Bagged concrete mix is convenient, but when you need a lot of concrete, or you want to choose the proportions yourself, you mix it from its raw materials: portland cement, sand, coarse gravel and water. This calculator converts the size of your pour and a mix ratio into the number of cement sacks, the cubic feet of sand and gravel, and the gallons of water.
How to use the concrete mix calculator
- Enter the pour size — length, width and thickness.
- Choose the mix ratio by volume, written cement : sand : gravel.
- Set the water-cement ratio and a waste allowance.
- Read the quantities of cement (cubic feet and sacks), sand, gravel and water.
Concrete mix formula
Dry volume = wet volume × 1.54
Part size = dry volume ÷ (cement + sand + gravel parts)
Cement = part size × cement parts · sand and gravel the same way
Water (gal) = cement (cu ft) × 94 lb × water-cement ratio ÷ 8.34 lb/gal
Because the dry ingredients are loose and full of air, they take up about 54% more room than the compacted wet concrete they make. A cubic foot of loose cement is taken as one 94 lb sack.
Worked example: a 10 × 10 ft slab, 4 in thick, 1:2:3 mix
- Wet volume: 10 × 10 × (4 ÷ 12) = 33.33 cu ft.
- Dry volume: 33.33 × 1.54 = 51.33 cu ft.
- Parts: 1 + 2 + 3 = 6, so one part is 8.56 cu ft.
- Cement 8.56 cu ft (9 sacks), sand 17.11 cu ft, gravel 25.67 cu ft.
- Water at 0.50: 8.56 × 94 × 0.5 ÷ 8.34 = 48.2 gallons.
Quantities for different ratios
A 10 × 10 ft slab, 4 in thick, no waste allowance, water-cement ratio 0.50.
| Ratio | Cement (cu ft) | Sand (cu ft) | Gravel (cu ft) | Sacks | Water (gal) |
|---|---|---|---|---|---|
| 1 : 2 : 3 | 8.6 | 17.1 | 25.7 | 9 | 48.2 |
| 1 : 2 : 4 | 7.3 | 14.7 | 29.3 | 8 | 41.3 |
| 1 : 1.5 : 3 | 9.3 | 14.0 | 28.0 | 10 | 52.6 |
| 1 : 3 : 6 | 5.1 | 15.4 | 30.8 | 6 | 28.9 |
| 1 : 2 : 2 | 10.3 | 20.5 | 20.5 | 11 | 57.9 |
Richer mixes (less sand and gravel per sack of cement) cost more and are generally stronger. Nominal ratios are starting points; for structural work follow the design strength and mix specified by an engineer.
Practical mixing advice
- Use a single container. A 5-gallon bucket is 0.67 cu ft, so one bucket-part approach scales easily.
- Account for moisture. Wet sand holds water and also “bulks”, so measure carefully and reduce the added water.
- Add water gradually. A stiff, workable mix is stronger than a soupy one; excess water weakens concrete.
- Cure it. Keep the slab damp for several days so it reaches its strength.
For the plain volume and bagged-mix alternative, use the Concrete Calculator. Sand and gravel can also be ordered by the ton or yard; see the Sand Calculator and Gravel Calculator. Foundations are covered by the Concrete Footing Calculator.
Frequently asked questions
What does a 1:2:3 concrete mix mean?
It is a ratio by volume: one part cement, two parts sand and three parts coarse aggregate (gravel). The parts must be measured with the same container, such as a bucket.
Why multiply the volume by 1.54?
Dry ingredients occupy more space than the finished concrete, because the water and the compaction fill the voids between particles. A dry-volume factor of about 1.54 is widely used (values from 1.50 to 1.57 appear in references).
How much does a sack of cement hold?
A US sack of portland cement weighs 94 lb and is treated as 1 cubic foot of loose cement, which is why cement volume in cubic feet equals the number of sacks.
How much water should I use?
The calculator multiplies the cement weight by the water-cement ratio and converts to gallons at 8.34 lb per gallon. A ratio around 0.45 to 0.55 is common; the sand’s own moisture reduces the water you add.
Is a site mix as good as ready-mix?
Hand-mixed concrete is fine for small non-structural jobs, but batching by volume is less accurate than a plant. For structural work, order ready-mix or use bagged product rated for the purpose.