IBU Calculator
Work out the bitterness of a beer recipe from hop weight, alpha acid percentage, boil time and batch size.
What it does
International Bitterness Units, or IBU, are the standard way brewers talk about how bitter a beer is. One IBU is one milligram of isomerized alpha acids in one liter of finished beer. The Beer Judge Certification Program lists IBU among the vital statistics of a style, so it turns up in every recipe and every competition entry.
Bitterness comes from the alpha acids in hops. Alpha acids do not taste bitter on their own, but boiling converts part of them into isohumulone, which is bitter and does not dissolve well. Boil time, hop weight, alpha acid percentage and batch size all decide how much of that conversion happens and how much bitter compound ends up in the glass.
The calculation on this page uses Tinseth's model, the standard approach in brewing software. It works out a gravity factor from the original gravity, then a utilization figure for each addition that depends on how long that addition boiled, then multiplies through by the weight of hops, the alpha acid fraction and the finished volume.
The result is an estimate of what the recipe should produce. A laboratory measurement using the standard method gives the true figure, and the gap between the two comes from hop form, whirlpool use, dissolved oxygen, the shape of the kettle and how the brewer handled the wort. The AHA notes that hop additions after the boil, such as a whirlpool, contribute bitterness that older calculation methods miss entirely.
The tool handles up to three additions, which covers most recipes. It also reports the total weight of alpha acids you are adding, which is a useful number to compare against what the beer actually tastes like.
How to use
- Pick metric or imperial units for the hop weight and the batch size.
- Enter the original gravity of the wort.
- Enter the finished volume of the beer.
- For each hop addition, enter the weight, the alpha acid percentage and the minutes it boiled.
- Leave the weight at zero to skip an addition you do not need.
- Read the total IBU and the figure for each addition, then compare with the style you are brewing.
Formula
- OG
- Original gravity of the wort.
- Fg
- Gravity factor, which rises with a stronger wort.
- t
- Boil time of that addition in minutes.
- W
- Weight of the hop addition in grams.
- a
- Alpha acid fraction, so 12% is written as 0.12.
- V
- Finished volume of the beer in liters.
The alpha acid percentage should be the measured value for the hop batch, taken from the supplier or the harvest. Old hops lose alpha acids, so the number on an old bag can be well above the current value. This model also treats every addition as a single charge, so a hop split into two additions gives a slightly different answer in the tool than in the kettle.
Worked example
Example: 30 g at 60 minutes plus 20 g at 10 minutes in 19 L
Defaults: metric units, original gravity 1.048, 19 liters finished, 30 g of 12% hops at 60 minutes and 20 g of 8% hops at 10 minutes.
- Gravity factor: 1.65 x 0.000125^0.048 = 1.0719.
- Utilization at 60 minutes: 1.0719 x (1 - e^(-2.4)) / 4.15 = 0.2348.
- First addition: 1000 x 30 x 0.12 x 0.2348 / 19 = 44.5 IBU.
- Utilization at 10 minutes: 1.0719 x (1 - e^(-0.4)) / 4.15 = 0.0851.
- Second addition: 1000 x 20 x 0.08 x 0.0851 / 19 = 7.2 IBU.
- Total: 44.5 + 7.2 = 51.67 IBU, with 5.2 g of alpha acids in the kettle.
- The tool shows 51.67 IBU, 60 min: 44.5 IBU · 10 min: 7.2 IBU, 5.2 g and a gravity factor of 1.0719.
Reference table
| Boil time | Bitterness from that addition | Note |
|---|---|---|
| 60 min | 44.5 IBU | Full kettle charge |
| 45 min | 40.8 IBU | Late charge |
| 30 min | 34.2 IBU | Flameout addition |
| 20 min | 26.9 IBU | Whirlpool range |
| 15 min | 22.1 IBU | Whirlpool range |
| 10 min | 16.1 IBU | Aroma addition |
| 5 min | 8.9 IBU | Flameout only |
Limits
- Calculates what the recipe should produce, while a lab test measures the beer you actually made.
- Assumes hops are added in one charge, so a split addition comes out slightly different.
- Has no term for hop form, whirlpool hops, oxygen pickup or how the brewer handled the wort.
- Only counts up to three additions, so very complex recipes need a spreadsheet or brewing software.
Common mistakes
- Entering 12 for alpha acid when the model wants 0.12, which puts the result 100 times too high.
- Using the finished volume of the wort rather than the volume of the beer in the fermenter.
- Forgetting that a whirlpool or flameout addition still adds bitterness that this model leaves out.
- Using the same alpha acid percentage for hops bought years apart.
- Reading the gravity difference between original and final gravity as bitterness, when that difference estimates alcohol instead.
Try it yourself
Quick checks. Answers sit tucked under each question.
A recipe uses 50 g of hops at 9% alpha acid boiled 30 minutes in 20 L at original gravity 1.050. What is the IBU?
The gravity factor is 1.0528 and the utilization at 30 minutes is 0.1825, so the addition contributes 1000 x 50 x 0.09 x 0.1825 / 20 = 39.89 IBU.
What happens to the example recipe if the late addition doubles from 20 g to 40 g?
The late addition goes from 7.2 IBU to 14.3 IBU and the recipe total moves from 51.67 IBU to 58.84 IBU.
How much bitterness does 25 g of 14% alpha hops at 60 minutes give in 19 L?
Using the same gravity factor of 1.0719 and utilization of 0.2348, the addition gives 1000 x 25 x 0.14 x 0.2348 / 19 = 43.26 IBU.
Questions
What is a good IBU for most beers?
Many amber and pale ales land between 30 and 50 IBU, light lagers sit near 10 to 15 and stouts often run 25 to 50. The style guidelines list ranges per style, and the number to aim for depends on the beer you are copying.
How is IBU different from EBC?
IBU measures bitterness in hops, while EBC is a color scale. They come from different parts of the beer, and one cannot be converted into the other.
Does a longer boil always mean more bitterness?
More bitterness for the same hops, yes, but the rate drops as the boil goes on, so the last 15 minutes add far less than the first 15. The table on this page shows the shape of that curve.
Why does my finished beer taste more bitter than the calculated IBU?
Calculations use the volume of the wort while hot, the alpha acid percentage on the label, and an average utilization. Hops that sat in the wort for a long time, a warm mash, or hops fresher than the label claims all push the real figure up.
Can I use this for a partial mash or a hop blend?
Yes, as long as each addition is entered with its own weight, alpha acid percentage and boil time. Use the average alpha acid of a blend if you do not know the individual values.
What about hops added after the boil?
A whirlpool or flameout addition does add bitterness, and the AHA explains why older methods miss it. This page does not model it, so treat the result as a floor for those recipes.
Sources
- Beer Judge Certification Program: Hops study guide on bitterness and IBU
- BJCP 2021 Beer Style Guidelines: format of a style description
- American Homebrewers Association: post boil and whirlpool hop additions and bitterness