Concrete Calculator
Size up a slab pour — length, width, and thickness turn into cubic yards for ready-mix, plus a bag count if you're mixing it yourself.
How it works
A slab's volume is length times width times thickness, but thickness is in inches while the rest is in feet, so we divide the thickness by 12 first. A 10-by-10 slab at 4 inches works out to about 33 cubic feet.
Ready-mix is sold by the cubic yard, and a yard is 27 cubic feet, so we divide by 27. That 10-by-10 pad comes to roughly 1.23 cubic yards — the number the concrete supplier will ask for.
Mixing from bags instead? An 80-pound bag yields about 0.6 cubic feet, so we divide the volume by that and round up. Bags add up fast on anything bigger than a small pad, which is where ready-mix starts to win.
Frequently asked questions
How many bags of concrete are in a cubic yard?
Around 45 of the 80-pound bags, since each yields roughly 0.6 cubic feet and a yard is 27. That's a lot of mixing, which is why anything past a few cubic feet is usually cheaper and easier as ready-mix.
How thick should my slab be?
Four inches handles patios, walkways, and shed floors. Bump it to 5 or 6 inches for a driveway or anything that'll carry vehicle weight, and check your local code for the specifics.
Should I order a little extra concrete?
For a ready-mix delivery, ordering about 10 percent over is common insurance against uneven subgrade and spillage — running short mid-pour is a real problem. Bag jobs are easier to top up, so you can cut it closer.
Why does the bag yield matter?
Different bag sizes yield different volumes — an 80-pound bag gives about 0.6 cubic feet, a 60-pound bag around 0.45. The field is editable so you can match whatever's stacked on the store shelf.