Cement Calculator

Work out cement bags, sand and aggregate for a given volume and mix ratio.

Please note: Material estimates for planning only. Always confirm quantities against your drawings, local building codes and supplier specifications before ordering. Structural work requires a qualified engineer.

What the Cement Calculator does

Cement quantity follows from the concrete volume and mix ratio. The dry ingredients bulk up by roughly 54% relative to the finished concrete, then split by the ratio parts. Water is set by the water/cement ratio, which is the single biggest control on final strength.

Formula

Dry volume = Wet volume × (1 + Waste) × 1.54
Cement volume = Dry volume × Cement parts ÷ Total parts
Cement weight = Cement volume × 1,440 kg/m³
Water = Cement weight × Water/cement ratio

Inputs explained

Input	Unit	Required	Notes
Volume of concrete required	m³	Yes	Accepts more than 0.
Mix ratio	one of 4 options	Yes	—
Cement bag size	kg	Yes	Accepts 1 or more.
Waste allowance	%	Yes	—
Water/cement ratio	number	Yes	Accepts 0.1 or more, up to 1.
Price per bag	selected currency	Optional	Accepts 0 or more.
Currency	one of 10 options	Optional	—

How to use it

Choose Mix ratio and Currency.
Enter Volume of concrete required, Cement bag size, Waste allowance and Water/cement ratio.
Optionally add Price per bag.
Select Calculate.

Worked example

3 m³ of M20 concrete (1:1.5:3), 5% waste, 50 kg bags, 0.5 water/cement ratio.

Volume: 3
Mix: 1:1.5:3
Bag: 50
Waste: 5

Order 3.15 m³, dry 4.851 m³ → cement 0.882 m³ = 1,270 kg = 26 bags, sand 1.323 m³, aggregate 2.646 m³, water 635 litres.

Frequently asked questions

What does a 1:1.5:3 mix mean?

One part cement to 1.5 parts sand to 3 parts coarse aggregate, measured by volume. It is the common M20 mix used for slabs and columns.

Why does more water weaken concrete?

Excess water leaves voids as it evaporates. Every increment above what the cement needs to hydrate reduces the final strength measurably.