How IBU is calculated

BrewDesigner calculates International Bitterness Units (IBU) using the Tinseth formula, with corrections for kettle geometry and an optional system-specific utilization factor.

The formula

For each hop addition:

IBU_addition = (decimal_alpha × mass_g × 1000 / volume_L) × utilization
utilization   = bigness_factor × boil_time_factor

Where:

• bigness_factor = 1.65 × 0.000125^(wort_gravity − 1) — penalises high-gravity worts.
• boil_time_factor = (1 − e^(−0.04 × time_min)) / 4.15 — rises asymptotically with boil time.

The recipe’s IBU is the sum across all additions, optionally scaled by the equipment profile’s hop utilization factor (default 100%).

Worked example

Below, BrewDesigner calculates the IBU for a sample recipe. The numbers come from running calculateRecipeStats (the same function the app uses) at build time — so they match what you’d see in the recipe page.

Why we trust these numbers

The card above is rendered by the same calculateRecipeStats function that powers the recipe page in the app. If the formula ever changes, this page changes too — there’s no copy of the math in prose to drift out of sync.