Ordering and mixing
Water–cement ratio and mixing water
- where
- total cement (kg)
- cement content (kg/m³)
- volume of concrete (m³)
- total free water (litres)
- free water–cement ratio
- water per cement bag (litres)
- cement bag size (kg)
Explain this equation
Works out how much mixing water goes with the cement for a given water–cement ratio.
Cement content times volume gives the cement. The water–cement ratio is the mass of free water per mass of cement; one kilogram of water is one litre, so the ratio times the cement mass gives litres of water. The same ratio times a bag gives water per bag.
A simple ratio by mass.
Sense check: 300 kg of cement at 0.55 needs about 165 litres of water, about 13.75 litres per 25 kg bag.
Watch out: "free" water excludes what the aggregates absorb; wet sand already carries water, so add less. Extra water makes concrete easier to place but weaker and less durable.
Works out the free mixing water for a given cement content and water–cement ratio, in total and per bag of cement.
Enter your values to see the result.
Good to know
The water–cement ratio is the mass of free water divided by the mass of cement; 1 kg of water is 1 litre. Lower ratios give stronger, more durable and less permeable concrete. BS 8500-1 and BS EN 206 set a maximum ratio and minimum cement content for each exposure class and design life; specify to those, not by eye. Wet sand and aggregate already carry free water, so the water you add from the hose should be less than this figure.
Common mistake
Adding water at the mixer or on site to make concrete easier to place. Every extra litre raises the ratio and lowers the strength; use a plasticiser if more workability is needed.
Limits
Free water only: water absorbed by dry aggregates and moisture already in damp aggregates are not included. Ratios outside 0.3–0.8 are rejected as outside normal practice.
Sources
- BS 8500-1:2023 Concrete. Complementary British Standard to BS EN 206. Method of specifying and guidance for the specifier
- BS EN 206:2013+A2:2021 Concrete. Specification, performance, production and conformity
- Hanson (Heidelberg Materials) ready-mixed concrete quotation examples: C35 at 280 kg/m³ cement, w/c 0.60; C40 at 300 kg/m³, w/c 0.55
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.