Cement Calculator
Give it an area, a thickness and a mix ratio, and it returns the cement you need in bags and volume — for concrete, mortar, plaster or floor screed. Standard nominal mixes like 1:2:4 and 1:1.5:3 are built in, and the dry-volume factor is applied automatically so nothing is under-ordered.
Cement required
| Wet volume | |
| Dry volume (bulked) | |
| Sand | |
| Coarse aggregate |
Rounding: bag counts round up to whole bags; volumes shown to two decimals (m³ to three) for display only.
Ratio estimating, the site-office way
Bagged-concrete arithmetic asks "how many bags fill this hole?" — ratio estimating asks a subtler question: of the dry materials I blend myself, how much is cement? The method is universal on sites that batch their own mixes. Wet volume comes from the geometry, a bulking factor inflates it back to loose dry ingredients, and the mix ratio then splits that dry heap into cement, sand and stone by proportion. One calculation hands you the entire shopping list.
The step people skip is the bulking factor, and it is not optional. Mixing water collapses the air trapped between dry particles, shrinking roughly one and a half volumes of ingredients into one volume of placed concrete. That 1.54 (or 1.33 for the finer mortar family) is the difference between a slab that finishes and a mixer that runs empty at two-thirds poured — which is exactly where under-estimated ratio jobs always stop.
How to use the cement calculator
- Choose the application. Concrete, mortar, plaster or screed — this sets the correct dry-bulking factor (1.54 for concrete, 1.33 for the others) and offers the ratios used for that work.
- Pick the mix ratio. Ratios read cement : sand (: aggregate). Structural concrete commonly uses 1:1.5:3 or 1:2:4; plaster and mortar sit around 1:4 to 1:6 depending on exposure.
- Enter area and thickness. Area in square feet or square metres; thickness in inches or millimetres, matching how each trade actually specifies it.
- Add waste if surfaces are rough. Plastering over uneven blockwork or filling deep keyways consumes real extra material; smooth formed concrete needs little.
- Read cement in bags, both standards. The tool reports 50 kg international bags and US 94 lb bags side by side, each rounded up — buy whichever your market stocks.
- Order sand and aggregate from the same run. The proportioned sand (and stone, for concrete) volumes come out with the cement so the whole materials list arrives from one calculation.
Key formulas
- Wet volume: Vwet = area × thickness
- Dry volume: Vdry = Vwet × 1.54 (concrete) or × 1.33 (mortar / plaster / screed)
- Cement share: Vcem = Vdry × c ÷ (c + s + a)
- Bags: ⌈Vcem ÷ 0.0347 m³⌉ for 50 kg; ⌈Vcem ÷ 1 ft³⌉ for 94 lb
Worked example
Plastering 200 ft² of wall at ½ in with a 1:4 mix: wet = 200 × (0.5 ÷ 12) = 8.33 ft³; dry = × 1.33 = 11.08 ft³ (0.3138 m³). Cement = one-fifth = 2.22 ft³ = 0.0628 m³ → 2 × 50 kg bags (or 3 × 94 lb). Sand = four-fifths = 8.87 ft³ (0.251 m³). The tool reproduces each line from the same inputs.
Common mix ratios
| Use | Ratio | Note |
|---|---|---|
| Structural concrete (≈M20) | 1:1.5:3 | Slabs, beams, columns |
| General concrete (≈M15) | 1:2:4 | Paths, non-structural pads |
| Lean concrete (≈M10) | 1:3:6 | Blinding, fill |
| External plaster / mortar | 1:4 | Wet areas, exposure |
| Internal plaster / mortar | 1:6 | Dry internal walls |
| Floor screed | 1:3–1:4 | Sharp sand, semi-dry |
Nominal-mix conventions from standard estimating practice; engineered structures use designed mixes specified by strength, not ratio.
Things to keep in mind
- Cement is perishable. Bags absorb moisture from air; stock stored months on a damp floor loses strength before it ever meets water deliberately.
- Measure by volume with a gauge box. "Shovels" vary by operator and enthusiasm; a simple box batched per ratio keeps every mix the same.
- Sand moisture bulks sand. Damp sand occupies up to a quarter more volume than the same dry sand — batching wet sand by volume silently leans the mix.
- Cure what you place. Plaster and screed gain strength wet; misting for the first days prevents the surface cracking that gets blamed on the ratio.
- Designed mixes override nominal ones. Where drawings specify a strength class, the ratio comes from the engineer or the batching plant, not a table.
Frequently asked questions
Why is dry volume larger than wet volume?
Loose cement, sand and stone contain air between particles; mixing water collapses those voids, so the finished wet volume is smaller than the dry ingredients that produced it. Estimators run the logic backwards — multiply the wet volume you need by a bulking factor, 1.54 for concrete and about 1.33 for mortar and plaster — to find how much dry material to buy. Skip the factor and every order comes up a third short.
How many 50 kg cement bags are in one cubic metre of cement?
Portland cement bulks at roughly 1,440 kg per cubic metre, so a 50 kg bag occupies about 0.0347 m³ — call it 28.8 bags per cubic metre of cement. Remember that is cubic metres of cement, not of finished concrete: a cubic metre of 1:2:4 concrete needs only the cement fraction of its dry volume, which works out near 6–7 bags.
Which mix ratio should I use for plaster?
Internal walls in dry rooms are commonly plastered at 1:6; external faces and wet areas tighten to 1:4 for durability and better adhesion. Ceilings often use 1:3 or 1:4 in a thinner coat. Richer is not automatically better — over-cemented plaster shrinks and crazes as it cures, which is why the leaner internal ratio survives decades on the wall.
What thickness of plaster or screed is normal?
Plaster typically goes on at 12 mm (about half an inch) per coat on walls, 6–10 mm on ceilings, with badly out-of-plumb masonry taking a second coat rather than one thick pass. Floor screeds run far thicker — 40 mm bonded, 50–75 mm unbonded — which is why the same calculator produces such different cement counts between the two trades.
Does the water amount change how much cement I need to buy?
No — water affects strength and workability, not the purchase list, because it adds no solid volume worth ordering. What matters for buying is the dry proportioning this tool performs. On site, keep the water-cement ratio disciplined anyway: extra water is the easiest and most damaging "improvement" a mix ever receives, cutting strength sharply as it rises.
Can I use this for block-laying mortar quantities?
Yes, indirectly: estimate the mortar volume as area × joint thickness across bed and head joints, or roughly 0.35 ft³ of mortar per 10 standard blocks, then run that volume through at your chosen ratio (1:4 or 1:6). For brick walls specifically, the brick calculator on this site handles the count and the pre-mixed mortar bags in one pass.
Last updated: 24 July 2026