Concrete Calculator
Calculate cubic yards, cubic feet, and the number of 40, 60, or 80 lb bags for slabs, footings, and columns — with a waste allowance built in.
How the Concrete Calculator works
This concrete calculator tells you how many yards of concrete a pour needs, plus the bag count if you are mixing by hand. Choose the shape you are pouring — a slab, a footing, a rectangular column, or a round column — and enter its dimensions. The tool returns the volume in cubic feet and cubic yards, with a waste allowance already built in.
Each shape uses its own volume formula, and the calculator handles the unit conversions so you can enter slab thickness or footing width in inches while length stays in feet. It then converts cubic feet to cubic yards, the unit a ready-mix truck is ordered in.
If you plan to use bagged concrete, it also divides the volume by the yield of an 80, 60, or 40 lb bag so you know how many bags to load in the truck. That makes it a true concrete bag calculator as well as a ready-mix estimator.
How to calculate how many yards of concrete you need by hand
Every concrete estimate is a volume calculation, and the trick is converting everything to the same unit. For a slab, multiply length by width in feet, then by the thickness converted from inches to feet. A 10 by 10 ft slab at 4 in thick is 10 times 10 times (4 divided by 12), which is about 33.3 cubic feet.
A footing is length times width times depth, with width and depth converted from inches to feet. A rectangular column is its width times depth times height, again converting the inch dimensions. A round column is pi times the radius squared times height, where the radius is the diameter in inches divided by 24 to get feet.
Once you have cubic feet, divide by 27 to get cubic yards, since a cubic yard is a 3 by 3 by 3 ft block. Our 33.3 cubic foot slab is about 1.23 cubic yards.
Finally add about 10% for waste — spillage, an uneven subgrade, and over-excavation all consume extra concrete. For bags, divide the cubic feet by the bag yield: 0.60 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag, and 0.30 for a 40 lb bag.
Tips for an accurate concrete estimate
Concrete is unforgiving — running short mid-pour means a cold joint, so order with margin and measure the ground you actually have.
- Always keep the 10% waste allowance; a dug-out footing is never as crisp as the drawing and the subgrade is rarely level.
- Check the subgrade before you order — soft or low spots quietly add cubic feet that a clean measurement misses.
- For anything past about one cubic yard, ready-mix from a truck is usually cheaper and far less labor than bag mixing.
- Order ready-mix by the quarter-yard and round up; you cannot send a truck back for a few more shovelfuls.
- For round columns, measure the actual sonotube diameter — a small error gets squared in the formula and skews the volume.
From cubic yards to a priced estimate
The yardage is the take-off; the client wants a price. GreenlitBid turns a concrete take-off like this into a priced, client-ready line item — material, forming, placement, and finishing labor combined with your rates so the pour reads as one clear figure in a professional estimate.
Questions
How do I calculate cubic yards of concrete?+
Find the volume in cubic feet — for a slab that is length x width x thickness, with the thickness converted from inches to feet — then divide by 27 to get cubic yards. This calculator does the unit conversions for you for slabs, footings, and columns.
How much extra concrete should I order?+
Plan on about 10% extra to cover spillage, uneven subgrade, and over-excavation. This tool adds a 10% waste allowance to every result, so the cubic yards and bag counts shown are what to actually order.
How many bags of concrete are in a cubic yard?+
A cubic yard is 27 cubic feet. An 80 lb bag yields about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. That works out to roughly 45 eighty-pound bags, 60 sixty-pound bags, or 90 forty-pound bags per cubic yard.
Should I use bagged concrete or order ready-mix?+
Bagged concrete is practical for small jobs — footings, posts, small pads. For anything past roughly one cubic yard, ordering ready-mix from a truck is usually cheaper and far less labor. Ready-mix is typically sold by the quarter-yard.
What thickness should a concrete slab be?+
Four inches is standard for patios, walkways, and shed floors. Use five to six inches for driveways or anything that carries vehicle loads. Enter your planned thickness in the slab inputs to size the pour.
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