Concrete Mix Calculator
The whole recipe for site-mixed concrete by strength grade: cement bags, sand and aggregate in yards and tons, and the water to add.
The whole recipe, by grade
Site-mixed concrete is ordered by strength, and each strength has a nominal recipe the trade has used for generations: fixed proportions of cement, sand and coarse aggregate, and a maximum water-cement ratio that makes the strength real. This calculator takes the grade and the pour size and returns the complete batch list — cement bags, sand and aggregate in yards and tons, and the water to add in liters and gallons.
Dry ingredients = wet volume × 1.54, split by the grade's ratio · Water = cement weight × the grade's water-cement ratio
Example: a 9 × 9 ft slab, 4 in thick, is exactly 1 cu yd of wet concrete. At the 3,000 psi grade (1 : 1.5 : 3), the batch is 8 bags of cement, 0.42 cu yd of sand (0.57 tons), 0.84 cu yd of aggregate (1.18 tons) and about 154 liters of water (40.7 gal) — before any waste allowance.
The grades
| Grade | Ratio | Water-cement ratio | Suited to |
|---|
| ≈1,500 psi (M10) | 1 : 3 : 6 | 0.60 | Blinding, leveling fill, non-structural |
| ≈2,200 psi (M15) | 1 : 2 : 4 | 0.55 | Footings, paths, mass concrete |
| ≈3,000 psi (M20) | 1 : 1.5 : 3 | 0.50 | Slabs, driveways, general structural work |
| ≈3,600 psi (M25) | 1 : 1 : 2 | 0.45 | Beams, columns, high-wear floors |
The water figure is a maximum, not a target to exceed: every extra liter beyond the ratio buys workability with strength. Wet aggregate and sand bring their own moisture, so add about three quarters of the water first, mix, and judge the slump before adding the rest. These nominal mixes suit ordinary work; anything structural, engineered or inspected belongs to a designed mix from a ready-mix plant.
Frequently asked questions
What do the psi figures mean?
Pounds per square inch of compressive strength at 28 days — the standard way concrete strength is specified. A driveway wants about 3,000–4,000 psi; a blinding layer under a footing barely needs 1,500.
Why is the water capped?
Strength is governed by the water-cement ratio more than by anything else. Too much water leaves pores when it evaporates, and the concrete cures weaker and dustier. Adding water at the truck or the mixer to make placing easier is the commonest strength mistake on small jobs.
How do these site mixes compare with ready-mix?
A ready-mix plant batches by weight with tested aggregates and delivers a certified grade. A careful site mix to these ratios is honest concrete for domestic work, but it varies batch to batch — for anything an inspector signs off, order the truck.
How much does one batch of a 1-bag mix make?
A batch built on one 94-lb bag at 1 : 1.5 : 3 yields roughly 5.5 cu ft of wet concrete — about two big wheelbarrow loads. The calculator's figures scale that batching up to your whole pour.
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