Concrete Mix Ratio Calculator
Turn a slab size into bags of cement, sand, gravel and water. Pick a proven mix ratio, adjust for waste and wet sand, and read the recipe in buckets and mixer batches
Concrete Job Details
Bags, buckets & mixer
Estimate based on nominal ratios by volume. Structural elements (columns, beams, elevated slabs, building foundations) must follow a mix design from an engineer.
How to use this tool
- Choose Metric or US units, then enter the slab as length × width × thickness, or switch to a known volume.
- Pick a mix ratio (cement : sand : gravel) that matches the job, or enter your own parts.
- Set the waste allowance, the water/cement ratio and the moisture of your sand.
- Choose your cement bag size and measuring bucket, then read bags, sand, gravel, water and the recipe per mixer batch.
Worked example
A 12 × 10 ft slab, 4 in thick, is 40 ft³ or about 1.48 yd³; with 5% waste the tool plans for 1.56 yd³. With a 1:2:4 mix it needs about 831 lb of cement (9 bags of 94 lb), 18.5 ft³ of sand (roughly 28 buckets of 5 gal) and 37.0 ft³ of gravel, plus 49.8 gal of water at a 0.50 water/cement ratio. The tool multiplies the wet volume by 1.54 to get the dry volume, because loose materials pack together once wet. Nominal ratios suit small non-structural jobs; structural work is designed to a standard such as ACI 211.1, EN 206 or ABNT NBR 12655.
Why this tool exists and where its numbers come from
Loose cement, sand and gravel take more volume than the finished concrete, so a slab size has to be turned into quantities with an allowance. The tool uses nominal volume ratios; for structural work, standards such as ACI 211.1 call for a designed mix instead. The 1.54 dry-volume factor is a common rule of thumb, not a value taken from a standard.
Sources
- ACI 211.1: Standard Practice for Selecting Proportions for Normal-Weight Concrete (abstract)
- Brick & Bolt: How to calculate cement, sand and aggregates (dry volume = wet volume × 1.54, a site-estimation rule of thumb, not a design standard)
- InfraLens handbook: Material quantities per unit of work (dry volume factor 1.52–1.54)
For the full method, see the Methodology section just below.
Methodology
Nominal concrete mixes are written as parts by volume of cement : sand : gravel. Because loose materials pack together once water is added, the calculator first works out the dry volume you need, then splits it by the ratio.
One cubic meter of concrete, four nominal mixes
Quantities for 1 m³ of finished concrete with 0% waste, a water/cement ratio of 0.50, dry sand and 50 kg cement bags, as the tool calculates them (dry-volume factor 1.54).
| Mix | Cement (50 kg bags) | Sand / gravel (m³) | Water |
|---|---|---|---|
| M10 · 1:3:6 | 222 kg (5 bags) | 0.46 / 0.92 | 111 L |
| M15 · 1:2:4 | 317 kg (7 bags) | 0.44 / 0.88 | 158 L |
| M20 · 1:1.5:3 | 403 kg (9 bags) | 0.42 / 0.84 | 202 L |
| M25 · 1:1:2 | 554 kg (12 bags) | 0.39 / 0.77 | 277 L |
How to read it: richer mixes need far more cement but slightly less aggregate. Going from M15 to M25 raises cement from 317 kg to 554 kg, about 75% more, while sand and gravel fall by only about 12%. Water follows the cement, so it is the first thing to correct for damp sand: with 3% moisture, M15 needs 158 − 21 = 137 L instead of 158 L.
Limits: these are nominal volume ratios for small non-structural jobs, with the tool's typical densities. Structural elements need a mix design from an engineer. Bags are rounded up, and real sites need extra for waste.
Concrete Job Essentials
Gear that makes hauling, mixing and protecting yourself on a pour easier
VEVOR Poly Garden Dump Cart (4 Cu. Ft., 600 lb)
Poly-bed dump cart rated for 600 lb with 10-inch all-terrain wheels. Carries sand, gravel and fresh mix across uneven ground.
Rubbermaid Commercial 7-Gallon Bus/Utility Box
Sturdy 7-gallon plastic tub with reinforced handles and bottom. Handy for measuring and mixing small batches.
DEWALT DPG82 Anti-Fog Dual Mold Safety Goggles
Anti-fog dual-mold goggles. Fresh cement is caustic, so splash and dust protection matters on every pour.
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Frequently asked questions
Why does the calculator multiply my volume by 1.54? ▼
Loose cement, sand and gravel take up more space than the finished concrete: once water is added, the grains pack together and the volume shrinks. A widely quoted rule of thumb puts the dry-to-wet factor at roughly 1.52–1.54 (it is not taken from a standard and varies with the materials), so a cubic meter of finished concrete needs about 1.54 m³ of dry materials. Skipping this step is the most common reason people run short.
Can I use a 1:2:4 mix for a structural slab or foundation? ▼
Only for small, non-critical work. A nominal ratio does not account for your actual cement, sand and gravel. Brazil's NBR 12655, for example, calls for rational, experimental mix design for class C20 and above. For columns, beams, elevated slabs and building foundations, use the mix specified by the structural engineer or order ready-mix.
My sand is damp. How much water should I actually add? ▼
Set the sand moisture in the form. The free water in the sand is subtracted from the water total. For a 1:2:4 mix at a 0.50 water/cement ratio, sand at 6% moisture already supplies roughly a quarter of the water (about 42 of 158 liters per m³).
Are the ratios by volume or by weight? ▼
By volume. 1:2:4 means one bucket of cement, two of sand and four of gravel, all measured with the same bucket. Weights differ because cement is far denser per liter than sand or gravel, and damp sand swells and takes more space, so weigh your materials whenever you can.
How many bags of cement do I need per cubic yard of concrete? ▼
For a 1:2:4 mix, about 317 kg of cement per m³ (≈ 534 lb per yd³), which is roughly 5.7 bags of 94 lb. A 1:1.5:3 mix needs about 403 kg/m³ (≈ 680 lb per yd³, roughly 7.2 bags of 94 lb). Choose your bag size in the form.