Just as a side note, I usually speak about boiloff amount. I have an electric system and can dial in the heat very repeatably. On a 5 or 10 gallon batch, I get the same amount of boiloff (1 gal/hr). If you state an amount, both volumes work. If you try and state a percent value, then it changes for every different volume.

+1! I don't know why folks want to describe boil-off rate as a percentage. Given the same conditions, you will boil off the same amount of water whether you are boiling 10 gallons or 5. It is almost completely a function of the surface are of wort in your kettle (kettle diameter), boil vigor (soft simmer to violent rolling boil) and atmospheric conditions especially if you boil outdoors).

I have ground out marks on my stainless brewing spoon at 1 gallon intervals with a Dremel and use that to check pre/post boil volumes. Boil-off volume is almost always within a pint or two on my system.

If you want to make gradations on your spoon or a stick of some kind to measure volume in you kettle, heres the formula to give you a number in inches from the bottom of the kettle:

(V * 231) / 3.141593 * (R * R)

V = Volume in Gallons (Change this number for each mark you intend to make)

R = Kettle radius in inches (half the total width of your kettle)

Here's how it worked on

**my** kettle to fill to 7 gallons:

V = 7 gallons

R = 6.8125 inches

(7 * 231) / 3.141593 * (6.8125 * 6.8125) = 11.09 (11 3/32) inches

So the 7 gallon mark on my spoon is 11 and a heavy 16th inches up the handle from the tip of the spoon.