Concrete calculator
Measure the pour, pick a shape, and this calculator gives you the volume in cubic yards plus the number of bagged mix to buy. It also shows when a ready-mix truck makes more sense than bags.
How the concrete calculator works
Concrete is sold by volume. The calculator converts your measurements into cubic feet, adds a waste allowance, then divides by 27 to get cubic yards, which is how ready-mix suppliers price it. For bagged mix it divides the same volume by the yield printed on the bag.
- Slab or footing: length × width × thickness. Thickness is entered in inches and converted to feet.
- Round column or post hole: π × radius² × depth, multiplied by the number of holes. Use the hole diameter, not the post size.
- Stairs: each step is treated as a solid block that gets taller as you go up, which is how poured stairs are formed.
Bag yields used here match the figures printed on Quikrete and Sakrete products: an 80 lb bag makes about 0.60 cubic feet, a 60 lb bag 0.45, and a 40 lb bag 0.30.
Formula
cubic yards = (length ft × width ft × thickness in ÷ 12) ÷ 27
bags = cubic feet ÷ yield per bag, rounded up to a whole bag.
Worked example: a 10 × 12 ft patio at 4 inches thick is 10 × 12 × 0.333 = 40 cubic feet. With 10 percent waste that is 44 cubic feet, or 1.63 cubic yards. In 80 lb bags, 44 ÷ 0.6 = 74 bags, weighing almost 3 tons. That is a clear case for ordering ready-mix.
Tips before you pour
- Ready-mix suppliers usually have a 1 cubic yard minimum and charge a short-load fee under about 4 yards. Ask when you call.
- Round up. Running out mid-pour creates a cold joint that is weaker and looks bad. Leftover concrete can fill a post hole or a stepping stone form.
- Subgrade matters more than thickness. Compact 4 inches of gravel under a slab and the 4 inch pour will outlast a 6 inch pour on soft dirt. Use the gravel calculator for the base.
- For fence and deck posts, the hole should be about three times the post width and a third of the post length deep, below the frost line in cold climates.
Common sizes, already worked out
Common questions
How many 80 lb bags of concrete are in a cubic yard?
45 bags. A cubic yard is 27 cubic feet and each 80 lb bag yields 0.60 cubic feet, so 27 ÷ 0.6 = 45. It takes 60 of the 60 lb bags or 90 of the 40 lb bags.
How thick should a concrete slab be?
4 inches is standard for patios, walkways, and shed floors. Use 5 to 6 inches for driveways and anything that will carry a vehicle, and add rebar or wire mesh for spans over about 10 feet.
How much does a cubic yard of concrete weigh?
About 4,000 pounds, or 2 tons. Each 80 lb bag of mix weighs 80 pounds dry and a bit more once water is added, so the bag count here is a good proxy for what you will be lifting.
Is it cheaper to buy bags or order ready-mix?
Bags cost less for small jobs under about half a yard, since there is no delivery fee. Above a yard, the cost per cubic yard of bagged mix is typically two to three times the ready-mix price, before counting your time to mix 45 or more bags.