Why does gravity rise during the boil?

Water evaporates from the kettle but the dissolved sugars do not, so the same amount of sugar ends up in less liquid. The wort becomes more concentrated, and its gravity rises in direct proportion to how much the volume shrinks.

Gravity points times volume is conserved. New gravity points equal old gravity points multiplied by the pre-boil volume divided by the post-boil volume. The ratio of the two volumes is the concentration factor. An SG of 1.050 is 50 points, so the points are what scale, not the whole SG number.

What is a concentration factor?

It is simply pre-boil volume divided by post-boil volume. Boiling 7 gallons down to 6 gives a factor of about 1.167, so a pre-boil 1.045 wort (45 points) finishes near 1.053 (53 points). A higher factor from a longer, hotter boil concentrates the wort more.

How do I find my evaporation rate?

Boil a known volume of water for an hour, then measure the loss; that is your hourly evaporation rate for that kettle and burner. Typical homebrew kettles lose around 1 to 1.5 gallons per hour. Once known, the tool derives the post-boil volume from it and your boil time.

Can I use this to correct a wort that is too weak or too strong?

Yes. If your pre-boil gravity is low, a longer boil concentrates it toward target; if it is already high, you can shorten the boil or add water. The concentration factor tells you exactly how the gravity will move so you can plan additions before flameout rather than guessing.

What is the Wort Boil Concentration Factor?

Computes the concentration factor and new original gravity as wort volume falls during the boil. Predicts post-boil OG from pre-boil gravity and either a target final volume or an evaporation rate and boil time, so brewers can hit their numbers. It runs free in your browser on Gera Tools, with nothing uploaded.

Wort Boil Concentration Factor

Name: Wort Boil Concentration Factor
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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The boil is where pre-boil gravity becomes original gravity. As water evaporates, the sugars left behind concentrate and the gravity climbs. This calculator predicts exactly where your gravity will land, so you can hit your numbers instead of discovering a miss after flameout.

How it works

Sugar does not evaporate, so the total dissolved extract is fixed. Expressed in gravity points (an SG of 1.050 is 50 points), the product of points and volume is conserved through the boil:

points_pre × volume_pre = points_post × volume_post

Rearranging gives the post-boil gravity:

points_post = points_pre × (volume_pre / volume_post)

The ratio volume_pre / volume_post is the concentration factor. Boil 7 gallons down to 6 and the factor is 1.167, lifting a 1.045 wort (45 points) to about 1.053 (53 points).

Two ways to get the final volume

You may already know the post-boil volume, in which case enter it directly. More often you plan a boil by its evaporation rate and time: a kettle that loses 1 gallon per hour over a 90-minute boil sheds 1.5 gallons. The tool derives the post-boil volume from rate times time, then applies the concentration formula.

Worked example

Suppose you measure a pre-boil gravity of 1.048 (48 points) in 8 gallons, and you want to finish at 6.5 gallons. The concentration factor is 8 ÷ 6.5 = 1.231. Post-boil gravity points are 48 × 1.231 = 59 points, giving an OG of 1.059. You evaporated 1.5 gallons to get there.

Alternatively, if your target OG is 1.055 (55 points), the required post-boil volume is 48 × 8 ÷ 55 = 6.98 gallons — meaning you need to boil off about 1.02 gallons from your 8-gallon starting point.

Why evaporation rate matters for recipe design

A vigorous rolling boil typically drives off 10–15% of kettle volume per hour on a home system. A too-gentle boil can leave DMS (dimethyl sulfide) in lagers and pale ales, while a too-aggressive boil concentrates colour and bitterness. For recipes that call for a specific pre-boil volume and gravity, knowing your kettle’s evaporation rate lets you scale the grain bill correctly before you even heat the water.

If your evaporation rate is higher than expected, the wort concentrates faster and you risk overshooting OG. In that case, plan to add pre-boiled (sanitised) water at flameout. The concentration factor tells you in advance how much to prepare: target volume = current points × current volume ÷ target points.

Tips and notes

Measure your evaporation rate once by boiling a known volume of water for an hour and noting the loss; it is repeatable for that kettle and burner. If a pre-boil reading comes in low, a longer or more vigorous boil concentrates it toward target; if it comes in high, shorten the boil or add water at flameout. Because the concentration factor is known in advance, you can plan these corrections rather than chase the gravity. All calculation runs locally in your browser.