Converting between yards and bags
A bag of concrete is sold by the weight of dry mix inside it, but a pour is measured by the space it has to fill. The number that joins the two is the bag yield: the volume of wet concrete one bag produces once water is added. Yield is printed on the sack and it is not proportional to weight, which is why doubling the bag weight does not halve the bag count.
- bag yield — cubic feet of mixed concrete per sack: 0.60 for 80 lb, 0.45 for 60 lb, 0.375 for 50 lb, 0.30 for 40 lb
- waste — added only when going from volume to bags, never in reverse
- 27 — cubic feet in a cubic yard, if your volume is in yards
Worked example: one cubic yard
Reversed, 45 sacks of 80 lb give you 27 cubic feet back — one yard exactly. That reverse figure carries no waste allowance, because the sacks already in your vehicle are a fixed quantity. Padding them would tell you that you can fill more space than you actually can.
Why the allowance only applies one way
Waste is a purchasing decision, not a property of the material. When you are working out what to buy, an allowance protects you against a short load. When you are working out what a stack of sacks will place, you want the honest figure — the pour will consume whatever it consumes, and a padded number would leave you short at the far end of the form.
Common mistakes
- Treating weight as volume. Two 40 lb sacks do not equal one 80 lb sack. Two 40s yield 0.60 ft³ and one 80 yields 0.60 ft³ — that pair happens to match, but 60 lb sacks break the pattern entirely.
- Trusting a rounded "45 bags per yard" for every size. That number belongs to the 80 lb sack only.
- Buying to the exact figure. Merchants sell whole sacks and a split sack is a wasted sack, so the count always climbs to the next whole one.
- Ignoring what 45 sacks weigh. A cubic yard in 80 lb sacks is 3,600 lb of dead lift, well past what most cars will legally carry in one trip.
When these numbers do not apply
- Specialist mixes. Fast-setting, fibre and post mixes publish their own yields, sometimes lower than standard mix at the same sack weight.
- Sacks that have been stored badly. Mix that has taken moisture on a damp floor yields less and gains lumps that will not break down.
- Volumetric or ready-mix deliveries. Those are sold by the yard directly, so no conversion is involved. They are also denser than bag mix — about 150 lb/ft³ against roughly 140 — so do not carry a weight from this page onto a delivery docket.
Yield and bag count by bag size
| Bag | Yield (ft³) | Bags per cubic yard | Covers at 1 in (ft²) |
|---|---|---|---|
| 80 lb bag | 0.600 | 45 | 7.2 |
| 60 lb bag | 0.450 | 60 | 5.4 |
| 50 lb bag | 0.375 | 72 | 4.5 |
| 40 lb bag | 0.300 | 90 | 3.6 |
| 90 lb bag | 0.675 | 40 | 8.1 |
Yields are the mixed volume each bag places, taken from the manufacturer data in this site's constants file. One cubic yard is 27 cubic feet.
