Concrete calculator
Volume in cubic yards, plus the number of bags you actually need to buy. Works for slabs, footings, pads and post holes.
- Volume
- — ft³
- Bags needed
- —
- Estimated cost
- —
How the calculation works
Convert thickness to feet, multiply the three dimensions, then divide by 27 to reach cubic yards:
yd³ = (length × width × thickness ÷ 12) ÷ 27
Bags are the same volume divided by the yield printed on the bag — 0.60 ft³ for 80 lb, 0.45 ft³ for 60 lb, 0.30 ft³ for 40 lb. The result is rounded up, because you cannot buy most of a bag and a short pour ruins a slab.
Worked example
A 10 ft × 10 ft slab at 4 inches: 10 × 10 × 0.333 = 33.3 ft³ raw. With 10% waste that is 36.7 ft³, or 1.36 yd³. In 80 lb bags: 36.7 ÷ 0.60 = 61.1, so 62 bags.
Common slab sizes
| Slab | 4 in thick | 6 in thick | 80 lb bags (4 in, +10%) |
|---|---|---|---|
| 8 × 8 ft shed | 0.79 yd³ | 1.19 yd³ | 40 |
| 10 × 10 ft patio | 1.23 yd³ | 1.85 yd³ | 62 |
| 12 × 12 ft pad | 1.78 yd³ | 2.67 yd³ | 88 |
| 20 × 20 ft driveway | 4.94 yd³ | 7.41 yd³ | 244 |
Frequently asked
How many 80 lb bags make a yard?
About 45. One cubic yard is 27 ft³ and an 80 lb bag yields 0.60 ft³, so 27 ÷ 0.60 = 45 bags. At that quantity, ready-mix delivered by truck is almost always cheaper than bagged mix.
Should I pour 4 or 6 inches?
Four inches is standard for foot traffic, patios and shed floors. Six inches is for vehicles, heavy equipment and anything that will see a loaded trailer.
Does rebar change the volume?
No. Rebar displaces a negligible amount — under 1% at normal spacing. The waste allowance covers it.