Cement Calculator
Bags of cement, and the sand and aggregate, for a volume of concrete, mortar or plaster at the mix ratio you use.
Result…
How it's calculated
Dry volume = wet volume × (1 + waste %) × dry volume factor Cement volume = dry volume × cement part ÷ sum of parts Cement weight = cement volume × bulk density Bags = cement weight ÷ bag size, rounded up Sand (aggregate) = dry volume × its part ÷ sum of parts
Works out how much cement, sand and aggregate to buy for site-mixed concrete, or cement and sand for mortar and plaster, by the volume-batching method used in quantity surveying: the finished wet volume is scaled up by a dry volume factor (1.54 for concrete, 1.33 for mortar is the usual convention) and shared out by the mix ratio. The cement volume is turned into weight with its bulk density (1440 kg/m³ is the figure usually used) and then into bags of the size you buy. For the volume of a slab or column, start with the concrete calculator.
Your numbers, step by step
About the Cement calculator
This cement calculator tells you how many bags of cement to buy, and how much sand and aggregate, for site-mixed concrete, mortar or plaster at the mix ratio you use. Enter the finished volume, or the length, width and thickness, and a ratio such as 1:2:4 for concrete or 1:6 for mortar. It follows the volume-batching method used in quantity surveying: the wet volume is multiplied by a dry volume factor (1.54 for concrete and 1.33 for mortar are the usual conventions), shared out by the ratio, and the cement is turned into weight with its bulk density, usually taken as 1440 kg/m³. Every factor is a field you can change, and each step shows your own numbers. For the volume of a slab or column first, use the concrete calculator; for wall plaster by area, the plaster calculator.
How to use it
- Choose “Concrete” or “Mortar or plaster”, and metric or imperial units.
- Under “I know the”, pick “Volume” and enter the “Wet volume”, or pick “Dimensions” and enter the length, width and thickness.
- Type the “Mix ratio”, for example 1:2:4 (cement : sand : aggregate) or 1:6 (cement : sand).
- Check the “Dry volume factor”, “Cement bulk density” and “Bag size” against your supplier's figures.
- Add an “Extra for waste” and read the bags, sand and aggregate.
Worked example
A slab 4 m × 3 m × 100 mm is 1.2 m³ of concrete; with 5% extra it is 1.26 m³. At a dry volume factor of 1.54 the dry materials are 1.9404 m³. A 1:2:4 mix has 7 parts, so cement is 1.9404 × 1 ÷ 7 = 0.2772 m³, which at 1440 kg/m³ weighs about 399.2 kg: 7.98 bags of 50 kg, so buy 8 bags. Sand is 0.5544 m³ and aggregate 1.1088 m³.
Updated
Questions people ask
How many bags of cement are in 1 cubic metre of 1:2:4 concrete?
With the usual 1.54 dry volume factor and 1440 kg/m³, cement is 1.54 ÷ 7 = 0.22 m³, or 316.8 kg: 6.34 bags of 50 kg, so 7 bags with no allowance for waste.
Why multiply by 1.54 to get the dry volume?
Loose cement, sand and stone contain air. When water is added the voids fill and the mix shrinks, so you need roughly 1.54 times the finished volume of dry material for concrete and about 1.33 times for mortar. These are rules of thumb, and some engineers use slightly different factors, which is why the field can be changed.
What mix ratio should I use for mortar or plaster?
Ratios such as 1:4, 1:5 and 1:6 (cement : sand) are common, with richer mixes for stronger work. The right ratio depends on the job and your local building practice, so follow your drawings, your engineer or the product's instructions.
Is a mix ratio by volume or by weight?
This calculator treats the ratio as parts by volume, which is how site mixes like 1:2:4 are usually written. If your specification gives a ratio by weight, the bag count will differ, so ask your engineer or supplier for the volume equivalent.
How accurate is the cement estimate?
It is a planning estimate. Damp sand bulks up, the real bulk density of cement varies, and spillage depends on the site. For structural concrete, use the mix design and quantities from a qualified engineer.