Baker percentages, weights, volumes, and such
If specific units are not given (i.e. you just have a ratio), then you should always go by weight, not volume.

Everything that happens in baking, every chemical reaction, is based on the actual number of molecules of a particular ingredient, which corresponds to its weight.  Volume is simply a rough approximation used in many home cooking/baking recipes.

Note that if using metric measurements (g or mL) then the weight vs. volume measurements actually are the same for water.  So consider using metric for baking, if you can, because that way the conversions are much easier and you can measure out your liquids in a measuring cup without having to do any conversion math.