Concrete Calculator

Slab size in, concrete out — cubic yards for the truck, bag counts for the store.

FREE TOOL · WORKS OFFLINE · NO SIGN-UP

4 inches is the standard thickness for patios and walkways; driveways usually need 5–6 inches.

Cubic yards (ready-mix)
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Cubic feet
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80 lb bags (0.60 cu ft each)
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60 lb bags (0.45 cu ft each)
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40 lb bags (0.30 cu ft each)
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🏗️ How to Use This Concrete Calculator

  1. 1.Measure the slab. Length and width in feet — measure the inside of your forms, not the outside, for the most accurate volume.
  2. 2.Enter the thickness in inches. Most calculators force you to convert inches to feet first; here you type 4 and the conversion is done for you.
  3. 3.Choose bags or a truck. Under about half a cubic yard, bags are usually cheaper. Above that, ordering ready-mix by the cubic yard saves hours of mixing.
  4. 4.Bag counts are rounded up. You cannot buy a fraction of a bag, so every count here is the number of whole bags to put in the cart.
  5. 5.Add a little extra. Pros order 5–10% more than the exact figure for spillage, uneven ground and form bulge — running short mid-pour leaves a weak cold joint.

❓ FAQ

How do I calculate how much concrete I need for a slab?

Multiply length × width × thickness, keeping the units consistent. For a 10 ft × 10 ft slab that is 4 inches thick: the thickness is 4 ÷ 12 = 0.333 ft, so the volume is 10 × 10 × 0.333 = 33.33 cubic feet. Divide by 27 to get cubic yards: about 1.23 cu yd.

How many bags of concrete do I need?

Divide the cubic feet by the yield of one bag: an 80 lb bag makes about 0.60 cu ft, a 60 lb bag about 0.45 cu ft, and a 40 lb bag about 0.30 cu ft. For 33.33 cu ft with 60 lb bags that is 33.33 ÷ 0.45 = 74.1, so you round up to 75 bags. Always round up — running short mid-pour is worse than having a bag left over.

How many cubic feet are in a cubic yard?

There are 27 cubic feet in one cubic yard, because a yard is 3 feet and 3 × 3 × 3 = 27. Ready-mix trucks sell concrete by the cubic yard, while bags are rated in cubic feet, so converting between the two tells you whether bags or a truck is the better buy.

Is this concrete calculator free?

Yes — 100% free, no sign-up. Your measurements stay in your browser; nothing is uploaded, saved or tracked.

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