Hay guys I just figured this out (all the smart guys already know this) My brew pot is 13.5 inches round inside dimensions and 17.5 inches tall inside dimensions. the radius of my pot is 13.5 / .5 = 6.75 inches I want to know how many inches it takes to get 5 gallons in my pot the formula is.....Pie x radius x radius ...(3.14 x 6.75 x 6.75 = 143.06625... or just 143 for my pot) to find the gallons the formula is..... inches x 143 Divide by 231 = gallons sooo 8 inches tall x 143 Divided by 231 is 4.95 gallons or close enough to 5 gallons in my pot 9.75 inches tall x 143 Divided by 231 is 6 gallons of water. etc. now I can use my measuring stick to figure out how many gallons of water are in my boil pot. or just use the long plastic spoon I normally use for mashing with markings on it.

Take a piece of lead free 1/8 solder about 20 inches long. Fold in half a bend a hook at the fold to hang on the inside your pot. Cut the two ends in the pot at what ever pre boil and post boil levels you need. It will become part of your kettle.

You aren't accounting for "rounds" at the bottom of your pot or for any valves, false bottoms, screens or so but your math is sound and should provide you a close enough approximation. The formula for volume of a cylinder, the approximation of a brew pot, is pi*radius squared*height. 231 square inches is a gallon. There's another method: Pour a gallon of water into your pot, etch a line at the waterline. Repeat. That would account for any loss of volume to rounds, etc.

Jmac the math method is nice...but if ya got time and like DIY i would recommend doing a home electro-etch on the inside. doesnt take but a couple hours and is permanent- looks "pro" too

that does look pretty good. seems to be a bit more legible than the markings on my kettle (they're more readable from the outside)

I have a stainless kettle spoon and have engraved the lines and numbers on it. Why have a separate dipstick? This is already in the kettle. Of course it only is accurate for that size kettle, but, you know. Anyway, here is a shortcut: http://kotmf.com/tools/dipstick.php Nosybear does math for fun, but this calculator saves us "Non Engineer" types from the drudgery. I do like the electro-etch design though.

a site glass is by far the easiest way to read levels, you can also put a digital temp probe through a T like mine https://www.brewhardware.com/category_s/1819.htm

cheapest option, go to the hardware store and buy a metal ruler with no cork backing or anything on it. Something long enough to measure the whole pot too. Then i pour in 2 quarts at a time, measure the millimeters of water in it, and continue to record measurements 2 quarts at a time. There's a "chart" hanging on my fridge. I sanitize my ruler with everything else on brew day to get quick wort measurements.