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Water to Cement Ratio Calculator

Calculate water/cement ratio from mix masses. Indicative strength class and BS EN 206 maximum w/c by exposure class. Free online concrete mix tool.

Last updated 25 August 2026

About this tool

Calculate the water/cement ratio from the water and cement masses in a concrete mix. The w/c ratio is the single most important factor controlling concrete strength and durability.

Results include an indicative strength class (from Abrams' law relationship) and a reference table of maximum w/c ratios by BS EN 206 exposure class. Use this to check mix compliance or estimate strength from a trial mix.

Pairs with the Concrete Mix Calculator for full mix design. Per BS EN 206 and BS 8500-1.

How to use this tool

1. Enter the free water content — total water minus aggregate absorption, in kg per m³ of concrete.

2. Enter the cement mass — total cementitious content (cement + additions) in kg per m³.

3. Read the w/c ratio and strength indication — check against the exposure class table for durability compliance.

Technical information

w/c = water mass / cement mass

Strength–w/c relationship follows Abrams' law (1918): lower w/c gives higher strength. Indicative classes assume CEM I with standard curing.

BS EN 206 specifies maximum w/c ratios by exposure class to ensure durability: XC1 ≤ 0.65, XC3/XC4 ≤ 0.55, XD3/XS3 ≤ 0.45.

Limitations

Strength class indications are approximate. Actual strength depends on cement type, aggregate quality, admixtures, and curing conditions. Always verify with cube/cylinder tests.

The w/c ratio should use free water content (total water minus aggregate absorption). Using total water will overestimate the effective w/c ratio.

Revision history

25 August 2026: Initial release

Disclaimer

This tool is provided for educational and general information purposes only. It is not a substitute for professional engineering advice, design or verification.

Diggy and its contributors are not licensed engineering consultants and no results generated by this tool should be used directly for construction, design or safety-critical decisions.

All values and outputs are based on published empirical correlations and should be independently checked and confirmed by a qualified geotechnical engineer before use.

By using this tool, you accept full responsibility for how you interpret and apply the information provided.

Spotted an error? Let us know.

Water mass (kg)

Free water content — total water minus aggregate absorption.

Cement mass (kg)

Mass of cementitious material (cement + additions if using equivalent cement content).