Two numbers get you there: the volume of your pour, and how much a bag of concrete makes. Work out the volume in cubic feet (or cubic yards), divide by the yield of your bag size, and round up. That is the whole method — everything below is just filling in the numbers.
Step 1: work out the volume
For a slab or footing, multiply length × width × thickness — with the thickness in feet. A 4 inch slab is 4⁄12 = 0.33 ft thick.
Example: a 10 × 10 ft slab at 4 inches is 10 × 10 × 0.33 = about 33 cu ft, which is 33 ÷ 27 = 1.23 cubic yards (0.94 m³). For a round post hole, use π × radius² × depth.
Step 2: divide by the bag yield
Bagged concrete is sold by weight, but what matters is the volume each bag makes once mixed. The standard yields:
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb (18 kg) | 0.30 cu ft | 90 |
| 60 lb (27 kg) | 0.45 cu ft | 60 |
| 80 lb (36 kg) | 0.60 cu ft | 45 |
So a cubic yard (27 cu ft) takes 45 × 80 lb bags, 60 × 60 lb, or 90 × 40 lb. For the 1.23 yd³ slab above, that is about 56 × 80 lb bags (1.23 × 45) — which is exactly the point at which most people stop and call for ready-mix.
Concrete Calculator
Skip the arithmetic — enter your slab, footing, tube, curb or stairs and get bags, ready-mix and a cement/sand/stone breakdown.
Bags or ready-mix?
Bags make sense for small jobs — a few post holes, a mower pad, a repair. Once you pass roughly 1 cubic yard (about a 10 × 10 ft slab at 4 inches), mixing 45+ bags by hand is slow, inconsistent, and usually more expensive than a ready-mix truck. Ready-mix also pours in one go, avoiding the cold joints you get when a hand-mixed slab sets in stages.
Always add a waste allowance
Order about 5–10% extra. Sub-grade is never perfectly level, some concrete sticks to the mixer and wheelbarrow, and running short mid-pour means a cold joint or a second trip. It is far cheaper to have half a bag left over than to be one bag short.
Working in metric?
The method is identical: volume in cubic metres (length × width × thickness in m), then divide by the bag yield. A 40 kg premix bag makes about 0.018 m³, and a 50 kg bag of cement plus sand and stone at a 1:2:4 mix yields roughly 0.13 m³ of concrete. The calculator does both systems — just flip the units switch.