Concrete Mix Ratio Calculator
Enter the wet volume you need and the mix ratio; get cement bags, sand and gravel volumes, and the water for a 0.5 water-cement ratio — with the shrinkage factor shown.
Last updated: 2026-09-22
How the calculation works
- Wet volume is multiplied by 1.54 because cement and aggregates compact into voids when water is added — you must batch more dry material than the final volume.
- The dry volume is split among ingredients in proportion to the selected ratio.
- Cement converts to 94 lb bags at 1 ft³ bulk per bag (the standard US unit).
- Water follows the water-cement ratio: 0.5 by weight is the common structural default — every extra gallon above it trades strength for workability.
Formula
Dry vol = wet vol × 1.54 (voids between sand & gravel fill as water is added) cement = dry × c/(c+s+g) sand = dry × s/(c+s+g) gravel = dry × g/(c+s+g) cement bags = cement_ft³ / 1.0 (94 lb bag = 1 ft³ bulk) water = cement_lbw × 0.5 (w/c ratio, by weight)
| Symbol | Meaning | Unit |
|---|---|---|
c:s:g | Mix parts by volume | — |
1.54 | Dry-volume / shrinkage factor | — |
w/c | Water-cement ratio by weight | — |
Worked example
Interpreting the result
The ratio controls strength more than anything except water. Dropping from 1:2:3 to 1:1.5:3 adds ~20% more cement for the same aggregate — roughly 25% stronger concrete. But the w/c ratio dominates: 0.45 vs 0.55 water swings strength by 15–20% in the opposite direction. If the mix is stiff, use a plasticizer or aggregate adjustment, not more water. Hand batches never hit the precision of a batch plant — treat the results as site-mix staging quantities.
Assumptions
- Volume batching with damp sand (bulked ~20%); oven-dry sand batches slightly richer.
- 94 lb bags of portland cement at 1 ft³ bulk each.
- w/c = 0.5 — suitable for most structural work exposed to weather.
Limitations
- Nominal mixes, not a true ACI 211 absolute-volume design for specified 28-day strength.
- Aggregate moisture, grading and absorption shift real batches a few percent.
- Air-entrained mixes (freeze-thaw climates) replace ~5% of volume with entrained air.
Water is the whole game
Ask any concrete technologist for the one thing that ruins more concrete than anything else and the answer is always water. Compressive strength tracks the water-cement ratio almost linearly: at w/c 0.40 expect 5000+ psi, at 0.50 about 3500–4000, at 0.60 closer to 2500, and at 0.70 the mix barely holds together. The temptation on site is real — a wet mix flows into forms, levels itself, and finishes easily. It also arrives at 28 days far weaker than the ratio promised. When workability fights strength, the compromise is a mid-range plasticizer, not the hose.
Frequently asked questions
What does 1:2:3 concrete mean?
1 part cement, 2 parts sand, 3 parts gravel by volume — the classic general-purpose site mix, roughly 3000 psi when mixed correctly. It's the starting point for slabs, walks and non-critical footings.
Why multiply by 1.54?
Dry cement and aggregate contain voids between particles. When water is added, materials settle into those voids and the wet volume is about 65% of the dry batch volume. 1/0.65 ≈ 1.54 is the correction so the finished volume matches what you ordered.
How much water should I add?
Enough for workability without weeping gray bleed water — the 0.5 w/c here is about 6 gallons per 94 lb bag equivalent. A soupy mix pours easy but loses 20-40% of its strength; a plasticizer adds flow without the strength loss.