How to use the cement calculator
- Enter the slab size or, if you already know it, the cubic yards of concrete.
- Pick a mix ratio. 1:2:3 suits most small slabs and footings.
- Read the shopping list: bags of Portland cement, sand and gravel in cubic yards and tons, and water.
The cement, sand and gravel formula
Wet volume = length × width × thickness (ft), plus waste
Dry volume = wet volume × 1.54
Each material = dry volume × its parts ÷ total parts
Cement bags = cement cu ft ÷ 1 (94 lb bag = 1 cu ft)
Water = cement weight × water-to-cement ratio ÷ 8.34 lb per gallon
Worked example
One cubic yard of 1:2:3 concrete
- 27 cu ft × 1.54 = 41.6 cu ft of dry materials, in 6 parts.
- Cement 6.93 cu ft = 7 bags of 94 lb.
- Sand 13.9 cu ft = 0.51 cu yd; gravel 20.8 cu ft = 0.77 cu yd.
- Water at 0.5: 651 lb of cement × 0.5 = 326 lb ≈ 39 gallons.
Common mix ratios
| Mix | Typical use | Cement bags per cu yd |
|---|---|---|
| 1 : 1.5 : 3 | Stronger work: driveways, posts | about 7.6 |
| 1 : 2 : 3 | General: slabs, paths, footings | about 7 |
| 1 : 2 : 4 | Footings, mass concrete | about 6 |
| 1 : 3 : 6 | Lean fill, blinding layers | about 4 |
Common mistakes to avoid
- Forgetting the dry factor. Without it you run short by about a third.
- Too much water. Soupy concrete is easy to place and much weaker.
- Using masonry sand. Concrete needs coarse concrete sand.
- Mixing a large slab by hand. Batches poured too far apart leave weak joints.
Frequently asked questions
How many bags of cement for a cubic yard of concrete?
For a 1:2:3 mix, about 7 bags of 94 lb Portland cement per cubic yard (27 cu ft × 1.54 ÷ 6 = 6.9 cu ft of cement), plus about 0.51 cu yd of sand and 0.77 cu yd of gravel.
What does a 1:2:3 concrete mix mean?
One part cement, two parts sand and three parts gravel, measured by volume, for example with the same bucket. It is a common general-purpose mix for slabs and footings.
Why multiply by 1.54?
When water is added, sand fills the gaps between stones and cement fills the gaps between sand grains, so the mixed concrete takes up roughly 35% less space than the dry materials. 1.54 is the usual allowance.
How much water do I add?
About half the weight of the cement for a 0.5 water-to-cement ratio: for 7 bags (658 lb) that is about 330 lb, or 39 gallons. Add it gradually; sand is often already damp. Too much water weakens concrete.
Is cement the same as concrete?
No. Cement is the powder that binds concrete. Concrete is cement plus sand, gravel and water. If you want everything in one bag, use premixed concrete and our concrete calculator.