To find how much concrete a slab needs, multiply length by width by thickness. Do all three in feet, then divide by 27 to get cubic yards. In metric, multiply all three in meters to get cubic meters.
Ready-mix concrete is sold by volume. An order that is too small leaves a section of slab unfinished. An order that is too large costs money for concrete you cannot use.
The formula in feet and yards
Convert the thickness from inches to feet first. Divide the inches by 12. A 4 inch slab is 0.333 feet. A 6 inch slab is 0.5 feet.
Then use this:
Length (ft) x width (ft) x thickness (ft) = cubic feet
Cubic feet / 27 = cubic yards
There are 27 cubic feet in one cubic yard. That is 3 feet by 3 feet by 3 feet.
Worked example: a 12 by 20 foot driveway pad
The pad is 12 feet wide and 20 feet long. The slab is 4 inches thick.
- Area: 12 x 20 = 240 square feet.
- Thickness: 4 / 12 = 0.333 feet.
- Volume: 240 x 0.333 = 80 cubic feet.
- Yards: 80 / 27 = 2.96 cubic yards.
The slab needs just under 3 cubic yards. With 10 percent added for waste, the order is about 3.25 cubic yards.
Make the same pad 6 inches thick and the math changes. 240 x 0.5 = 120 cubic feet. 120 / 27 = 4.44 cubic yards. Our guide on how thick a concrete slab should be explains which depth fits which project.
The formula in meters
Metric is simpler because no conversion is needed beyond turning millimeters into meters. A 100 mm slab is 0.1 meters.
Length (m) x width (m) x thickness (m) = cubic meters
A slab 4 meters wide and 6 meters long at 100 mm thick: 4 x 6 x 0.1 = 2.4 cubic meters. With 10 percent added, the order is about 2.65 cubic meters.
To convert between the two, one cubic meter is about 1.31 cubic yards.
Quick reference for common thicknesses
One cubic yard covers about 81 square feet at 4 inches thick. It covers about 65 square feet at 5 inches. It covers about 54 square feet at 6 inches.
Divide your square footage by the number for your thickness to get yards. A 400 square foot patio at 4 inches needs about 4.9 cubic yards before waste.
Shapes that are not rectangles
L-shaped or split areas. Break the slab into rectangles. Calculate each one. Add the results together.
Triangles. Multiply base by height, divide by 2, then multiply by thickness.
Round slabs. Multiply 3.14 by the radius, then by the radius again. Then multiply by thickness. The radius is half the diameter.
Round piers and sonotube footings. Use the same round formula, with the depth of the hole as the thickness. A 12 inch diameter pier, 4 feet deep, has a radius of 0.5 feet. 3.14 x 0.5 x 0.5 = 0.785 square feet. 0.785 x 4 = 3.14 cubic feet. That is about 0.12 cubic yards for one pier. Ten piers need about 1.2 cubic yards.
Footings and thickened edges. Calculate the footing as its own rectangle. Length times width times depth. Add it to the main slab total.
How much extra to order
Add 5 to 10 percent to the total. Several things use up concrete beyond the plain math:
An uneven base. A low spot in the gravel or soil takes more concrete to fill. A base that is dug a half inch too deep across a 240 square foot pad adds about 10 cubic feet.
Forms that bow. Wet concrete pushes forms outward. A small bow adds volume.
Spillage and leftover in the truck chute. Some concrete never reaches the forms.
Use 10 percent for a slab on uneven or soft ground, or for a shape with many cuts and corners. Use 5 percent for a flat, well-compacted base with square forms. Your contractor knows the site and will often adjust the number.
Check the real depth
Lumber sizes cause errors. A 2x4 form measures 3.5 inches deep, not 4. A slab poured to the top of 2x4 forms holds about 12 percent less concrete than a true 4 inch slab, if the base is level. A 2x6 measures 5.5 inches.
Measure the depth from the top of the compacted base to the top of the form. Use that number in the formula.
How many bags of concrete
Bags work for small jobs such as a step, a set of fence post holes, or a small pad. The yield is printed on each bag. An 80 pound bag of premixed concrete commonly yields about 0.6 cubic feet. Check the label on the brand you buy, because yields differ.
Divide your cubic feet by the yield per bag. The 12 by 20 foot pad above needs 80 cubic feet. 80 / 0.6 = 133 bags. With 10 percent added, that is about 147 bags.
Each 80 pound bag must be lifted, opened, and mixed. A slab that size is a ready-mix job. As a rough guide, anything beyond a few dozen bags is easier and more consistent from a truck. Mixing by hand also makes it hard to finish in one continuous pour, and cold joints and color changes can show up between batches.
Ordering from a ready-mix supplier
Give the supplier the total volume, the strength rating your contractor specified, and whether the mix needs air entrainment. Exterior concrete in a freezing climate needs it. Your contractor will usually place this order.
Ask about minimum load charges. Small orders often cost more per yard, and some suppliers add a fee for a partial truck. Ask also how long the crew gets to unload before waiting charges begin. Have the forms, base, and crew ready before the truck arrives.
After the pour, the slab needs time before it takes weight. Our guide on how long until you can walk or drive on new concrete gives the timeline.
Checklist before you order
- Measure length and width at several points. Walls and forms are rarely perfectly square.
- Confirm the thickness with your contractor, and measure the actual form depth.
- Add footings, thickened edges, and piers as separate volumes.
- Add 5 to 10 percent for waste.
- Round up to the next quarter or half yard, based on what the supplier sells.
- Ask your contractor to check your number against theirs.