How the concrete calculator works
Concrete is ordered by volume. Work out the volume of the space you are filling, add a little extra for waste, then either order that many cubic yards (or cubic metres) of ready-mix, or divide by the yield of one bag.
Every measurement must be in the same unit before you multiply. In US units that usually means turning inches into feet (divide by 12). Then:
A cubic yard is 3 ft × 3 ft × 3 ft, which is why you divide by 27. The steps formula treats the stairs as solid: the bottom step is one rise high, the second is two rises high, and so on, and the optional platform at the top is as high as the top step.
Worked examples
A patio slab. 10 ft × 10 ft, 4 in thick.
- Thickness: 4 ÷ 12 = 0.333 ft.
- Volume: 10 × 10 × 0.333 = 33.33 ft³.
- Cubic yards: 33.33 ÷ 27 = 1.23 yd³.
- With 10% extra: 1.23 × 1.10 = 1.36 yd³ (36.7 ft³).
- In 80 lb bags: 36.7 ÷ 0.60 = 61.1, so buy 62 bags.
Eight fence posts. Each hole is 10 in wide and 2 ft 6 in deep.
- Radius: 5 in = 0.417 ft. Depth: 2.5 ft.
- One hole: π × 0.417² × 2.5 = 1.36 ft³. Eight holes: 10.9 ft³.
- With 10% extra: 12.0 ft³, which is 27 bags of 60 lb or 20 bags of 80 lb.
- The post itself takes up some of the space, so you will usually have a bag or two left over.
A metric driveway section. 6 m × 3 m, 15 cm thick.
- Volume: 6 × 3 × 0.15 = 2.7 m³. With 5% extra: 2.84 m³.
- That is far too much to mix from bags (over 240 bags of 25 kg), so order about 3 m³ of ready-mix.
Bag yields
Premixed concrete bags are sold by weight, but what matters is how much space the mixed concrete fills. These are the yields printed by Quikrete on its standard concrete mix:
| Bag | Yield per bag | Bags per cubic yard | Bags per m³ |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | about 118 |
| 60 lb | 0.45 ft³ | 60 | about 79 |
| 80 lb | 0.60 ft³ | 45 | about 59 |
| 20 kg | about 0.0094 m³ | about 82 | about 107 |
| 25 kg | about 0.0117 m³ | about 65 | about 85 |
The kilogram sizes are estimated at the same density as the 80 lb bag (about 2,136 kg of dry mix per cubic metre of placed concrete). Other brands, high-strength mixes and fast-setting post mixes yield different amounts, so read the bag before you buy. The calculator always rounds bags up.
Typical thickness
| Job | Common thickness |
|---|---|
| Garden path, shed base | 3–4 in (75–100 mm) |
| Patio | 4 in (100 mm) |
| Driveway for cars | 4–6 in (100–150 mm) |
| Garage floor | 4–6 in (100–150 mm) |
| Footings | Set by local code and soil; often 8–12 in deep |
These are typical figures, not design advice. Footings, structural slabs and anything carrying heavy loads should follow your local building code or an engineer’s drawing.
Tips before you pour
- Add 5–10% extra. Ground is never perfectly flat and forms bow slightly. Running short in the middle of a pour can leave a weak joint.
- Measure the real depth. Check the depth in several places once the sub-base is compacted. A slab planned at 4 in that is really 4.5 in needs 12.5% more concrete.
- Split complicated shapes. Break an L-shaped patio into rectangles and add the volumes. The area calculator helps with circles and triangles.
- Bags or truck? Mixing more than about a cubic yard (45 bags of 80 lb) by hand is heavy work. Ready-mix suppliers usually deliver in steps of a quarter or half cubic yard, and some have a minimum order.
- Convert units easily. If your plans are in litres or cubic metres and the supplier works in cubic yards, the volume converter switches between them.
Related tools
Planning flooring or paving first? The square footage calculator adds up several areas, and the roofing calculator works out roof area and shingle bundles from a house footprint.