How this calculator works
Pick the shape you are pouring — a flat slab, a strip footing, or one or more round columns (like deck footings or fence post bases) — enter its dimensions, and this calculator works out the volume in cubic yards or cubic meters, plus how many pre-mixed 60 lb or 80 lb bags that volume takes. Slabs and footings both use length times width times thickness; round columns use the cylinder formula, radius squared times pi times height, multiplied by how many you are pouring.
Ready-mix truck orders and bagged concrete are bought differently. A truck is priced by the cubic yard and delivery is usually rounded up to a quarter or half yard, so contractors commonly add 5 to 10% to the calculated volume to cover form spillage, uneven subgrade and minor measurement error. Bagged concrete is priced per bag, so the bag counts below are already rounded up to the next whole bag — buying the exact rounded number, with no extra waste margin, is the more common approach for small DIY pours.
The formula
Slab or footing volume = length × width × thickness (thickness converted to feet: inches ÷ 12)Round column volume = π × (diameter ÷ 2)² × height × number of columnsCubic yards = cubic feet ÷ 27 exactly; bags needed = ceiling(cubic feet ÷ bag yield)Bag yields used here — 0.45 ft³ for a 60 lb bag and 0.60 ft³ for an 80 lb bag — are the standard published yields for common pre-mixed concrete bags; check your specific product's label, since yields vary slightly by brand and mix.
Worked example: a 10 ft × 10 ft slab, 4 in thick
- Thickness in feet: 4 ÷ 12 = 0.3333 ft.
- Volume: 10 × 10 × 0.3333 = 33.33 cubic feet.
- In cubic yards: 33.33 ÷ 27 = 1.2346 cubic yards.
- In 60 lb bags: 33.33 ÷ 0.45 ≈ 74.07, rounded up to 75 bags (or 33.33 ÷ 0.60 = 55.6, rounded up to 56 bags of 80 lb).
Frequently asked questions
Should I add extra for waste?
Yes, for a ready-mix truck order — 5 to 10% above the calculated volume is a common allowance for spillage, an uneven subgrade, and rounding up to the truck's minimum load or nearest quarter yard. For bagged concrete the bag count is already rounded up to a whole bag, which usually covers small measurement error on its own.
60 lb bags or 80 lb bags — which is cheaper per cubic foot?
Compare the price per bag against its yield (0.45 ft³ for 60 lb, 0.60 ft³ for 80 lb) rather than the bag price alone, since the larger bag usually — but not always — costs less per cubic foot. 80 lb bags are also considerably heavier to lift and mix by hand.
How deep should a slab or footing be?
A typical shed or patio slab is 4 inches thick; driveways poured for vehicle traffic are commonly 5 to 6 inches. Footing depth depends on your local frost line and load requirements — check your local building code or a structural engineer for anything load-bearing.
Do I need to account for rebar or wire mesh?
No — this calculator estimates the concrete volume only. Rebar or welded wire mesh reinforcement takes up a negligible fraction of the pour's volume and is calculated separately based on your reinforcement spacing.
Why does a round column need diameter and not radius?
Diameter — the distance straight across the column — is what you actually measure with a tape measure on formwork or an existing post. The calculator halves it internally to get the radius that the volume formula needs.