Concrete Calculator
Work out how much concrete a slab needs, in cubic yards and bags.
Runs in your browser — nothing is sent or saved.
How to use the concrete calculator
- Choose feet and inches, or metres and centimetres.
- Enter the slab length, width, and thickness.
- Read the volume in cubic yards and cubic metres.
- Check the bag estimate and add the waste margin.
How much concrete you need
Concrete is measured by volume, so this concrete calculator multiplies length by width by thickness to get the amount, whether you are pouring a concrete slab or filling forms. It reports the cubic yards of concrete, the concrete yardage to order, and the concrete bags for smaller jobs. It is usually ordered in cubic yards in the US and cubic metres elsewhere, and the tool shows both, plus cubic feet. Thickness is the one to watch: a slab only a few inches thick still adds up over a large area, and getting it wrong is the most common sizing mistake.
Ready-mix or bags
For a large pour, ready-mix delivered by the cubic yard or cubic metre is normal. For small jobs, bagged concrete is easier, and the tool estimates how many 80, 60, or 40 pound bags you would need. Bag yields are approximate and printed on the bag, so treat the counts as a guide. Bagged concrete gets expensive and heavy fast, which is why bigger jobs switch to ready-mix.
Always add for waste
Order a little more than the exact volume. Some concrete is lost to spillage, uneven ground, and over-dig, and running short mid-pour is a real problem because a cold joint forms if the next batch arrives late. A waste margin of about 10% is a common allowance, which the tool shows, and rounding bag counts up is wise for the same reason.
A worked example
A slab 10 feet by 10 feet at 4 inches thick is about 1.23 cubic yards. Adding a 10% margin brings it to roughly 1.36 cubic yards, so you would order 1.5 cubic yards to be safe.
FAQ
How much concrete do I need for a slab?
Multiply length by width by thickness. The tool does this and shows cubic yards and cubic metres, plus a bag estimate, then add about 10% for waste.
How many bags of concrete are in a cubic yard?
It depends on bag size, but roughly 45 of the 80 pound bags make a cubic yard. The tool estimates bag counts for your exact volume in 80, 60, and 40 pound sizes.
Why should I add a waste margin?
Some concrete is always lost to spillage and uneven ground, and running short mid-pour risks a weak joint. A margin of about 10% is a common allowance.
Does thickness really matter that much?
Yes. Over a large area, even an inch of extra thickness adds a lot of volume, so measure it carefully. It is the most common source of sizing errors.
Is my information private?
Yes. Everything is calculated in your browser and nothing you enter is uploaded or saved.