How bitter will the beer be?

What IBU means

International Bittering Units measure the iso-alpha acids dissolved in a beer, in milligrams per liter. The alpha acids in hops are not bitter themselves; boiling turns them into iso-alpha acids, which are. A lab measures them by spectrophotometer. A home brewer predicts them: the weight and alpha acid of the hops are known, and the open question is how much of that alpha acid is converted and kept, which depends mostly on how long the hops boil and how strong the wort is.

The formula, step by step

Glenn Tinseth fitted curves to his own measurements and published the result as two factors: utilization = bigness factor × boil time factor. The bigness factor, 1.65 × 0.000125^(gravity − 1), shrinks as the wort gets stronger, because dense wort holds fewer iso-alpha acids. The boil time factor, (1 − e^(−0.04 × minutes)) ÷ 4.15, rises quickly at first and levels off, at about $25.2% after two hours in 1.050 wort. Then IBU = utilization × mg/L of added alpha acid, where mg/L = alpha acid fraction × hop weight × 1,000 ÷ liters (or × 7,490 ÷ gallons with ounces).

Worked example (John Palmer's). Five gallons; 1.5 oz of 6.4% hops boiled 60 minutes and 1 oz of 4.6% hops boiled 15 minutes; boil gravity 1.080. The first addition holds 143.8 mg/L of alpha acid at 17.6% utilization, which is 25.3 IBU. The second holds 68.9 mg/L at 8.7%, 6.0 IBU. The total is 31 IBU, the figure Palmer reaches by table lookup.

Limits of the method

It is a model fitted to one brewer's data. Tinseth says the constants are "empirically derived to fit my data" and that he makes no guarantees if you change them. Palmer adds that utilization numbers are an approximation and that variables such as boil vigor, wort chemistry and losses during fermentation are hard to pin down; the value of the model is that brewers who use the same one can compare their beers.

The gravity you enter matters. The same hops give 31 IBU in a 1.080 boil and 41 in a 1.050 boil. Tinseth wants the average gravity over the whole boil; Palmer uses the initial boil gravity, which he says overstates the total by 1 to 3 IBU.

Hop age and storage. Palmer notes that hops stored warm or in thin plastic bags can lose about half their bitterness potential in a few months. Use the alpha acid from the package date if you can, or a lower figure for old hops.

Late and dry hops. Additions at 0 minutes, in a whirlpool, or as dry hops are counted as zero here (the boil time factor is zero at zero minutes). They contribute flavor and aroma and some bitterness that this formula does not model.

Frequently asked questions

What is the Tinseth formula for IBU?

Glenn Tinseth's formula multiplies two things: the milligrams of alpha acid you put in each liter of beer, and a utilization fraction that says how much of that alpha acid ends up as bitter iso-alpha acid. The alpha acid is the hop weight times its percentage alpha acid, times 1,000 for grams and liters (7,490 for ounces and gallons), divided by the volume of the finished beer. The utilization is a gravity term, 1.65 × 0.000125^(gravity − 1), times a time term, (1 − e^(−0.04 × minutes)) ÷ 4.15. For 1.5 oz of 6.4% hops boiled for 60 minutes in 5 gallons at a boil gravity of 1.080 the alpha acid is 143.8 mg/L, the utilization 17.6% and the bitterness 25.3 IBU.

Which gravity should I use?

Tinseth says to use an average gravity of the wort for the whole boil, which changes as the volume boils away. John Palmer instead uses the initial boil gravity, because it is simpler and conservative; he says that overstates the bitterness by 1 to 3 IBU. If you boil only part of the wort, as in an extract batch, use the gravity of the boil, not of the finished beer: Palmer's example boils 6 pounds of extract in 3 gallons, so the boil is 1.080 even though the beer is 1.048. The same hops give 31 IBU at 1.080 and 41 IBU at 1.050.

Do pellets give more bitterness than whole hops?

A little, but no source we could open gives a number. Tinseth says his table is optimized for fresh whole cones loose in the boil and can be adjusted for other forms. In Palmer's table of hop forms, pellets give a "small increase in isomerization due to shredding". The Hop FAQ (Norm Pyle, 1995) has Al Korzonas advising 10% more hops when Rager's numbers, which are often applied to pellets, are used for whole hops in a bag. Many brewers add about 10% utilization for pellets, but we could not trace that figure to a primary source, so the calculator does not apply it by itself. Use the adjustment field if you want to: for the example hops at 1.050, +10% raises the total from 41.1 to 45.2 IBU.

Why does the Rager number differ?

Jackie Rager's method, published in Zymurgy, uses a hyperbolic-tangent fit of utilization against time, a correction that raises the divisor when the boil gravity is above 1.050 and a slightly different constant (7,462 instead of 7,490). It gives higher numbers for the 60-minute addition and lower for the 15-minute one: in the example above Rager gives 43.3 against 31.4 IBU. We took the Rager formula from the Hop FAQ and could not open Rager's original Zymurgy article, so it is shown for comparison only. Mark Garetz's method needs a table for each yeast and an iterative calculation, and is not included.

Can I change the formula to match my own beer?

Tinseth expects it. He says the number 4.15 controls the maximum utilization: "make it smaller if your kettle utilization is higher than mine". Palmer suggests 4 or 3.9 for a very vigorous boil, and raising it by a tenth or two if you get less. The only way to know your own number is to measure the beer in a lab, so use the formula to compare recipes and keep notes on the results. Hops kept warm in thin bags can lose half their bitterness in a few months (Palmer), so a package that is old can taste less bitter than its printed alpha acid says.

Sources: Glenn Tinseth, "Hop Utilization Page" (realbeer.com, no longer online; copy at web.archive.org, read October 28, 2026); John Palmer, How to Brew (free online edition), chapter 5 "Hops", sections "Hop Forms" and "Hop Bittering Calculations" (first edition); Norm Pyle (ed.), "The Hop FAQ" (1995, copy of the realbeer.com page), for the Rager and Garetz formulas and the pellet advice. Rager's article in Zymurgy and Garetz's book Using Hops were not opened. Neighbours: ABV calculator, beer color calculator, yeast pitch rate calculator. How we check our calculators.