Water Cement Ratio Calculator

Work out water cement ratio instantly with clear inputs, formula shown and shareable results.

About Water Cement Ratio Calculator

Water-cement ratio is the mass of free water divided by the mass of cement, and it governs strength and durability more than any other mix variable. Bolomey's relation, fc = K x (C/W - 0.5) with K around 22 for ordinary Portland cement, shows why: strength climbs steeply as the ratio falls.

Formulas

Water-cement ratio

w/c = water kg / cement kg (1 litre of water = 1 kg)

Bolomey strength

fc = 22 x (cement/water - 0.5) MPa

Worked example

With the default values below, Water Cement Ratio Calculator returns Actual water-cement ratio of 0.5000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Cement contentminimum 1 kg: 350 kg
Water addedminimum 1 L: 175 L
Target water-cement ratioaccepts 0.2–1: 0.45

Results

Actual water-cement ratio: 0.5000
Water for the target ratio: 157.5 L
Water to add or remove: -17.5 L
Indicative 28 day strength: 33.0 MPa

Step by step

1Values used
2Water-cement ratio
3Bolomey strength
4Actual water-cement ratio= 0.5000
5Water for the target ratio= 157.5 L
6Water to add or remove= -17.5 L
7Indicative 28 day strength= 33.0 MPa

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Mix design and strength verification must follow the applicable concrete standard with trial mixes and cube testing.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Mix design and strength verification must follow the applicable concrete standard with trial mixes and cube testing.

Frequently Asked Questions

How is the Water Cement Ratio figure worked out?

w/c = water kg / cement kg (1 litre of water = 1 kg). Water-cement ratio is the mass of free water divided by the mass of cement, and it governs strength and durability more than any other mix variable.

Does aggregate moisture count?

Yes. Free water on wet sand is part of the mix water. Ignoring it is the most common reason site concrete has a higher ratio than the design assumed.

What values do I need to enter?

This calculator takes 3 inputs: Cement content, Water added, Target water-cement ratio. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.