How much concrete one bag makes
Each bag of premixed concrete produces a set volume once water is added. Typical yields for standard mixes are:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | about 0.30 cu ft | 90 |
| 60 lb | about 0.45 cu ft | 60 |
| 80 lb | about 0.60 cu ft | 45 |
Yields vary a little by brand and mix, so check the bag. One cubic yard is 27 cubic feet, which is where the bags-per-yard figures come from.
The method
- Work out the volume in cubic feet: length × width × thickness, all in feet. Divide inches by 12 first.
- Divide the volume by the yield of your bag size.
- Add 5–10% for spillage and uneven ground, then round up.
Example: a 10 × 10 slab
A 10 × 10 ft slab 4 inches thick is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, or about 1.23 cubic yards.
| Bag size | Bags (exact) | With 10% extra |
|---|---|---|
| 80 lb | 55.6 | 62 |
| 60 lb | 74.1 | 82 |
| 40 lb | 111.1 | 123 |
That's about 5,000 lb of bags to move and mix by hand for the 80 lb option. At this size, ready-mix is worth pricing. Our guide to concrete slab thickness covers how thick different slabs should be.
Example: fence post holes
For a round hole, volume = π × radius² × depth. A 10 in diameter hole 3 ft deep holds about 1.64 cubic feet. A 4 × 4 post (3½ × 3½ in actual) takes up about 0.26 cubic feet of that, leaving roughly 1.38 cubic feet of concrete, or about 2.3 bags of 80 lb mix per post. Round up to 3 bags per hole, or work out the total for the whole fence and then round up.
The fence calculator works out post count and concrete for a fence run, and the concrete footing calculator handles round and square footings.
Bags or ready-mix?
As a rule of thumb, pours under about one cubic yard are usually done with bags. Above that, ready-mix delivered by truck is often cheaper and far less work. Many suppliers charge a short-load fee for small orders, so compare:
- Bag total = number of bags × price per bag, plus mixer rental.
- Ready-mix total = cubic yards × price per yard, plus delivery and short-load fees.
The concrete calculator shows both the bag count and the cubic yards, and lets you enter your local prices.
Tips for a bagged pour
- Have all the bags on site before you start. Running out mid-pour leaves a weak cold joint.
- Mix to the water amount on the bag. Too much water weakens the concrete.
- Rent a small drum mixer for anything over about 10 bags.
- Keep the slab damp for several days while it cures, especially in hot or windy weather.