If you consider your kettle to be a simple cylinder, the volume of the liquid in the kettle would be:
</b>where "r" is the inside radius of the kettle and h is the height of the liquid in it.
If you use a yardstick, this will give you a volume in cubic inches, which you can convert to gallons with the rule:
1 gallon = 231 cubic inches
I took the cheater method and filled my kettle with a 4 gallons of water, stuck my Big Spoon in it, and marked the waver level on the spoon.
Added another 1 gallon of water and made another mark.
Repeat until you have a spoon that can serve as a dip-stick and tell you how much is in the kettle.