Concrete Calculator
Work out concrete for a slab, footing or post holes - as bags, as ready-mix yards, and what each route costs.
How it works
- Slab and footing volume = length x width x depth
- Post hole volume = pi x (diameter / 2)^2 x depth, times the number of holes
- Cubic yards = cubic feet / 27
- Bags = volume (with waste) / yield per bag (80 lb = 0.6 cu ft, 60 lb = 0.45, 40 lb = 0.3)
Worked example
A 10 ft x 10 ft slab at 4 inches thick: 10 x 10 x 0.333 ft = 33.3 cu ft, which is 1.24 cubic yards. Add 10% for waste and you need 1.36 cubic yards, so you order 1.5 cubic yards of ready-mix (about $225 at $150 a yard) or 62 bags of 80 lb mix (about $372 at $6 a bag). For this size, ready-mix wins.
Assumptions to know
- Bag yields are the standard printed yields: 80 lb = 0.6 cu ft, 60 lb = 0.45 cu ft, 40 lb = 0.3 cu ft.
- Ready-mix is rounded up to the nearest 0.25 yard for ordering; minimum-load and delivery fees are not included.
- Reinforcement, forms and gravel base are not included.
Results are planning estimates only. Always check the coverage and instructions on the actual product you buy, and measure your own space carefully before ordering. When in doubt, a little extra material is cheaper than a second trip or a stalled job.
Frequently asked questions
How many bags of concrete do I need for a 10 x 10 slab?
A 10 ft x 10 ft slab, 4 inches thick, is about 1.24 cubic yards (33.3 cubic feet). With 10 percent extra that is about 62 bags of 80 lb mix, or 1.4 cubic yards of ready-mix.
Bags or ready-mix - which is better?
Under about half a cubic yard, bags are usually simpler and cheaper. Above 1 cubic yard, ready-mix delivery is usually better value and far less work, even with a short-load fee.
How thick should a concrete slab be?
4 inches is typical for patios, sidewalks and sheds. Driveways often use 4 to 6 inches with reinforcement. Local code and soil conditions decide - check before you pour.
Why add a waste allowance?
Ground is never perfectly level, forms bow a little, and some concrete stays in the wheelbarrow. 5 to 10 percent extra avoids running short mid-pour, which is the worst time to find out.