Cement Calculator
Work out how much cement a mix needs from the volume and ratio: 94-lb bags (and 42.5-kg bags), plus the sand and aggregate that go with them.
From wet concrete to dry ingredients
Bags, sand and stone are bought dry, but the pour is measured wet — and dry ingredients occupy more room than the concrete they make, because loose cement, sand and aggregate carry air between their grains. Estimators bridge the gap with the standard dry-volume factor of 1.54: multiply the finished volume by 1.54 to get the total dry ingredients, then split that total by the mix ratio. A 94-lb bag of cement is 1 cu ft of cement, which is why the bag count falls straight out of the cement share.
Dry ingredients = wet volume × 1.54 · Cement = dry total × cement parts ÷ total parts · 1 bag (94 lb) = 1 cu ft
Example: a 10 × 10 ft slab, 4 in thick, is 33.3 cu ft of wet concrete. At a 1 : 2 : 3 mix the dry ingredients total 51.3 cu ft; cement is one sixth of that — 8.6 cu ft, or 9 × 94-lb bags before waste, 10 bags with the 10% allowance in — with about 0.64 cu yd of sand and 0.95 cu yd of aggregate to go with them.
Choosing a ratio
| Ratio (cement:sand:stone) | Character | Typical work |
|---|
| 1 : 1.5 : 3 | Rich, high strength | Beams, structural repairs, precast |
| 1 : 2 : 3 | General purpose | Slabs, driveways, steps |
| 1 : 2 : 4 | Moderate | Footings, foundations, paths |
| 1 : 3 : 6 | Lean | Blinding layers, mass fill, non-structural |
Sand and aggregate come out in cubic yards and tons for the delivery truck; for small pours, the same volumes in 50-lb or 60-lb bags can be counted with the sand and gravel calculators. Metric bags of cement are 42.5 kg in most of the world (some regions use 50 kg — the count here uses 42.5 kg and says so on the result).
Frequently asked questions
Why multiply by 1.54?
Dry sand, cement and stone are full of voids; once mixed and compacted into wet concrete they settle into about 65% of their loose volume. The 1.54 factor (a working range of 1.50–1.57 is quoted in estimating texts) restores the true dry quantity so you do not run short mid-pour.
Is a 94-lb bag really 1 cubic foot?
Yes — cement is packed to a standard bulk density of 94 lb per cubic foot, so the 94-lb bag is the industry's unit cube. A 42.5-kg bag is 0.96 of one, which is why the metric count runs slightly higher.
Does the ratio include water?
No. Ratios are by volume of the three dry ingredients. Water is added separately — about 25 to 30 liters per bag of cement for a workable general-purpose mix, less for higher strength. The concrete mix calculator works the water out grade by grade.
Can I strengthen a mix by just adding more cement bags?
Within limits — a richer ratio is stronger, but past the grade's design the extra cement mostly adds cost, heat and shrinkage cracking. Pick the ratio for the job rather than improvising one stronger.
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