 54 square feet at 6 inches thick. So a 10×12 ft patio at 4 inches needs roughly 1.5 cubic yards.
Can I pour a slab in sections on different days?
You can, but each joint between pours is a cold joint — a weak plane where cracks love to start. For structural slabs, pour in one go. For large patios, plan deliberate control joints instead of accidental ones.
How long before I can walk or drive on new concrete?
Walk on it after 24–48 hours, drive on it after about 7 days. Concrete keeps curing for 28 days to reach full strength — keep it damp for the first week in hot weather.
What is a "short load" fee?
Ready-mix companies often charge extra for orders under 3–4 cubic yards, since the truck still makes the trip. Always ask about the short-load fee when comparing bags vs. ready-mix for small pours.