How to use the ibu calculator
- Choose US units (gallons and ounces) or metric units (liters and grams).
- Enter the batch volume going into the fermenter and the original gravity, such as 1.055. These are examples, so use your own recipe.
- For each hop addition, enter the weight, the alpha acid percent from the package and the minutes the hops spend in the boil.
- Read the estimated total bitterness in IBU, the contribution of each addition and the bitterness to gravity ratio.
Formula
bigness factor = 1.65 x 0.000125^(original gravity - 1)
boil time factor = (1 - e^(-0.04 x minutes)) / 4.15
utilization = bigness factor x boil time factor
IBU of an addition = utilization x alpha acid fraction x grams x 1000 / liters
This is the Tinseth formula. Utilization is the share of the alpha acid that ends up bittering the beer. It rises with boil time and falls as the wort gets stronger.
Three additions in 5.5 gallons
A batch of 5.5 gallons is 20.82 liters, with an original gravity of 1.055. One ounce (28.35 g) of 12% alpha hops boiled for 60 minutes has a utilization of 22.1% and adds 36.0 IBU. Half an ounce of 6% hops at 15 minutes adds 4.5 IBU (10.9% utilization), and another half ounce at 5 minutes adds 1.8 IBU (4.4%).
The total is 42.3 IBU. With 55 gravity points, the bitterness to gravity ratio is 42.3 / 55 = 0.77. At a gravity of 1.050 and a full 60 minutes, the utilization is 0.2307, which matches the published table for the formula.
What the estimate is and is not
IBU stands for international bittering units, and one unit is meant to be about one milligram of bittering acids per liter. The formulas that estimate it are fitted to measurements, and different formulas can differ by several units for the same recipe. Pellets versus whole cones, hop age, kettle shape, pH, whirlpool or hop stand additions and how long the hops sit after the boil all change what ends up in the glass.
Treat the number as a way to compare recipes and plan changes, and not as a lab measurement. Taste matters more than any formula.
More brewing math
Once the beer has fermented, the ABV Calculator turns gravity readings into alcohol. For other kitchen and drink ratios, the Coffee Ratio Calculator scales brew strength, and the Unit Converter handles gallons, liters, ounces and grams.
More maker tools
For other hobby projects and builds, try the Cone Pattern Calculator, Circle Skirt Calculator and Circle From Three Points Calculator.
Assumptions and limits
- The Tinseth formula is used for every addition, with whole minutes of boil time.
- Hop weights, alpha acid values and times are examples until you replace them.
- The batch volume is the amount in the fermenter after the boil.
- Whirlpool, hop stand and dry hop additions are not modeled.
Frequently asked questions
How do I calculate IBU for a homebrew recipe?
Multiply the alpha acid fraction by the grams of hops and 1000, divide by the liters, and multiply by the utilization from the boil time and gravity. The tool does this for each addition and adds them.
What is hop utilization?
It is the share of the hop alpha acids that become bitterness in the beer. Longer boils raise it and stronger wort lowers it, so a 60 minute boil at 1.050 uses about 23% of the alpha acids.
Why does a high gravity beer get less bitterness from the same hops?
Sugar in a stronger wort makes the hop acids less likely to dissolve and isomerize, so the same hops give less bitterness. The Tinseth bigness factor models that.
Is a higher IBU always more bitter?
Not to the taste. Sweetness and body can hide bitterness, so the ratio of IBU to gravity points says more about balance than the IBU alone.