IBU Bitterness Calculator

How IBUs (International Bitterness Units) Are Calculated

IBU measures the bitterness of beer from hop alpha acids isomerized during the boil. The most widely used formula in homebrewing is Tinseth’s Formula.

Tinseth IBU Formula, in metric: IBU = (AA% / 100) × Hops(g) × Utilization × 1000 / Volume(L)

The inputs on this page are ounces and gallons, so it uses the imperial form: IBU = (AA% / 100) × Hops(oz) × Utilization × 7489 / Volume(gal)

The 7489 is not a fudge factor. One ounce of hops in one US gallon works out to 7,489 mg/L (28,349.5 mg ÷ 3.78541 L), and IBU is defined in milligrams of isomerized alpha acid per liter. The metric form uses 1000 for the same reason, converting grams per liter to milligrams.

Utilization depends on boil time and wort gravity: Utilization = Bigness × Boil Time Factor

• Bigness = 1.65 × 0.000125^(gravity − 1)
• Boil Time Factor = (1 − e^(−0.04 × minutes)) / 4.15

Worked Example: 28g of Cascade hops (6% AA), 60-minute boil, 20-liter batch, OG 1.050:

• Bigness = 1.65 × 0.000125^0.050 = 1.65 × 0.638 ≈ 1.053
• Boil Time Factor = (1 − e^(−2.4)) / 4.15 = (1 − 0.0907) / 4.15 ≈ 0.219
• Utilization = 1.053 × 0.219 ≈ 0.231
• IBU = (6/100) × 28 × 0.231 × 1000 / 20 = 19.4 IBU

That 23% utilization at 60 minutes and OG 1.050 is the figure most brewing software quotes, and it is worth memorizing as a sanity check on any IBU calculator including this one.

IBU Style Reference:

• American Lager: 5–15 IBU
• Pale Ale: 30–50 IBU
• IPA: 40–70 IBU
• Double IPA: 65–100 IBU
• Imperial Stout: 50–90 IBU

Key Variables:

• Higher alpha acid hops = more bitterness per gram
• Longer boil = higher utilization
• Higher gravity wort = lower utilization (sugar inhibits isomerization)
• Pellet hops: ~10% more efficient than whole leaf hops