Concrete Water Content
The free water content is the starting point of an IS 10262:2019 mix design — it sets the workability of the mix before the cement content is worked out. This page covers the standard’s approximate water values by aggregate size and slump, and how shape and admixtures adjust them. Open the calculator with these values already filled in.
Approximate free water content
IS 10262:2019 Table 4 gives the approximate free water content for a 25–50 mm slump, using angular coarse aggregate:
| Nominal maximum aggregate size | Water content |
|---|---|
| 10 mm | 208 kg/m³ |
| 20 mm | 186 kg/m³ |
| 40 mm | 165 kg/m³ |
Values are for angular aggregate at 25–50 mm slump; see the adjustments below for other conditions.
Adjusting for slump, shape and admixture
For slump above 50 mm, increase the water content by about 3% for every additional 25 mm of slump. Aggregate shape also matters — relative to angular aggregate, sub-angular aggregate needs about 10 kg/m³ less water, gravel with some crushed particles about 20 kg/m³ less, and rounded gravel about 25 kg/m³ less. A water-reducing admixture (plasticiser or superplasticiser) reduces water further still — use the reduction actually achieved in your trial rather than a generic figure.
Worked example
Take 20 mm angular aggregate and a 100 mm slump. The base value is 186 kg/m³. The slump is 50 mm above the 50 mm reference, i.e. two 25 mm steps, so add 6% — giving about 197 kg/m³ before any admixture. If a superplasticiser gives, say, a 20% reduction on trial, the water content comes down to about 158 kg/m³.
Questions this page answers
Why does bigger aggregate need less water?
Larger aggregate particles have less surface area per unit volume, so less water is needed to wet the surfaces and achieve the same workability.
Does higher slump always mean more water?
Yes, unless the extra workability is achieved instead with a water-reducing admixture, which can offset or reverse the increase.