Concrete Calculator
Enter the slab, footing, or post-hole dimensions — the volume and bag counts update as you type, and the link in your address bar always reflects your numbers.
How this calculator works
Concrete volume is straightforward geometry: multiply length by width to get the slab's footprint in square feet, then multiply by the thickness (converted from inches to feet) to get cubic feet. Dividing by 27 converts cubic feet to cubic yards, which is how ready-mix concrete is priced and ordered. Bag counts use the manufacturer's typical published yield — a standard 60 lb bag yields about 0.45 cubic feet of mixed concrete, and an 80 lb bag yields about 0.6 cubic feet — so we divide your total cubic feet by that yield and round up, since you can't buy a partial bag. Actual yield can vary a little by brand, so treat the bag count as a close estimate rather than an exact cutoff.
Worked example
A 10 ft × 10 ft slab poured 4 inches thick works out to 10 × 10 × (4 ÷ 12) = 33.33 cubic feet, which is 33.33 ÷ 27 ≈ 1.23 cubic yards. In bags, that's 33.33 ÷ 0.45 ≈ 75 bags of 60 lb mix, or 33.33 ÷ 0.6 ≈ 56 bags of 80 lb mix — pick whichever bag size is easier for you to carry, since both cover the same volume once mixed. Neither total includes a waste buffer, so add your own margin as described in the FAQ below.
Frequently asked questions
Why do concrete bags come in different sizes like 60 lb and 80 lb?
Bag size is mostly about handling weight and how much water you mix in. An 80 lb bag packs more dry mix per bag, so you add proportionally less water per cubic foot of yield than juggling more small bags — fewer bags to lift and open for the same job, at the cost of each one being heavier to carry. Both sizes cure to the same concrete once mixed correctly; pick whichever weight you can comfortably handle.
Should I buy extra concrete for waste?
Most pros pad their order by roughly 5-10% to cover an uneven subgrade, spillage, and forms that aren't perfectly square — this calculator gives you the raw volume for the dimensions you entered, not a padded number, so you can decide the buffer that fits your job. For a small, well-formed slab, 5% is often plenty; for irregular ground or a first-timer pour, lean toward 10%.
How thick should a concrete slab be?
It depends on what's sitting on it. A pedestrian walkway or patio is commonly poured around 4 inches thick, while a driveway that carries vehicle weight is typically 5-6 inches. These are common ranges, not a single correct answer — your local building code, soil conditions, and whether heavier vehicles will cross it can all push the number up, so check with your local permitting office for anything structural.
Does this calculator account for rebar, mesh, or forms?
No — it only calculates the volume of concrete itself, in cubic yards and bag counts. Rebar, wire mesh, gravel base, and the lumber for forms are separate materials you'll need to plan and buy on top of what this tool gives you.
Can I use this for footings and post holes, not just slabs?
Yes. The math is the same rectangular-volume calculation regardless of shape name — enter the length and width of the footing (or the hole's footprint) and its depth as the thickness, and the cubic yardage and bag counts come out the same way. For round post holes, use the hole's diameter as both length and width for a rough (slightly over-generous) estimate.