Homebrew Calculator (ABV from gravity, IBU Tinseth)

Understanding the homebrew ABV and IBU calculator — concept and principle

Homebrewers measure the specific gravity of wort before fermentation (original gravity, OG) and of the beer afterwards (final gravity, FG) with a hydrometer or refractometer. From these two numbers you can estimate alcohol content and how completely the yeast fermented the sugars. The calculator also estimates bitterness in International Bitterness Units (IBU) from a hop addition using Glenn Tinseth's widely used formula.

The drop in gravity from 1.050 to 1.010 means about 5.25% alcohol.

How it is calculated

ABV ≈ (OG − FG) × 131.25.
Apparent attenuation = (OG − FG) ÷ (OG − 1) × 100.
Tinseth utilisation U = 1.65 × 0.000125^(OG − 1) × (1 − e^(−0.04 × t)) ÷ 4.15, with t the boil time in minutes.
IBU = U × (alpha acid % ÷ 100 × hop grams × 1000 ÷ litres).

The last bracket is the added alpha acid in mg/L.

The principle

Fermentation turns dissolved sugar into ethanol and carbon dioxide. Sugar makes wort denser than water, while ethanol is lighter, so the fall in gravity tracks how much sugar was converted; 131.25 is an empirical factor that works well for ordinary-strength beers. The attenuation is called "apparent" because ethanol lowers the reading below what the remaining sugar alone would give. In Tinseth's model, hop alpha acids isomerise into bitter compounds with boil time (the exponential term levels off after about an hour), and higher wort gravity reduces how much is utilised (the "bigness" factor).

Worked example

ABV = (1.050 − 1.010) × 131.25 = 0.040 × 131.25 = 5.25 %.
Attenuation = 0.040 ÷ 0.050 × 100 = 80 %.
Bigness = 1.65 × 0.000125^0.050 ≈ 1.0528; time factor = (1 − e^(−2.4)) ÷ 4.15 ≈ 0.2191; U ≈ 0.2307.
Alpha acids = 0.06 × 30 × 1000 ÷ 20 = 90 mg/L.
IBU = 0.2307 × 90 ≈ 20.76.

Each point is the calculator re-run with only “Original gravity” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Original gravity	ABV
0.263	-98.11%
0.525	-63.66%
0.787	-29.2%
1.05	5.25%
1.31	39.7%
1.57	74.16%
2.1	143.06%

How to read the result

Typical ale yeasts reach roughly 70–80 % apparent attenuation. As benchmarks from the BJCP style guidelines, American light lager is about 8–12 IBU, American pale ale about 30–50 and American IPA about 40–70. Perceived bitterness also depends on malt sweetness: the same IBU tastes more bitter in a dry beer than in a sweet, strong one.

Common mistakes

Reading the hydrometer at a temperature far from its calibration temperature without correcting.
Using refractometer readings after fermentation without an alcohol correction.
Entering pre-boil volume or forgetting late hop additions, which contribute little IBU.

Keep in mind: Tinseth is an empirical model; real IBU varies with hop form, boil vigour, kettle geometry and fermentation. Here OG and batch volume are used as the boil gravity and volume, a common simplification.

Frequently asked questions

Is (OG − FG) × 131.25 accurate for strong beers?

It drifts above about 7–8 % ABV; more complex formulas are used for high-gravity beers.

Why does IBU barely increase after 60 minutes?

The time factor approaches its maximum; longer boils add little bitterness.

Do dry hops add IBU?

Very little by this formula, because they are not boiled.

Sources: Glenn Tinseth, "Glenn's Hop Utilization Numbers" (1997). · John Palmer, How to Brew (Brewers Publications). · BJCP Beer Style Guidelines (2021).

Checked 8 Nov 2026. Typical coverage rates and standard formulas. Always check the product label and local building rules; buy 5–10% extra for waste.

Your numbers