You could take what you know is a 5 gallon container and pour water from it into the larger ones.
The ones I have hold about 60 quarts of soil to the brim, I use a whole 55 quart bag in each pot.
That makes them about 15 gallons each.

To me I think right around 15 gallons is perfect, plenty big for a good root system but not so big as to be impossible to move.

I have 5, 15 gallon pots and one 95 gallon "Vegtrug" on my deck.

Check out a chart

Here is a link that might be useful: Litres to gallons /gal to litres

Yeah, I've more or less guess-timated how big my containers are. I've been using 30L bags of soil, which us apx 8 gallons, so my big pots are a bit bigger than that (probably about 10-12, one of them might be 15.) In any case, if my tomatoes don't get as huge as they could be, I'm not going to make a fuss as long as they produce something.
Garden is more or less established (after a long afternoon of work).
Thanks
Peg

Assuming the pot is straight-sided, I think you could average out the distances across the top and bottom of the pot and then calculate the pot volume as though it were a cylinder of the averaged width. Alter as appropriate for pots that are different shapes.

It's probably easier to just do the maths, since your photographed examples will need to be checked against what you have available anyway. Heck, there are probably tables somewhere on the internet that give volumes for certain pot sizes.

Here's a handy calculator

Here is a link that might be useful: Volume Calculator

Here's a great site.

Hope it helps.

Tony

Here is a link that might be useful: Online Conversion Chart