Concrete Mix Ratio Calculator
Enter your slab size and a cement:sand:gravel ratio to get the bags of cement, sand, gravel, and water you need — plus a printable materials estimate.
By weight. 0.45–0.55 is typical for general-purpose concrete.
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Concrete Mix Ratio Calculator
Short answer
For a 10×10 ft slab at 4 in thick with a 1:2:4 mix, you’d need about 8 bags of cement, roughly 1,470 lb of sand, 3,080 lb of gravel, and about 45 gallons of water. Change the ratio for a stronger or leaner mix, or plug in your own slab dimensions above.
Common mix ratios
| Ratio (C:S:G) | Typical use | Approx. strength |
|---|---|---|
| 1 : 3 : 6 | Mass fill, non-structural | ~2000 psi |
| 1 : 2 : 4 | Slabs, paths, general purpose | ~3000 psi |
| 1 : 2 : 3 | Footings, driveways | ~3000–3500 psi |
| 1 : 1.5 : 3 | Structural, load-bearing | ~3500–4000 psi |
How the formula works
- Find the wet (poured) volume: length × width × thickness.
- Multiply by 1.54 to get the dry, loose volume — this accounts for air gaps between dry particles that close up once mixed and compacted.
- Split the dry volume across the ratio: cement’s share is cement parts ÷ total parts, and the same for sand and gravel.
- Convert cement volume to bags (≈1 ft³ per 94 lb bag) and round up.
- Convert sand and gravel volumes to weight using typical bulk densities, and find water from the cement weight and your water-cement ratio.
Worked example
A 10 ft × 10 ft slab, 4 in thick, mixed 1:2:4, water-cement ratio 0.5:
Wet volume = 10 × 10 × (4/12) = 33.3 ft³. Dry volume = 33.3 × 1.54 = 51.3 ft³. Total parts = 7. Cement volume = 51.3 × 1/7 = 7.3 ft³ → 8 bags (752 lb). Sand = 51.3 × 2/7 = 14.7 ft³ ≈ 1,467 lb. Gravel = 51.3 × 4/7 = 29.3 ft³ ≈ 3,080 lb. Water = 752 × 0.5 = 376 lb ≈ 45 gal.
Assumptions this calculator makes
- Dry-to-wet volume factor of 1.54 (standard industry rule of thumb).
- One 94 lb (US) bag of Portland cement ≈ 1 ft³ loose volume.
- Sand density ≈ 100 lb/ft³; gravel density ≈ 105 lb/ft³ (both vary by moisture and source).
- No allowance for waste, spillage, or uneven subgrade — add 5–10% extra for real-world jobs.
Common mistakes
- Using wet (as-poured) volume for the material split instead of dry volume — this under-orders every ingredient.
- Forgetting to round cement up to a whole bag, then coming up short mid-pour.
- Using a water-cement ratio that’s too high, which weakens the finished slab and increases shrinkage cracking.
- Mixing up sand and gravel bulk densities with concrete’s finished density (~150 lb/ft³) — they’re not the same number.
FAQ
What does a concrete mix ratio like 1:2:4 mean?
It’s the proportion by volume of cement to sand to coarse aggregate (gravel). A 1:2:4 mix has 1 part cement, 2 parts sand, and 4 parts gravel, mixed with enough water to reach a workable consistency.
Why is the dry volume bigger than the wet (poured) volume?
Loose, dry cement, sand, and gravel contain air gaps between particles. When mixed with water and compacted, those gaps close up and the material shrinks. A factor of about 1.54 is the standard allowance for this bulking.
How many bags of cement are in one cubic foot?
A standard 94 lb (US) bag of Portland cement is commonly treated as roughly 1 cubic foot of loose volume, which is the approximation this calculator uses.
Should I round up the number of bags?
Yes. This calculator rounds cement bags up to the next whole bag, since partial bags aren’t sold and having a small surplus is safer than running short mid-pour.
What water-cement ratio should I use?
A water-cement ratio around 0.45 to 0.55 by weight is typical for general-purpose concrete. Lower ratios (around 0.4) give higher strength but are harder to place; higher ratios are easier to work but weaker and more prone to cracking.