# Standard Water to Cementitious Material Ratios

If you’re mixing your own concrete, you’ll need to remember that the relationship between water and cement (w/cm), is extremely important. Concrete forms when cement and aggregate meet water, initiating a chemical process that results in concrete. Adding about 23% of the cement volume in water can initiate this process. But before deciding how much water to add, you should understand how the water-cement ratio is associated with the strength of concrete.

## Why the Water-Cement Ratio In Concrete Materials Ratio Matters

The water-cement ratio is the weight of the water in your concrete relative to the weight of cement. Normally, according to IS code 10262 (2009), your nominal mix should have a ratio between 0.4 and 0.6. However, depending on the type of concrete, the compressive strength you need and your environment, you may want a higher or lower ratio.

If you add more water, you may have an easier time working with your cement. The problem is that if you add too much water, once the aggregates settle, the water will evaporate, which leaves voids in the concrete. The more and bigger voids you have, the weaker your concrete is. You need to add the right amount of water for cement workability without leaving you in a situation where your concrete is too weak when it hardens.

## How to Calculate the Concrete Water-Cement Ratio

So what is the calculation for the water to cement ratio? How do you figure it out? Fortunately, you don’t have to. Either follow the specific instructions provided by the cement producer, choose a mix somewhere between the 0.4 and 0.6 standard or consult the following water-cement ratio formula table to find the ratio you need:

Compressive Strength at 28 days (PSI) W/CM for Non-Air Entrained Concrete W/CM for Air-Entrained Concrete
2000
0.82
0.74
3000
0.68
0.59
4000
0.57
0.48
5000
0.48
0.40
6000
0.41
0.32
7000
0.33
XXX

This chart should serve you well in most situations, but when contractors are concerned about the specific environmental conditions their concrete may be subjected to, there are other factors to consider.

If your concrete is likely to be frequently exposed to water and you want it to have low permeability, your water to cement ratio should be no greater than .50. If you expect your concrete to be exposed to freezing and thawing, especially as a result of deicers, keep your ratio no higher than .45. If you want your concrete to be more resistant to corrosion that may occur as a result of deicing chlorides, seawater, saltwater or other corrosive fluids, keep your ratio at the bottom of the nominal standard at .40.