IBU Calculator
Bitterness for a whole hop schedule by the Tinseth and Rager models side by side, with the factors behind each number shown rather than hidden inside the result.
Average gravity during the boil, not final OG. Higher gravity lowers utilization.
Hop schedule
| Weight (oz) | Alpha acid % | Boil (min) | IBU | |
|---|---|---|---|---|
| 17.3 | ||||
| 8.6 |
The two models fit the same messy reaction differently and routinely disagree by 20–30%. Neither is more correct; pick one and stay with it.
Tinseth's two factors, for the 60 minute addition
bigness = 1.65 × 0.000125^(1.050 − 1) = 1.0528
boil time (60 min) = (1 − e^(−0.04 × 60)) ÷ 4.15 = 0.2191
utilization = 0.2307 = 23.1%
Rager's utilization, for the 60 minute addition
%util = 18.11 + 13.86 × tanh((60 − 31.32) ÷ 18.27) = 30.82%
GA (boil gravity 1.050) = 0 (at or below 1.050)
How the Tinseth model works
Hops do not make beer bitter by sitting in it. The alpha acids have to isomerise, a heat-driven reaction that is slow, incomplete, and hindered by dissolved sugar. Tinseth models the fraction that actually converts — the utilisation — as the product of two factors, then multiplies it by how much alpha acid you added per unit volume.
The bigness factor handles gravity: 1.65 × 0.000125^(gravity − 1). The boil time factor handles contact time: (1 − e^(−0.04 × minutes)) ÷ 4.15. Multiply them for utilisation, and the rest is bookkeeping — weight, alpha acid percentage, and batch volume.
Rager takes a different route to the same number: a single utilisation curve fit to Jackie Rager's original boil-time lookup table — 18.11 + 13.86 × tanh((minutes − 31.32) ÷ 18.27) — plus a separate gravity adjustment that only kicks in above 1.050 boil gravity, rather than gravity being baked into the curve itself the way Tinseth does it. That structural difference, not just different constants, is why the two models can disagree in size and even in which one reads higher, depending on boil time.
Worked example
One ounce of 5% alpha acid hops, boiled 60 minutes, in 5 gallons at 1.050 boil gravity.
Tinseth utilisation works out at 23.1%, giving 17.3 IBU. Rager's utilisation for the same 60 minute boil is 30.8%, giving 23.0 IBU — the same hops, same recipe, a noticeably higher number simply because the two models fit the underlying reaction differently.
Move those same hops to a 15 minute addition and Tinseth's contribution falls to 8.6 IBU — roughly half the 60 minute figure. Rager falls further, to 6.1 IBU, a bit over a quarter of its own 60 minute figure. The two models don't just disagree by a fixed amount — at 60 minutes Rager reads about a third higher than Tinseth here, but at 15 minutes it actually reads lower, because the two curves have different shapes, not just different scales.
What utilisation looks like across the range
Both effects in one table: time along the top, gravity down the side, utilisation as a percentage in each cell.
| Boil gravity | 10 min | 20 min | 30 min | 45 min | 60 min | 90 min |
|---|---|---|---|---|---|---|
| 1.040 | 9.1% | 15.3% | 19.4% | 23.2% | 25.2% | 27.0% |
| 1.050 | 8.4% | 14.0% | 17.7% | 21.2% | 23.1% | 24.7% |
| 1.060 | 7.6% | 12.8% | 16.2% | 19.4% | 21.1% | 22.6% |
| 1.070 | 7.0% | 11.7% | 14.8% | 17.7% | 19.3% | 20.6% |
| 1.080 | 6.4% | 10.7% | 13.5% | 16.2% | 17.6% | 18.8% |
| 1.090 | 5.8% | 9.8% | 12.4% | 14.8% | 16.1% | 17.2% |
Common mistake
Entering original gravity instead of boil gravity on a partial-boil batch. If you boil 3 gallons of concentrated wort and top up to 5, the wort the hops actually sit in is far denser than the finished beer. Using the diluted OG will overstate your utilisation and leave the real beer less bitter than the recipe promised.
Keep going
- Bitterness is usually judged against strength -- the bitterness-to-gravity ratio -- so the ABV calculator supplies the other half of that balance. ABV Calculator
Frequently Asked Questions
Why does late-addition hopping add so little bitterness?
Bitterness comes from isomerising alpha acids, and that reaction needs sustained heat. The boil time factor is (1 − e^(−0.04 × minutes)) ÷ 4.15, which rises steeply early and then flattens — so a 60 minute addition contributes many times what the same hops give at 10 minutes, and a flameout addition contributes essentially nothing on this model despite adding plenty of aroma.
Why does a stronger beer get less bitter from the same hops?
Alpha acids isomerise less efficiently in sugar-rich wort. Tinseth captures that with the bigness factor, 1.65 × 0.000125^(gravity − 1), which falls as gravity rises. Brew the identical hop schedule at 1.080 instead of 1.040 and you lose roughly 30% of the bitterness.
Should I use boil gravity or original gravity?
Boil gravity — the average gravity of the wort while the hops are actually in it. On a full-volume boil the two are close. On a partial boil topped up with water afterwards, boil gravity is much higher than final OG, and using OG instead will overstate your bitterness significantly.
Why do different calculators give different IBUs for the same recipe?
Because they use different models. Tinseth, Rager and Garetz each fit the utilisation curve differently. Rager tends to read higher for a recipe dominated by a long boil addition, but the gap isn't a fixed ratio — as this page's worked example shows, Rager can actually read lower than Tinseth for a single short late addition, because the two curves have different shapes rather than just different scales. None is definitively correct — they are empirical fits to a messy reaction. What matters is picking one and staying with it so your recipes stay comparable to each other.
Does this handle whirlpool, dry hop or first wort hopping?
Not as distinct modes. Dry hopping adds aroma but essentially no measurable IBU, so it is correctly excluded. Whirlpool and first wort additions do contribute bitterness that a plain boil-time model handles only approximately — enter an equivalent boil time if you want an estimate, and treat the result as rough.
Is measured IBU the same as calculated IBU?
No. Laboratory IBU measures absorbance of bitter compounds in the finished beer, while these models predict isomerisation from the recipe. Real losses to trub, yeast and packaging mean measured values often come in below calculated ones, particularly in heavily hopped beers.
Sources. Both Glenn Tinseth's and Jackie Rager's original publications circulated through Norm Pyle's Hops FAQ at realbeer.com, which has since been retired — the domain now redirects to a site-closure notice. The Tinseth formula components used here were taken from Solid Beer's write-up of the Tinseth and Rager formulas, retrieved 19 September 2026. The Rager utilisation curve, gravity adjustment and full IBU equation — previously left out here because a usable form of the utilisation table couldn't be found — were retrieved from Diesel's Brewing Spreadsheet reference page, which cites the same Hops FAQ as its source, and independently corroborated word for word — including its own worked example, reproduced exactly by this calculator — by Homebrew Academy's IBU calculator page, both retrieved 22 September 2026.
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