Concrete calculator
Size in, volume and bags out.
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Example
Example: a 12 × 12 ft patio slab, 4 in thick, in premix bags or a hand mix
A 12 × 12 ft patio slab poured 4 inches thick, with the default 10 % extra. Choose Slab, type 12, 12 and 4 in US units, and tick “Mix it myself” to see a hand mix as well.
- Volume: 12 ft × 12 ft × 4 in = 48 ft³ = 1.78 yd³; with 10 % extra, 52.8 ft³ = 1.96 yd³.
- Dry premix: 52.8 ft³ × 137 lb/ft³ = 7,252 lb, divided by the bag weight and rounded up.
- Hand mix 1:2:3 by volume: cement, twice as much sand, three times as much gravel, and water about half the cement’s weight.
| To buy | Quantity |
|---|---|
| Concrete with 10 % extra | 1.96 yd³ |
| 80 lb premix bags | 91 (90.6 rounded up) |
| or 60 lb bags | 121 |
| or 40 lb bags | 182 |
| Cement for a hand mix | ≈ 1,218 lb → 13 × 94 lb |
| Sand | 27.1 ft³ ≈ 2,707 lb |
| Gravel | 40.66 ft³ ≈ 3,807 lb |
| Water | ≈ 73 gal |
Nearly 2 cubic yards and over 3½ tons of premix: at this size a ready-mix truck is usually cheaper than 91 bags, and saves mixing them. The yield printed on the bag you buy wins over the 137 lb/ft³ used here.
How it works
- Choose a slab, a strip footing or a round column, and type its size.
- Type how many if there are several, and the extra to allow (10 % by default).
- You get the volume and the bags of premix; tick “mix it myself” for cement, sand, gravel and water.
Frequently asked questions
- How do I calculate concrete for a slab?
- Length × width × thickness: a 10 × 10 ft slab 4 in thick is 33.3 cu ft, or 1.23 cubic yards — 1.36 cubic yards with 10 % extra, 63 bags of 80 lb. Add 5–10 % for uneven ground and spillage.
- How many bags of concrete do I need?
- Premix is taken at about 137 lb of dry mix per cubic foot of concrete, so a cubic yard takes 46.4 bags of 80 lb — buy 47 — or 62 bags of 60 lb. An 80 lb bag makes about 0.6 cu ft. Large pours are cheaper from a ready-mix truck.
- What is a 1:2:3 mix?
- One part cement, two parts sand and three parts gravel by volume, with water about half the cement’s weight: a general-purpose mix for footings and slabs.
- Is the result exact?
- It is an estimate from standard figures. Walls are rarely perfectly square and surfaces absorb differently, so buy a little extra from the same batch, and check the figures on the product you choose.
- How much concrete do I need for fence posts?
- Choose Round column and type the hole, not the post: 8 holes 10 in wide and 3 ft deep come to 13.09 cu ft — 0.53 cubic yards with 10 % extra, or 25 bags of 80 lb. The posts take up part of each hole, so a little is left over.
- Is anything uploaded?
- No. It is worked out in your browser, and nothing you type leaves your computer.
Made for a specific use
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