# Calculating Boil Off Rate

### Help Support Homebrew Talk:

#### johnodon

##### Well-Known Member
Hey all...just a quick question.

I want to start to dial my equipment in a little better (OK...I haven't done it at all yet). The figured the easy place to start was with my boil off rate. Is it OK to use plain water for this? Or does wort have a drastically different boil off rate than water?

TIA,

John

#### jfkriege

##### Well-Known Member
I have not noticed any sort of real measurable difference in boiloff rates.

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.

Do you have accurate volume measurement?

OP
OP

#### johnodon

##### Well-Known Member
I use the very scientific method of putting notches in a section of CPVC.

#### Catt22

##### Well-Known Member
I use the very scientific method of putting notches in a section of CPVC.

That's a perfectly OK method. Don't fret too much over the boil off rate. If you happen to boil off too much, you can always add some water near the end of the boil to make up for it. If you wind up with too much volume, boil a little longer. Obviously, you must take the wort gravity into account when concentrating or diluting the wort during the boil, but once you have a few batches behind you, you will get a feel for it and the adjustments, if any are needed, should be very minor. I'm to the point that I can eyeball the boiling wort and estimate the boil off fairly accurately. I check the volume during the boil and make burner adjustments accordingly. You can also slow down or speed up the boil to change the rate on the fly, but you already knew that I am sure.

##### Well-Known Member
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.

#### Bigscience

##### Well-Known Member
I always track mine every batch to make sure I boiled hard enough. I use a sight guage for initial volume. Since hops will take up room, I calculate the evaporation based on gravity. If I know the initial volume and gravity, I can monitor it during the boil with a refractometer and adjust if necessary.

#### a1lawng

##### Well-Known Member
I always track mine every batch to make sure I boiled hard enough. I use a sight guage for initial volume. Since hops will take up room, I calculate the evaporation based on gravity. If I know the initial volume and gravity, I can monitor it during the boil with a refractometer and adjust if necessary.

I've always kept things pretty simple so I've never really checked gravity. I'd like to be more scientific about my brewing, though, so I've bought a hydrometer to use on my next batch. This is off-topic from the OP's post, but it sounds like I can drop the hydrometer during the boil? I wasn't sure if temperature would affect the reading.

#### LovesIPA

##### Well-Known Member
I've always kept things pretty simple so I've never really checked gravity. I'd like to be more scientific about my brewing, though, so I've bought a hydrometer to use on my next batch. This is off-topic from the OP's post, but it sounds like I can drop the hydrometer during the boil? I wasn't sure if temperature would affect the reading.

Temperature definitely affects the reading. And you don't put the hydrometer into the boil kettle. You need a sample vessel. You can buy one specifically for brewing, or you can buy a plastic 100ml graduated cylinder which is what I use. Once you read the gravity, see what the temperature is. Then use a hydrometer temperature correction program on the web (google for one) to see what the actual gravity is.

If you leave a hot sample around and check the gravity occasionally, you will see the gravity increase as the sample cools.

#### Bigscience

##### Well-Known Member
Refractometer is what you want.

#### acidrain

##### Well-Known Member
Yes, it's OK to use plain water to find your boil off volume.

#### cheesehed007

##### Well-Known Member
I just checked the boil off on my new set up. I'm down 1 gallon in 30 min. Does 2 gal. An hour seem like a lot? The temp in my garage is in the mid 20s about 50% humidity outside ... Just trying to dial in my new setup. Thanks!

Sent from my van, down by the river.

#### sabitu

##### Well-Known Member
2 gal/hr in cold weather with low humidity is in the normal range. My boil off rate is 1 gal/hr in the spring/summer and it jumps up to 1.5 gal/hr in the winter due to the lower humidity. So keep in mind your evaporation rate may decrease in warmer weather.

#### cheesehed007

##### Well-Known Member
2 gal/hr in cold weather with low humidity is in the normal range. My boil off rate is 1 gal/hr in the spring/summer and it jumps up to 1.5 gal/hr in the winter due to the lower humidity. So keep in mind your evaporation rate may decrease in warmer weather.

Thanks for the reply! I never really did a test with my old set up but it seemed to be in the .75 gal/hour range. I was doing extract with that. I'm now moving on the BIAB. Cheers!

Sent from my van, down by the river.

#### jprank2

##### Member
This equation that was posted a ways back:

h=231*V/(pi*r^2)

starts to lose accuracy as you get more water in the kettle for kettles that taper outwards. I noticed this while making my mash paddle ruler tonight. It may be better to measure using water.

#### DaveCS

##### Well-Known Member
Just commenting on this older thread as it is useful. Checked mine today - 3100 ml in 60 min (approx. 52ml per min during a rolling boil) - I find this helpful as it will help me determine how much to boil off based on starting volume, loss due to mash etc. (I do BIAB so my starting volumes always are larger... and no sparge).

Cheers,
Dave

#### mj1angier

##### Supporting Member
HBT Supporter
2 gal/hr in cold weather with low humidity is in the normal range. My boil off rate is 1 gal/hr in the spring/summer and it jumps up to 1.5 gal/hr in the winter due to the lower humidity. So keep in mind your evaporation rate may decrease in warmer weather.

This.
Here in North Carolina, winter can be cold/ low humidity one day and 70 with tropical front the next, lol.

In Beersmith I made two equipment profiles- one for high humidity and one for low. But I do brew in a garage so it changes a lot. If in house, not so much

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