Hops

This is the guide for an IBU calculator, a hop schedule editor, or a hop inventory that wants to know how much alpha acid is left in an old bag.

The functions to start with

• calculateIbu answers "how bitter is this hop schedule?" It takes the additions, the wort volume and gravity, and the boil length, and returns the total and each addition's share. Boil, first-wort and whirlpool additions all count. Dry hops and keg hops are accepted and add nothing.
• agedAlphaAcidPercent answers "what is this hop's alpha acid now?" from the label's figure, how long it's been stored, how cold, and how it's packed.

calculateRecipe calls calculateIbu itself, so you only need it directly when you're working on hops without the rest of the recipe.

A bag from the back of the fridge

Cascade that's been open in the fridge for a year, used in a whirlpool after a 60-minute Magnum charge:

// Cascade loses about half its alpha in six months at room temperature.
const cascadeAlpha = agedAlphaAcidPercent(6.5, {
  alphaLostSixMonths: 0.5,
  months: 12,
  storageC: 4,
  packaging: 'wrapped',
});
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const hops: HopAddition[] = [
  {
    name: 'Magnum',
    amountKg: 0.015,
    alphaAcidPercent: 12,
    form: 'pellet',
    use: 'boil',
    timeMinutes: 60,
  },
  {
    name: 'Cascade',
    amountKg: 0.05,
    alphaAcidPercent: cascadeAlpha,
    form: 'pellet',
    use: 'whirlpool',
    timeMinutes: 20,
    whirlpoolTempC: 85,
  },
];

const bitterness = calculateIbu({
  hops,
  volumeL: 21,
  boilGravity: 1.05,
  boilTimeMinutes: 60,
});
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The 6.5 % on the label has dropped to 4.0 %. Cascade is a poor keeper, losing about half its alpha in six months at room temperature, and a fridge and a squeezed-shut bag slow that down without stopping it.

The schedule comes to 24.3 IBU. The Magnum gives 21.7 of it. The Cascade gives 2.5, because a 20-minute stand that starts at 85 °C and cools is worth only 2.5 minutes at a full boil.

The working

Each entry in contributions carries its own IBU, its utilization, and the boil minutes it was worth. A whirlpool addition also has a standDescription, a sentence you can show as it is: "Cascade, 20 min from 85 °C cooling: worth 2.5 boil minutes, 2.5 IBU."

For the whole recipe, calculateRecipe's explain.ibu names the model (tinseth-1997 by default), lists the volume and gravity it used, and cites Tinseth, Malowicki's isomerization kinetics and the mIBU method the whirlpool model comes from. The README prints that explanation in full.

Traps

• Amounts are in kilograms. A 15 g addition is amountKg: 0.015.
• timeMinutes means different things. For a boil addition it's the time left in the boil. For a whirlpool it's the length of the stand. First-wort hops get the whole boil whatever it says, and dry hops use dryHopDays.
• The volume and gravity are post-boil by default. That's what Tinseth measured against. CalculationOptions can change both (ibuVolumeBasis, bitternessGravityBasis), but numbers then stop matching other calculators.
• Pellets get a bonus. Pellet and cryo hops are multiplied by 1.10 over whole cones, a common default that you can override with hopFormMultipliers.
• Iso-alpha extract is a different number. An iso-extract addition reads isoAlphaPercent, not alphaAcidPercent. They measure different things, and reading one as the other makes the beer about a fifth as bitter as intended.

Going further

• Gravity, IBU and color has the three IBU models and the whirlpool model, with their sources.
• In the reference, the Hops category has the pieces: the Tinseth, Rager and Garetz utilization curves, the whirlpool integration in standEffectiveBoilMinutes, and the storage model in hopAlphaRemaining.