Enter measurements above 0 to see results.
How the Concrete Calculator works
The concrete calculator tells you how much concrete a slab, footing or round post hole needs: in cubic yards for ordering ready-mix, and in 80, 60 and 40 lb bags of premix. Enter length, width and thickness (or diameter and depth for a round hole), how many you are pouring, and a waste allowance.
Slab volume in cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12. A round hole is π × (diameter ÷ 2)² × depth. Multiply by the quantity and add the waste percentage, then divide cubic feet by 27 for cubic yards. Bags = cubic feet ÷ the bag yield (about 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb, 0.30 for 40 lb), rounded up to whole bags.
Example: a 10 ft × 10 ft patio, 4" thick. 10 × 10 × 4 ÷ 12 = 33.33 cu ft, or 1.23 cu yd. With 10% waste that is 36.67 cu ft (1.36 cu yd). In bags: 36.67 ÷ 0.60 = 61.1, so 62 bags of 80 lb, or 36.67 ÷ 0.45 = 81.5, so 82 bags of 60 lb.
Waste covers an uneven subgrade, forms that bow out and spillage; 5% to 10% is typical, more for dug footings with rough sides. Bag yield varies a little by brand and mix, so check the bag. Break odd shapes into rectangles and add them up, and measure the thickness in several spots: a slab that averages 4 1/2" instead of 4" needs 12.5% more concrete.
Concrete Bag Chart: common sizes at a glanceFrequently asked questions
How many bags of concrete do I need?
Work out the volume in cubic feet and divide by the bag yield: about 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb and 0.30 for 40 lb. Round up to whole bags. A 10 ft × 10 ft slab 4" thick is 33.33 cu ft, which is 56 bags of 80 lb before waste.
How many 80 lb bags of concrete are in a yard?
A cubic yard is 27 cu ft, so at 0.60 cu ft per bag it takes 45 bags of 80 lb. That is the same as 60 bags of 60 lb or 90 bags of 40 lb. Past about a yard, many crews find ready-mix easier than mixing that many bags.
How much concrete do I need for a post hole?
Volume = π × (diameter ÷ 2)² × depth. A hole 12" across and 4 ft deep is 3.14 × 0.5² × 4 = 3.14 cu ft, so 6 bags of 80 lb (5.2 rounded up) without waste. The calculator figures the whole hole; the post takes up a little of that space, which works as extra.
How thick should a concrete slab be?
4" is standard for patios, sidewalks and shed floors, and driveways are often 4" to 6" depending on the vehicles. IRC R506.1 sets a 3 1/2" minimum for concrete floor slabs on the ground. Heavier loads, poor soil or your local code may call for more, so check before you pour.
How much extra concrete should I order?
Plan on 5% to 10% extra for an uneven base, forms that bow out and what stays in the mixer or wheelbarrow. Dug footings and post holes with rough sides can take more. The calculator adds 10% by default; change the waste field to suit the job.