How the concrete calculator works
Concrete is sold by volume, so the calculator finds the volume of the space you are filling, adds a little extra for waste, and converts the answer into cubic yards, cubic metres and bags.
Slabs, patios and footings
Multiply the length by the width by the thickness. Because thickness is usually measured in inches, divide it by 12 first to turn it into feet:
Volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12
Round columns and post holes
A round hole is a cylinder. Halve the diameter to get the radius, then multiply π by the radius squared and by the depth:
Volume (ft³) = 3.1416 × (diameter (in) ÷ 24)² × depth (ft)
Turning volume into an order
- Divide cubic feet by 27 to get cubic yards.
- Multiply cubic feet by 0.0283 to get cubic metres.
- Divide cubic feet by the bag's yield: an 80 lb bag makes about 0.60 ft³, a 60 lb bag 0.45 ft³ and a 40 lb bag 0.30 ft³. Always round bags up.
Worked example
You are pouring a 12 × 8 ft patio, 4 inches thick.
- Volume: 12 × 8 × 4 ÷ 12 = 32 ft³.
- Add 10% extra: 32 × 1.10 = 35.2 ft³.
- Cubic yards: 35.2 ÷ 27 = 1.30 yd³.
- Bags: 35.2 ÷ 0.60 = 58.7, so buy 59 bags of 80 lb concrete.
At about 1.3 cubic yards, it is worth getting a ready-mix quote before buying 59 heavy bags.
Tips before you pour
- Measure the real depth. Dig-outs are rarely even. Take several depth measurements and use the average, or the deepest if the difference is large.
- Include footings and thickened edges. Calculate them separately and add the results together.
- Mind the gravel base. Most slabs sit on 4 inches of compacted gravel. Don't include it in the concrete thickness.
- Buy one or two spare bags. Unopened bags can usually be returned, and running out mid-pour leaves a weak joint.
Frequently asked questions
How many bags of concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab holds 33.3 cubic feet (1.23 cubic yards) of concrete. That is 56 bags of 80 lb concrete with no extra, or 62 bags once you add 10% for waste.
How thick should a concrete slab be?
Patios, walkways and shed bases are commonly poured 4 inches thick. Driveways and slabs that carry vehicles or heavy loads are usually 5 to 6 inches. Local building codes can require more, so check before you pour.
How much extra concrete should I order?
Add 5 to 10% to cover spillage, uneven ground and forms that bow slightly. Use the higher end for small jobs mixed by hand or ground that is not perfectly level. Running short in the middle of a pour is far worse than having some left over.
How many cubic feet are in a cubic yard?
One cubic yard is 27 cubic feet (3 ft × 3 ft × 3 ft), which is about 0.76 cubic metres. Ready-mix suppliers in the US sell concrete by the cubic yard.
Should I use bags or order ready-mix concrete?
Bags suit small jobs such as post holes, steps and repairs. Once a job needs about 1 cubic yard or more, a ready-mix truck is usually cheaper and much less work. Many suppliers charge a short-load fee for small orders, so ask for a quote.
Calculations are exact for the measurements you enter and use typical bag yields. Bag yields vary slightly by brand, so check the label, and follow local building codes for structural work. Read the full disclaimer and how we check our formulas.