Concrete Calculator
Enter a slab or footing's dimensions, pick a waste factor, and get cubic yards plus bag counts for 40, 60, and 80 lb mix. Results update as you type. Need floor area or flooring instead? Try the square footage calculator or the flooring calculator.
Enter the slab's footprint length and width, and its thickness in inches.
- Total volume
- 0 ft³
- With waste factor
- 0 ft³
- Cubic yards (with waste)
- 0 yd³
- 40 lb bags
- 0
- 60 lb bags
- 0
- 80 lb bags
- 0
How to measure for concrete
For a slab, measure the length and width of the poured area at ground level in the same units you plan to use throughout, and measure the form's thickness (or the depth you plan to excavate and fill) separately in inches, since slab thickness is almost always specified in inches even when the footprint is measured in feet. Multiply length by width by thickness (converted to feet) to get cubic feet, the same way you would for a square footage calculation with a third dimension added.
For a footing or a column base, measure the length and width of the formed hole in feet and its depth in inches, just like a slab. For a round post hole or cylindrical form, measure the diameter and enter that same number for both length and width; this treats the circle as its bounding square, which slightly overstates volume and gives you a safe margin rather than coming up short. If you are pouring several identical footings or columns, enter the count in the sections field instead of adding each one by hand.
Thickness matters more than most people expect, because volume scales directly with it. A slab that is 5 inches thick instead of the planned 4 inches needs 25 percent more material across that same footprint, so check your form height or screed guides in more than one spot rather than assuming they are uniform.
Add extra waste on top of the raw volume for anything other than a clean, level pour. The default 10 percent covers minor spillage, a form that is not perfectly square, and small over-excavation. Push that to 15 or 20 percent when the sub-grade is uneven or poorly compacted, when you are hand-mixing in small batches that vary slightly, or when the pour happens in several separate stages, since each of those adds material you cannot see until you are mixing the next batch.
As a practical, general rule of thumb rather than a precise cutoff, bagged concrete tends to be more economical for small jobs under roughly 1 cubic yard, while ready-mix delivery from a concrete truck is usually cheaper per cubic yard, and much less physical labor, once a pour gets close to or above that amount. Call a local ready-mix supplier for a delivered price before buying a pallet of bags for anything in that range, since delivery minimums and short-load fees vary a lot by area.
Worked example
A slab measures 10 ft by 10 ft, poured 4 inches thick, with a 10% waste factor, 1 section.
Thickness in feet = 4 ÷ 12 = 0.333 ft.
Volume = 10 × 10 × 0.333 = 33.33 cubic feet.
With 10% waste: 33.33 × 1.10 = 36.67 cubic feet of material to order.
Cubic yards = 36.67 ÷ 27 = 1.36 cubic yards.
40 lb bags (0.30 ft³ each) = 36.67 ÷ 0.30 = 122.2 exact, rounded up to 123 bags.
60 lb bags (0.45 ft³ each) = 36.67 ÷ 0.45 = 81.5 exact, rounded up to 82 bags.
80 lb bags (0.60 ft³ each) = 36.67 ÷ 0.60 = 61.1 exact, rounded up to 62 bags.
Frequently asked questions
How much concrete do I need for a 10x10 slab?
A 10 by 10 foot slab poured 4 inches thick is 10 times 10 times 0.333 feet, which is 33.33 cubic feet. With a 10 percent waste factor that is 36.67 cubic feet, or about 1.36 cubic yards. In bags, that is 123 bags of 40 lb mix, 82 bags of 60 lb mix, or 62 bags of 80 lb mix, each rounded up since you cannot buy a partial bag.
How many bags of concrete make a cubic yard?
One cubic yard is 27 cubic feet. Using published yields of 0.30 cubic feet per 40 lb bag, 0.45 cubic feet per 60 lb bag, and 0.60 cubic feet per 80 lb bag, that works out to 90 bags of 40 lb mix, 60 bags of 60 lb mix, or 45 bags of 80 lb mix per cubic yard. All three land on roughly 3,600 lb of dry mix per cubic yard, which is a useful sanity check on any bag count.
Should I use bagged concrete or order ready-mix delivery?
As a general rule of thumb, bagged concrete mixed on site is usually more economical for small pours under about 1 cubic yard, while ready-mix delivery from a truck is usually cheaper per cubic yard, and far less labor, once you cross roughly that amount. This is practical guidance rather than a precise threshold, since local delivery minimums, truck access to your site, and short-load surcharges all shift the breakeven point. Get a delivered price for comparison on anything near or above a cubic yard before buying a pallet of bags.
How much waste factor should I add for a concrete pour?
Start with 10 percent extra for a straightforward slab or footing poured on a reasonably level, well-compacted sub-grade. Add more, often 15 to 20 percent, when the sub-grade is uneven or not fully compacted, since low spots and voids soak up extra material you cannot see until you are already pouring. Multiple separate pours, hand mixing where batches vary, and tight access that makes placement harder are also good reasons to round your waste factor up rather than down.
How do I measure a footing or column for concrete?
For a rectangular footing, measure the length and width of the formed hole or form in feet and the depth in inches, the same way you would measure a slab. For a round column or post hole, measure the diameter and use that figure for both the length and width fields, which gives a slightly conservative (higher) volume than the true cylinder, a safe direction to round. Multiply by however many identical footings or columns you are pouring using the sections field, since a single measurement times quantity is faster and less error-prone than adding rows by hand.
Why does slab thickness matter so much for concrete volume?
Volume scales directly with thickness: doubling a slab from 4 inches to 8 inches doubles the concrete you need for the same footprint, while area errors only scale with one dimension at a time. A small measuring mistake on thickness, say guessing 4 inches when the form is actually 5, understates material by 25 percent on that section alone. Check your form height or string line in multiple spots rather than assuming it is uniform, especially on sloped or uneven ground.