Concrete Calculator — Cubic Yards & Bags
Enter your length, width and thickness to get the concrete you need in cubic feet, cubic yards and bags (40/60/80 lb), with an optional waste allowance.
Calculator
A 12 × 12 ft slab at 4" needs about 48.0 cu ft — roughly 1.78 cubic yards, or 80 bags of 80 lb mix (includes 0% waste). Order ready-mix in quarter-yard steps.
This is the calculator most people mean when they search “how much concrete do I need?” It takes a rectangular slab, footing or pad and returns the volume three ways — cubic feet, cubic yards for ready-mix, and the number of 40, 60 or 80 lb bags — so you can decide whether to order a truck or buy bags. Concrete is sold by the cubic yard from a ready-mix plant and by the bag at the store, and this tool speaks both languages.
Everything is a plain identity on your measurements: no prices, no brand assumptions, nothing that goes stale. Add a small waste allowance (spillage, uneven subgrade, over-excavation) and the result already includes it.
Formula
Volume in cubic feet is area × thickness, with thickness converted from inches to feet:
cu ft = length_ft × width_ft × (thickness_in ÷ 12)
cu ft (with waste) = cu ft × (1 + waste%)
cubic yards = cu ft ÷ 27 (1 yd³ = 27 ft³)
bags = ceil(cu ft ÷ bag yield), using labeled yields of 0.60 ft³ (80 lb), 0.45 ft³ (60 lb) or 0.30 ft³ (40 lb).
Worked example
A 12 × 12 ft slab at 4 in thick:
- Area = 12 × 12 = 144 sq ft
- Volume = 144 × (4 ÷ 12) = 48 cu ft
- Cubic yards = 48 ÷ 27 = 1.78 yd³
- Bags (80 lb) = ceil(48 ÷ 0.60) = 80 bags
Add a 10% waste allowance and the volume becomes 52.8 cu ft, or about 1.96 yd³ — the reason plants let you order in quarter-yard steps.
Ordering ready-mix vs bags
As a rough rule of thumb, once a pour passes roughly a cubic yard (about 40 × 80 lb bags) mixing bags by hand stops being worth the sweat, and a short-load ready-mix delivery usually wins on both time and finish quality. Under that, bags let you pour in stages. Whichever route you pick, buy or order a little extra: you cannot splice a second batch into a slab that has already started to set, and coming up short mid-pour is the one mistake you cannot fix later.
Thickness matters more than people expect — going from 4 in to 5 in on the same slab adds 25% to the volume. Measure the thickness you will actually pour, not the nominal one, and remember that an uneven or over-excavated subgrade quietly eats concrete.
Reference table
How far one cubic yard of concrete goes, by slab thickness (labeled planning typicals):
| Thickness | Coverage per yd³ |
|---|---|
| 3 in | 65 sq ft |
| 4 in | 46.7 sq ft |
| 6 in | 35 sq ft |