Concrete Calculator
Estimate concrete in cubic yards and cubic metres, compare 40 lb, 60 lb and 80 lb bag quantities, add a waste allowance, round a ready-mix order and optionally estimate material cost.
Calculate concrete quantity
Slab dimensions
Footing dimensions
Post-hole dimensions
Wall dimensions
Order assumptions
How much concrete do I need?
Answer first: calculate the project volume in cubic feet, divide by 27 to get cubic yards, then add a realistic allowance for site variation and round upward to the way your ready-mix supplier sells concrete. For bagged concrete, divide the required cubic feet by the yield printed on the bag.
This calculator handles four common project shapes: rectangular slabs, continuous rectangular footings, round post holes and concrete walls.
Concrete slab formula
Length (ft) × width (ft) × thickness (ft) ÷ 27 = cubic yards. If thickness is entered in inches, divide it by 12 first.
Worked example: 20 × 20 ft slab at 4 inches
20 × 20 × (4 ÷ 12) ÷ 27.
A 0.25 yd³ order increment would round this planning quantity to 5.5 yd³.
Ready-mix yards vs concrete bags
Ready-mix concrete is commonly ordered by volume, while bagged mixes are sold by bag weight. Bag weight alone does not tell you exact finished volume; the product label's yield is what matters.
AcreSketch uses approximate planning yields of 0.30 ft³ for a 40 lb bag, 0.45 ft³ for a 60 lb bag and 0.60 ft³ for an 80 lb bag. Those values are only for early estimating. Replace them mentally with the actual yield printed on the product you plan to buy before placing an order.
Concrete for footings and post holes
For a rectangular footing, multiply total length by width and depth after converting all dimensions to feet. For a round post hole, calculate the cylinder volume using the hole radius and depth, then multiply by the number of holes.
Post-hole volume can change significantly if the augered hole is wider at the bottom, soil collapses into the hole, or posts displace part of the concrete. Use field measurements and the installation method required for the project.
Should I add extra concrete?
Form dimensions, excavation, subgrade variation, spillage and supplier order increments can make the real quantity differ from the clean mathematical volume. The calculator lets you choose your own allowance rather than assuming one number is correct for every job.
For structural work, follow the project drawings and concrete specification. This calculator estimates quantity only; it does not design reinforcement, footing dimensions, concrete strength, curing or structural capacity.
Frequently asked questions
How many cubic feet are in one cubic yard?
One cubic yard contains 27 cubic feet.
How many 80 lb bags make one cubic yard?
Using a planning yield of about 0.60 cubic feet per 80 lb bag, one cubic yard is about 45 bags. Actual product yield can differ, so use the bag label before buying.
Does the calculator include waste?
Yes. You choose the waste or site allowance percentage, and the calculator shows both raw volume and adjusted volume.
Can I estimate ready-mix cost?
Yes. Enter a price per cubic yard. The estimated material cost uses the rounded order quantity and does not automatically include short-load fees, delivery, pumping, tax, labor or finishing.
Planning a driveway?
Estimate the concrete volume here, then compare base-material needs with the Gravel Calculator.