Concrete Calculator
Calculate concrete volume for slabs and round columns in cubic feet and cubic yards, with a waste allowance and 40, 60 or 80 lb bag counts.
Updated Formula covered by automated tests
Concrete is sold by volume, but you measure a patio, shed base or fence post hole in feet and inches. This calculator turns those measurements into cubic feet and cubic yards, adds an allowance for waste, and tells you how many 40, 60 or 80 lb bags of premixed concrete it would take if you mix it yourself.
Use the slab mode for anything with a flat rectangular footprint: patios, walkways, shed pads, footings and short walls. Use the column mode for round forms such as cardboard tube forms (sonotubes) for deck piers and fence posts. For an L-shaped patio, split it into two rectangles and add the results.
The cubic-yard figure is what a ready-mix supplier will ask for on the phone. The bag counts help you decide whether a project is small enough to mix by hand or with a rented mixer, or big enough that a truck delivery makes more sense.
The formula
- L, W
- Length and width of a slab or footing, in feet
- D
- Slab thickness in inches; dividing by 12 converts it to feet
- d
- Column diameter in inches; d ÷ 24 is the radius in feet
- H
- Column height in feet
- waste%
- Extra concrete ordered to cover spillage, uneven subgrade and form bulge, typically 5 to 10 percent
- bag yield
- Cubic feet of mixed concrete one bag produces: about 0.30 for 40 lb, 0.45 for 60 lb and 0.60 for 80 lb
- ⌈ ⌉
- Round up to the next whole bag, since stores do not sell partial bags
A slab is a rectangular box, so its volume is length times width times thickness. The only trap is units: plans give thickness in inches and the other two sides in feet, so the inches must be divided by 12 before multiplying. A cylinder works the same way with the circular area π r² in place of length times width.
A cubic yard is a cube 3 feet on each side, so it holds 3 × 3 × 3 = 27 cubic feet. Ready-mix plants in the US batch and price by the cubic yard, usually in quarter-yard steps, which is why the result is shown in both units.
Bag yields come from the manufacturers: an 80 lb bag of standard premix makes roughly 0.6 cubic feet once water is added, which means about 45 bags per cubic yard. Because the yield is printed on each bag, check the brand you buy and adjust if it differs.
Worked example
Daniel is pouring a 10 ft by 12 ft backyard patio, 4 inches thick, and wants 10 percent extra in case the gravel base is a little low in spots.
- 1 Convert the thickness to feet: 4 ÷ 12 = 0.333 ft.
- 2 Net volume: 10 × 12 × 0.333 = 40 cubic feet.
- 3 Add 10 percent: 40 × 1.10 = 44 cubic feet.
- 4 Convert to cubic yards: 44 ÷ 27 = 1.63 cu yd.
- 5 Count 80 lb bags: 44 ÷ 0.60 = 73.3, rounded up to 74 bags. With 60 lb bags: 44 ÷ 0.45 = 97.8, so 98 bags.
Result: Daniel needs about 1.63 cubic yards (order 1.75 yd from a ready-mix plant) or 74 bags of 80 lb premix. At over 5,900 lb of bags, a small truck delivery is worth pricing.
Before you rely on the result
- The calculator assumes straight sides and a uniform thickness. Thickened slab edges, sloped footings and steps need to be calculated piece by piece.
- Bag yields vary slightly by brand and mix (high-strength, fast-setting and mortar mixes differ); read the yield on the bag you plan to buy.
- It does not size reinforcement, base gravel or joints. Structural footings and anything holding up a building should follow local code or an engineer’s design.
Frequently asked questions
How thick should a concrete patio or driveway be?
Four inches is the usual thickness for patios, walkways and shed floors that carry foot traffic. Driveways that carry cars are commonly poured 4 to 6 inches thick, and heavier vehicles call for more plus reinforcement. Your local building department may set a minimum, so check before you pour.
Is it cheaper to buy bags or order ready-mix?
For a fraction of a yard, bags win because ready-mix plants usually charge a short-load fee on small orders. Past roughly one cubic yard (45 bags of 80 lb), the labor of mixing and the cost of that many bags usually makes a truck the better deal. Get both prices for your area.
Why does my slab need more concrete than the math says?
Real subgrades are never perfectly flat. A base that is half an inch low across a 10 by 12 ft slab swallows another 5 cubic feet. Forms bow outward under the weight, some concrete stays in the mixer and wheelbarrow, and a little spills. That is what the waste allowance covers.
Can I use the column mode for a square post hole?
Not directly. For a square or rectangular hole, use the slab mode with the hole width and length in feet and the depth entered as inches (a 3 ft deep hole is 36 inches). Remember to subtract the volume of the post if it sits in the hole while you pour.
Sources
- Portland Cement Association (cement.org)
- American Concrete Institute (concrete.org)