Concrete Calculator

How to measure for concrete

Concrete is ordered by volume, so you need three measurements: length, width and thickness (depth). For slabs, measure the inside of the forms, not the area you hope to pour. For footings, measure the trench. For post holes and round columns (such as tube forms), measure the diameter of the hole or tube and the depth.

• Length and width go in feet. Convert inches to decimal feet (6 in = 0.5 ft).
• Thickness goes in inches. The calculator divides by 12 for you.
• Check depth in several spots. Subgrade is rarely perfectly flat, and a slab that is 4 in on one side and 5 in on the other needs noticeably more concrete than planned.

The formula

For slabs and footings: cubic feet = length × width × (thickness ÷ 12). For round columns: cubic feet = π × (diameter in inches ÷ 24)² × height in feet. Divide cubic feet by 27 to get cubic yards. Bags are figured from the yield printed on common premix bags: about 0.60 cu ft for an 80 lb bag, 0.45 cu ft for 60 lb and 0.30 cu ft for 40 lb.

Worked example

A 12 ft × 10 ft patio slab, 4 in thick:

• 12 × 10 × (4 ÷ 12) = 40 cu ft
• Add 10% waste: 40 × 1.10 = 44 cu ft, or about 1.63 cubic yards
• 80 lb bags: 44 ÷ 0.60 = 73.3, so 74 bags
• 60 lb bags: 44 ÷ 0.45 = 97.8, so 98 bags

Seventy-four 80 lb bags is nearly 3 tons of material to lift and mix by hand. That is why many people switch to ready-mix delivery once a job passes about a cubic yard.

Waste and overage

Add about 5–10% for spillage, uneven subgrade and forms that bow slightly. Running short in the middle of a pour is a real problem, because a cold joint forms where fresh concrete meets concrete that has started to set. Extra bags are cheap insurance, and unopened bags can often be returned or saved for a later project if kept dry.

Common mistakes

• Entering thickness in feet. The thickness box is in inches.
• Ignoring thickened edges. Many slabs have a deeper edge or footing around the perimeter. Calculate that as a separate footing and add it.
• Using nominal form size. A slab formed with 2×4s is about 3.5 in thick, not 4 in, unless the base is dug lower.
• Forgetting the gravel base. Slabs usually sit on a compacted gravel base, which is a separate purchase. Use the gravel calculator.
• Mixing too few bags at once. Have enough help and mixers so each section is placed and finished before the edges stiffen.

Buying tips

• Premix bags are commonly sold in 40, 50, 60 and 80 lb sizes. Always confirm the yield on your brand's label and enter the bag count from that figure.
• Fast-setting post mixes are handy for fence posts, but yields differ from standard mixes. Check the bag.
• Ready-mix trucks are ordered in cubic yards, often in quarter- or half-yard steps, and many suppliers have a minimum load or short-load fee. Call ahead.
• For posts, switch the calculator to Round column mode and set the quantity to your number of holes.