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Eventually there comes a time where just boiling 6 gallons for 60 minutes and taking it on faith that you have 5 gallons left doesn't cut it anymore.

What tools can I use to measure the volume in the kettle when its boiling? Should I make a correction for the increased volume due to temp? What is the correction?

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    Boiling water takes up 4% more volume than at 60ºF. You should correct for it. And hops absorb water, which is left behind on the way to the fermenter. To be quite accurate you have to account for all of it.
    – Dean Brundage
    Mar 5, 2010 at 19:32
  • To bad hops didn't absorb 4% water, then the two would cancel out!
    – brewchez
    Mar 5, 2010 at 19:46

2 Answers 2

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3 common ways include:

1. calibrated dipstick or spoon (easiest)

Before your next brew session, take a 1 gallon pitcher or some other known volume vessel. One gallon at a time, pour the water into your kettle, let it settle, then put the dipstick or spoon in. Mark the top of the wet line with a sharpie or craft knife. Repeat for each gallon. Measuring volume is now just a case of putting your dipstick in the boiling wort and reading off your volume.

2. calibrated sightglass (harder)

Drill a small hole at the bottom of your kettle and using a bulkhead fitting and street elbow create a 90° turn pointing upwards. Install a polycarbonate or glass tube into a compression fitting. Now when you pour liquid into your kettle the level will be revealed by a column of water in the polycarbonate tube. There are kits you can buy for this but beware using these on kettles as too much heat applied may melt the polycarbonate or boil the water in the tube. Glass tubes usually need some kind of protective shroud so you don't smash them. Sightglasses are perfect for vessels that are high up like a hot liquor tank where you typically can't look down into them. Calibrate as in option 1 - dipstick.

3. calibrated float-tube (novel)

I saw this idea recently and remain intrigued. Braze/solder a stainless or copper tube into your vessel with a hole at the bottom to allow the liquid into it. in the top of the tube insert some kind of buoyant rod, like a balsa wood dowel. As the liquid in the vessel rises it pushes the dowel up and this can then be calibrated in the same way but your markings are now in reverse, i.e. the 5 gallon mark is at the bottom of the dowel, not the top.

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    The calibrated float tube is a cool idea. I assume the float would be removed pre chill. Seems like something that may not get fully sanitized during the boil.
    – brewchez
    Mar 5, 2010 at 19:47
  • If it's in the boil the whole time it will be just as sanitary as, say, the walls of your pot that are not immersed. Steam and heat kills.
    – Homebrew Holli
    Mar 11, 2010 at 16:07
  • another flavor of idea 1 is to mark the inside of your boil kettle. With a gallon pitcher, I marked each gallon with a grease pencil. Later I emptied the kettle and with a dremel, etched marks on 3 columns. This method has the advantage over the graduated stick in that it's never separated from the kettle!
    – Dale
    Aug 1, 2012 at 21:39
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One more option:

If you have a cylindrical kettle, you can measure the volume quite accurately.

The base formula for volume in a cylinder is as follows:

V = πr2h

So to get the height for a given volume, you need to divide the Volume by (πr2).

Since most kettles have rounded corners in the bottom, your volume will be a little off, but that amount is something that is easy to find out. I guess it will be less than a litre in most cases.

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