Concrete Calculator (slabs, footings, post holes)
Work out the concrete a slab, footing, or post hole needs in cubic yards or metres, and how many bags that is.
Above about a cubic yard, ready-mix delivered by truck is usually cheaper and far less work than mixing bags.
Bags per cubic yard and cubic metre
| Bag | Yield | Per cubic yard | Per cubic metre |
|---|---|---|---|
| 80 lb bag (0.60 ft³) | 0.01699 m³ | 45 | 59 |
| 60 lb bag (0.45 ft³) | 0.01274 m³ | 60 | 79 |
| 40 lb bag (0.30 ft³) | 0.008495 m³ | 90 | 118 |
| 25 kg bag (12.5 L) | 0.0125 m³ | 62 | 80 |
| 20 kg bag (10 L) | 0.01 m³ | 77 | 100 |
Common slabs at 4 inches thick
| Slab | Cubic yards | Cubic metres | 80 lb bags |
|---|---|---|---|
| 4 × 4 ft pad | 0.198 | 0.151 | 9 |
| 8 × 8 ft shed base | 0.79 | 0.604 | 36 |
| 10 × 10 ft patio | 1.23 | 0.944 | 56 |
| 12 × 12 ft patio | 1.78 | 1.36 | 80 |
| 10 × 20 ft driveway | 2.47 | 1.89 | 112 |
| 20 × 20 ft garage | 4.94 | 3.78 | 223 |
Volumes are net — add your own waste allowance on top.
Working out concrete
Concrete is quantity surveying, not chemistry: work out the volume of the hole you are filling, add a bit for spillage and uneven ground, then convert that volume into whatever you are buying it in. The only trap is units — slabs are measured in feet but poured a few inches thick, so the thickness has to be converted before anything is multiplied.
Slab: length × width × thickness · Post hole: π × (diameter/2)² × depth
Worked example
A 10 × 10 ft patio at 4 inches thick is 10 × 10 × 0.333 = 33.3 ft³, which is 33.3 / 27 = 1.23 cubic yards. At 0.6 ft³ per 80 lb bag that is 56 bags before any allowance — and this is exactly the point where most people stop and order ready-mix instead. Add 10% for waste and it is 62 bags, or about 1.36 yd³ delivered.
Why 45 bags to the yard
An 80 lb bag of standard mix yields 0.6 ft³ of set concrete, and a cubic yard is 27 ft³, so 27 / 0.6 = 45 bags. The 60 lb bag yields 0.45 ft³ giving 60 to the yard, and the 40 lb bag yields 0.30 ft³ giving 90. Those three figures are worth remembering — they let you sanity-check any quote in your head. Note that the yield is much less than the dry weight suggests, because you are adding water and the mix compacts.
How much waste to allow
Ten percent is the usual allowance and it is rarely too much. Sub-grade is never perfectly flat, forms bow slightly under the weight of wet concrete, and some always ends up on the ground. For a footing dug into soft or crumbling soil, allow more — an over-dug trench can swallow a surprising amount. It is far better to have a spare bag than to be one short with a slab half-poured and setting.
Bags or ready-mix?
Mixing bags by hand is fine up to roughly half a cubic yard. Beyond that the labour becomes serious — a cubic yard is 45 bags weighing over 1.6 tonnes in total, each of which has to be lifted, mixed, and placed before the first batch starts going off. Ready-mix trucks usually have a minimum load and a waiting charge, but for anything patio-sized or larger they win comfortably.
Related
For the volume of an irregular shape, break it into rectangles and cylinders and add them up, or use thesolid volume calculator. Setting fence posts is handled by the fence calculator, which works out the concrete per hole for you.
Bag yields and ground conditions vary
The volumes here are exact geometry; the bag counts depend on the yield printed on the product you buy, which differs between manufacturers and between mix types. Check it rather than assuming the standard figure. The bigger uncertainty is the hole: sub-grade is never perfectly flat, forms bow under the weight of wet concrete, and a trench dug in soft or crumbling soil can swallow a surprising amount more than its nominal dimensions. That is what the waste allowance is for, and 10% is rarely too much. Seeestimating materials for the bags-versus-ready-mix arithmetic.