water + alcohol: % by volume or by weight?

[tici]

As in the title: is it ment by weight or by volume?
50% alcohol by volume is about 40% by weight, no matter if ethanol or methanol.

Thanks - Stefano

[Richard L]

Most common usage is done by volume - just easier to mix.

[tici]

Thanks! This means that the usual amount of alcohol is 40% by weight