What is the water-cement ratio?
The water-cement ratio (w/c) is the mass of effective water divided by the mass of cement in a concrete mix. It is the single most important number for concrete quality: the lower the w/c ratio, the stronger, denser and more durable the concrete.
How to calculate it
w/c = mass of water (kg) / mass of cement (kg)
Example: a mix with 50 litres of water and 100 kg of cement has a w/c ratio of 50 / 100 = 0.50. A litre of water weighs about one kilogram, so litres and kilograms can be treated as equal here.
Why does it matter so much?
Cement only needs part of the water for its chemical reaction (hydration). A w/c ratio of around 0.4 is enough for full hydration. Any water above that is there only for workability. That excess water later evaporates and leaves a network of fine capillary pores. The result of a high w/c ratio:
- lower compressive strength;
- more shrinkage and cracking;
- higher permeability, so water, chlorides and CO2 penetrate faster;
- earlier reinforcement corrosion and frost damage.
Use and standards
EN 206 and its national annexes, such as BS 8500 in the UK, set a maximum water-cement ratio for each exposure class. The more aggressive the environment, for example with de-icing salts or seawater, the lower the permitted ratio. When ordering ready-mix concrete you specify strength and exposure class, and the supplier designs the w/c ratio to suit.
Practical tips
- Never add water on site to delivered concrete. A few buckets of extra water noticeably raise the w/c ratio.
- Use a plasticiser if you need a more workable mix without extra water.
- Mixing yourself: measure water and cement for each batch and allow for moisture in the sand.
- Cure properly: keep young concrete damp or covered so the cement keeps enough water to hydrate.
Related terms
- Mixing water
- Water demand of fresh concrete
- Exposure class
- Concrete cover
- Compressive strength
- Plasticiser
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