How does one measure the capacity of a round bottom boiling flask?

Fill it and pour it into a liquid measuring jug to measure the capacity.

So do I fill it to the top, or is there an invisible line I need to know about? That’s what I want to know.

Oh – how to decide where the capacity is measured from? Got it, but don’t know the answer. Since there is no line, I would guess the top; but maybe do an imaginary line at the bottom of the neck and list both measurments in your write-up.

Good idea, that’s what I thought. Thanks for articulating my question better.

Cool.

For ease I would list it with a capacity that is a nice square number that happens to fill up most of the flask. That way you don’t have a “153.245ml flask” you have a “150ml flask”. But in general, any amount of liquid that fills it to the very top or almost the top will be enough for the consumer to get a good idea of how much it holds.