IBU Calculator (Tinseth & Rager)

How this works

International Bitterness Units (IBU) approximate the concentration of dissolved iso-alpha acid in your finished beer — roughly one IBU per milligram of iso-alpha acid per litre. Not all of a hop addition's alpha acid actually makes it into solution as bitterness; the fraction that does is called utilization, and it depends heavily on how long the hops boil and how strong (gravity) the wort is at the time.

This calculator runs your hop schedule through two independently developed utilization models, shown side by side.

Tinseth, developed by Glenn Tinseth and first published online in the mid-1990s, models utilization as the product of a "bigness factor" (which increases utilization at lower wort gravity) and a "boil time factor" (which rises toward a ceiling the longer hops boil, following an exponential decay curve). It's the most widely used default in modern brewing software.

Rager, developed by Jackie Rager, uses a different curve for boil-time utilization (a hyperbolic tangent function centred around 31 minutes) and applies a separate gravity correction only above 1.050 boil gravity. The two curves cross twice — Rager reads lower than Tinseth on additions roughly in the 5-30 minute range (the exact crossover shifts a few minutes earlier at high gravity, later at low), and higher outside that window, with the gap widening most on big, high-gravity, long-boiled beers. See Tinseth vs Rager: why they disagree for the worked numbers behind the swing.

Neither formula is simply "more correct" than the other — they're two different models built from different assumptions, and different brewing software defaults to different ones. Seeing both, and the BU:GU ratio derived from each, gives you a more honest picture than trusting a single number.

BU:GU ratio

The bitterness-to-gravity-unit ratio (total IBU divided by gravity points, where gravity points = (SG - 1) x 1000) is a quick way to judge a beer's balance independent of its absolute IBU number. A ratio around 0.3-0.5 typically reads as malt-forward, 0.5-0.8 as balanced, and above 0.8 as distinctly hop-forward — though "balanced" varies a lot by style, which is exactly what the style guideline pages are for.

Reading the result

If your calculated IBU is far higher than what you actually taste in the finished beer, remember that IBU measures dissolved iso-alpha acid concentration, not perceived bitterness — perception is also shaped by malt sweetness, carbonation, and how the beer's other flavour compounds interact with your palate. A beer can measure 60 IBU and taste less aggressively bitter than a 40 IBU beer with a drier, less malty finish.

Add hop additions one at a time and note how much each contributes — a small late addition at 5-10 minutes contributes far less bitterness than the same weight at 60 minutes, even though it's usually where most of a hop-forward beer's aroma actually comes from.

A worked example

A 19 L (5 gallon) batch at 1.052 boil gravity with a 28 g addition of a 12% alpha hop at 60 minutes, plus a 28 g addition of a 5% alpha hop at 10 minutes, lands at 46.1 IBU by Tinseth and 58.8 IBU by Rager — a fairly assertive hop schedule, closer to a hoppy American pale ale or a session IPA than a purely malt-forward beer. Push the 60-minute addition's alpha acid up toward 15-16% and the same weight contributes meaningfully more bitterness, since the alpha acid percentage scales the contribution close to linearly at a fixed weight and time.

Boil time is not the only lever

It's tempting to treat "more boil time equals more bitterness" as a simple linear relationship, but Tinseth's boil-time factor actually approaches a ceiling — the jump from a 45-minute to a 60-minute addition contributes noticeably less extra bitterness than the jump from a 10-minute to a 25-minute addition, because utilization is already most of the way to its maximum well before 60 minutes. This is part of why hop-bursting recipes (concentrating hops late rather than spread across the whole boil) can hit a target IBU with a smaller total hop weight than a more traditional single-60-minute-addition schedule.