IBU & Bitterness

What it means, how it works, and what makes our approach different

IBU stands for International Bitterness Units. It's how brewers measure hop bitterness. A light lager might sit around 10 IBU. A West Coast IPA pushes 60+. The number tells you how bitter the beer will taste, though malt sweetness and other flavors can balance it out.

In the recipe builder, IBU updates automatically every time you add, change, or remove a hop addition. Boil hops, whirlpool hops, first wort hops, dry hops. They all contribute, and the builder tracks every one of them.

Hops46 IBU

Centennial10.0% AA

Boil60 min18g18.7 IBU

Simcoe13.0% AA

Boil15 min14g7.4 IBU

Simcoe13.0% AA

Dry Hop5 days28g5.4 IBU

Cascade5.5% AA

Whirlpool20 min28g3.2 IBU

Every addition type (boil, whirlpool, dry hop) gets its own IBU contribution.

How We Calculate It

We use the Tinseth model, the industry standard formula that virtually every brewing calculator is built on. It accounts for two things: how your wort's gravity affects extraction, and how long the hops are in contact with hot wort.

Tinseth IBU

IBU = \frac{W \times \alpha \times U \times 75}{V}

W = hop weight (oz), α = alpha acid %, U = utilization factor, V = batch volume (gal).

The utilization factor is where the real physics lives. Heavier wort suppresses bitterness extraction, and longer boils increase it, but with diminishing returns past about 60 minutes.

Utilization

U = \underbrace{1.65 \times 0.000125^{\,(G - 1)}}_{\text{gravity}} \times \underbrace{\frac{1 - e^{-0.04t}}{4.15}}_{\text{time}}

Beyond the Boil

The Tinseth formula covers boil additions. But modern recipes use hops in a lot more places, and each one adds bitterness differently.

Whirlpool & Hop Stand

temperature-scaled

Isomerization doesn't stop when the flame goes off. It just slows down. At 80°C, you get about 29% of the extraction rate of a full boil. We scale the Tinseth utilization by a temperature factor that drops to zero below 60°C, where extraction becomes negligible.

Dry Hops

unique to Brewing.It

Dry hops do contribute measurable bitterness. During hop pellet processing, some alpha acids oxidize into compounds called humulinones. These dissolve into beer at room temperature. We model both the humulinone contribution and a small amount of non-isomerized alpha acid dissolution, with an extraction efficiency that decreases at very high dry-hop rates. A heavy dry hop (8 g/L, 12% AA) adds about 12 measurable IBU, enough to shift the balance in hop-forward styles.

First Wort Hops

First wort hopping means adding hops during lautering, before the boil. They get the full boil time plus extra contact, producing more IBU with a smoother, rounder perceived bitterness. We model this by adding 20 minutes to the boil time for utilization calculations, so a 60-minute boil treats FWH as 80 minutes of contact time.

Mash Hops

Hops added during the mash contribute very little bitterness. Most alpha acids wash out with the grain. We apply 20% utilization, which is the standard approach. Community experiments suggest the real number may be closer to 10–15%, so our figure is slightly generous.

Worked Example

Here's the math for a single hop addition: 1 oz of Cascade (7% alpha acid) boiled for 60 minutes in 5 gallons of 1.050 wort:

Gravity factor = 1.65 × 0.000125^0.050 = 1.053

Time factor = (1 − e^−2.4) / 4.15 = 0.219

Utilization = 1.053 × 0.219 = 23.1%

IBU = (1 × 7 × 0.231 × 75) / 5 = 24.2 IBU

Where This Comes From

The Tinseth model has been the industry standard since 1995. Our dry hop model is based on research showing that humulinones formed during pellet processing dissolve into beer without heat, contributing measurable bitterness.

See all the numbers come together in real time.

See this in the recipe builder