About this tool
The IBU Calculator is a free, in-browser homebrewing tool that estimates the bitterness of your beer in International Bitterness Units (IBU) from each hop addition. Enter your batch volume and boil gravity, pick the Tinseth or Rager model, choose whole or pellet hops, then fill in the weight, alpha-acid percentage, and boil time for up to three hop additions. It reports the IBU contributed by each addition, the total, and the bitterness-to-gravity (BU:GU) ratio. All math runs locally in your browser.
The Tinseth model computes hop utilization as U = bigness × boil-time factor, where bigness = 1.65 × 0.000125^(OG − 1) accounts for reduced extraction in high-gravity worts and the boil-time factor = (1 − e^(−0.04 × t)) / 4.15 captures how longer boils isomerize more alpha acids. IBU per addition = U × AA_decimal × grams × 1000 / liters. The Rager model instead uses U = 18.11 + 13.86 × tanh((t − 31.32) / 18.27) as a percentage, with a gravity adjustment (OG − 1.050) / 0.2 applied above 1.050. Selecting pellet hops multiplies utilization by about 1.10, since pellets extract more efficiently than whole cones.
As a worked example, 28 g of 6% alpha hops boiled 60 minutes in 20 L at OG 1.050 gives roughly 19 IBU under Tinseth and about 26 IBU under Rager — the two models legitimately differ, so pick one and stay consistent across recipes. Tinseth tends to read lower and is the more widely used default. Actual bitterness also depends on hop freshness, boil vigor, and whirlpool time, so treat the number as a well-calibrated estimate rather than a lab measurement.