Tell me the shape and dimensions of your pour and I’ll work out the volume in cubic yards and how many bags of concrete that is, with a 10% waste allowance built in.
Concrete calculator
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Estimate for planning, with a 10% waste allowance. Not an engineering spec. For structural footings or columns, follow your permit/engineer’s mix and dimensions.
How I calculate this
Everything starts as a volume in cubic feet, then converts to bags or cubic yards.
For a slab, that’s length times width times thickness, with thickness converted from inches to feet (divide by 12). For a wall footing, it’s length times width times depth, with the width and depth converted from inches. For a round column, it’s the area of the circle (pi times radius squared) times the height, times how many columns you’re pouring.
Then I add 10% for waste, spillage, and uneven sub-grade, because you do not want to be one bag short with a half-poured slab going off in the sun. To turn that into bags I divide by what each bag yields: an 80 lb bag makes about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. Cubic yards is just cubic feet divided by 27. I round bags up, always.
A worked example
Say you’re pouring a 10 by 10 foot slab, 4 inches thick. Volume is 10 x 10 x (4/12) = 33.3 cubic feet. Add 10% and you’re at 36.7 cubic feet, which is 1.36 cubic yards. Divide by 0.60 and you need 62 eighty-pound bags. The calculator shows that, plus what it would be in 60 and 40 lb bags.
A few things I’d tell a friend
Here’s the honest one: 62 bags is a brutal amount to mix by hand. Once your pour crosses about one cubic yard, bagged concrete stops making sense, both for your back and your wallet. Call a ready-mix supplier and have a truck deliver it. The calculator pops up a note when you cross that line, and I wrote a separate piece on concrete bags vs ready-mix to help you decide. Under a yard, bags are fine. Also: buy a couple extra bags beyond the estimate. Returning two dry, unopened bags is easy; stopping a pour to drive to the store is not.
FAQ
How many 80 lb bags make a cubic yard? About 45, since a cubic yard is 27 cubic feet and each 80 lb bag yields roughly 0.60. A 60 lb bag yields ~0.45 (about 60 per yard) and a 40 lb bag ~0.30 (about 90 per yard).
How thick should a slab be? Four inches is standard for patios, walkways, and shed floors. Driveways and anything bearing vehicle weight usually want 5 to 6 inches. Check local code for anything structural.
When should I order ready-mix instead of bags? Roughly once you pass one cubic yard. Below that, bags are practical; above it, a truck is cheaper and far less labor.
Does this include rebar or gravel base? No. This calculates the concrete only. Plan your gravel sub-base separately with the gravel calculator.
Built and maintained by Sam Walsh. Last updated June 2026. This is a planning estimate, not an engineering specification.
Need to measure the area first? See how to measure a space. Setting fence posts? The fence calculator links back here for concrete per post.