Water cement ratio is the ratio of the weight of water to the weight of cement used to prepare 1 unit of concrete. It is essentially the weight ratio and not the volume.
The water/cement ratio primarily governs the strength of concrete. More water added to make concrete higher will be the w/c ratio and the lesser will be the strength.
These facts were first studied in 1918 AD by Abrams. Abrams also told that the relation between the Water/Cement ratio and strength is not linear.
Following is a method to calculate water/cement ratio
If 2 bag of cement is used to prepare some random amount of concrete and for that 60 liters of water is consumed then the calculation is as below.
The weight of cement used to prepare concrete is – 100 kg
The weight of water used to prepare concrete is – 60 kg [as 60 liters water is used]
W/c = 60/100 = 0.6
The Water cement ratio has no dimension as it is a weight to weight ratio.
Following is an image giving an idea about the water/cement ratio and strength as per IS-10262:2019.