How to use the concrete calculator
Pick the shape of your pour, enter the dimensions, and the calculator gives the volume in cubic yards (the unit ready-mix is sold in), cubic feet and cubic metres, plus how many bags of pre-mix you’d need. A waste allowance is added by default because sub-grades are never perfectly level. Enter prices to compare ready-mix against bagged concrete.
The maths
volume (cu ft) = length (ft) × width (ft) × thickness (ft)
cubic yards = cubic feet ÷ 27Thickness is usually quoted in inches, so divide by 12 first. A 10 × 12 ft patio at 4 inches: 10 × 12 × (4 ÷ 12) = 40 cu ft = 1.48 cu yd.
For round columns and piers: π × (diameter ÷ 2)² × height. A 12-inch (1 ft) diameter pier 4 ft deep is π × 0.5² × 4 = 3.14 cu ft.
For stairs, each step is treated as a block the full width of the stairs, one run deep and as tall as its height above the ground — so step 3 of a 7-inch rise is 21 inches tall. The calculator sums the steps.
Recommended thickness
| Application | Thickness |
|---|---|
| Sidewalk, patio, shed base | 4 in |
| Residential driveway | 5–6 in |
| Garage floor | 4–6 in |
| Footing for a wall or deck post | 8–12 in, width ≥ 2 × wall thickness |
| Slab for heavy equipment | 6–8 in with rebar |
Always check local building codes for footings — the required depth depends on the frost line in your area.
Bags vs ready-mix
| Bagged mix | Ready-mix truck | |
|---|---|---|
| Best for | Up to ~1 cu yd (posts, small pads, repairs) | 1 cu yd and up |
| Cost | ~$5–7 per 80 lb bag ≈ $225–315 per yard | ~$150–200 per yard + delivery, short-load fee below 3–4 yd |
| Effort | Mixing 45 bags per yard by hand is a full day’s work | Poured in minutes; you need helpers ready to spread and finish |
| Consistency | Varies batch to batch | Uniform, and strength-rated (e.g. 3,000–4,000 psi) |
Before you pour
- Order 5–10% extra. Running out mid-pour creates a cold joint; leftover is a minor cost.
- Prepare the base: compacted gravel 4 inches deep under slabs, and forms braced to resist the weight of wet concrete (about 150 lb per cubic foot).
- Reinforce driveways and larger slabs with wire mesh or rebar; add control joints every 8–12 ft to manage cracking.
- Weather: avoid pouring below 40°F or in heavy rain; in hot weather, work fast and keep the surface damp while it cures.
- Measure the area first with the square footage calculator if the shape is irregular.
Frequently asked questions
How much concrete do I need for a 10×10 slab?
At the standard 4-inch thickness, a 10 × 10 ft slab is 33.3 cubic feet, or 1.23 cubic yards. With a 10% allowance for uneven ground and spillage, order 1.35 yards — or about 62 bags of 80 lb pre-mix.
How many 80 lb bags make a cubic yard?
About 45. An 80 lb bag yields roughly 0.6 cubic feet; a 60 lb bag 0.45; a 40 lb bag 0.3. That's why bagged mix stops making sense above about one cubic yard.
How thick should a slab be?
4 inches for patios, sidewalks and shed floors; 5–6 inches for driveways and garage floors that carry vehicles; footings are usually 8–12 inches thick and at least as wide as the wall is thick, below the frost line.
Should I order ready-mix or use bags?
Under about 1 cubic yard, bags are cheaper and simpler. Above that, a ready-mix truck saves hours of mixing and gives a consistent pour, though most suppliers charge a short-load fee for less than 3–4 yards.
Last updated August 27, 2026.