About this tool
IBU (International Bitterness Units) measures hop bitterness. Tinseth's formula (1995) accounts for boil time (longer = more isomerization), hop alpha acid percent, batch volume, and wort gravity (higher gravity reduces use). Most calculators use Tinseth as the standard.
How it works
Enter batch size and boil gravity, then up to 2 hop additions. The calculator computes use for each addition and totals the IBUs.
ABV, IBU, and SRM - what each measures
| Number | Stands for | What it measures | Typical range |
|---|---|---|---|
| OG | Original Gravity | Sugar before fermentation (density vs water) | 1.030 - 1.120 |
| FG | Final Gravity | Residual sugar after fermentation | 1.000 - 1.030 |
| ABV | Alcohol by volume | Alcohol % of finished beer | 3% - 12%+ |
| IBU | International Bitterness Units | Iso-alpha acids from hops in mg/L | 0 - 120 |
| SRM | Standard Reference Method | Color, light straw to opaque black | 1 - 40+ |
ABV formulas
The simple formula used by most calculators:
ABV = (OG - FG) x 131.25
More accurate for high-gravity beers (Daniels formula):
ABV = (76.08 x (OG-FG) / (1.775-OG)) x (FG / 0.794)
Above ~6% ABV the simple formula starts to underestimate by ~0.3-0.8%. Use the second formula for big stouts and barleywines.
IBU formulas
Bitterness from hops depends on alpha-acid content, boil time, and wort gravity. Two common formulas:
- Tinseth (most common): accounts for wort gravity reducing isomerization. Default in BeerSmith, Brewfather.
- Rager: older, doesn't penalize high-gravity worts as much.
Rule of thumb: an ounce of 6% alpha hops boiled 60 minutes in 5 gallons of 1.050 wort produces about 22 IBU (Tinseth).
SRM color reference
| SRM | Color | Example styles |
|---|---|---|
| 1-2 | Pale straw | Light American lager |
| 3-4 | Light yellow | Pilsner, witbier |
| 6-8 | Pale gold | Helles, blonde ale |
| 10-12 | Golden | American pale ale, English bitter |
| 14-17 | Light copper | American amber, ESB |
| 19-22 | Deep copper | American brown, Marzen |
| 25-30 | Brown | English brown, doppelbock |
| 35-40+ | Dark brown to black | Porter, stout, imperial stout |
Mash efficiency
Efficiency = sugar extracted / theoretical maximum. Most home brewers run at 65-75%; pro breweries hit 80-85%. Improving efficiency:
- Crush grain finer (but not flour - stuck mashes)
- Mash at 152-155°F (67-68°C) for balanced fermentability
- Stir mash every 15 min
- Use longer mash (75-90 min vs standard 60)
- Use larger volumes of sparge water (fly sparge vs batch)
- Higher mash pH (5.2-5.4) extracts better than 4.8 or 5.6
The formula explained
This calculator uses the following formula:
IBU = AA% × g × use × 1000 / (vol_l × 100); util = (1 - e^(-0.04t)) / 4.15 × bigness_factor
The reason this formula works is rooted in the underlying physics, finance, or biology of the problem. Behind every calculator is a published, peer-reviewed equation or a widely accepted convention. We do not invent formulas; we apply standard ones from textbooks, government tables, professional bodies, and academic literature.
If you are curious about the math, the simplest way to verify is to plug in two known numbers and compare against a known result. The calculator should match published examples to within rounding precision.
Frequently asked questions
IBU by beer style?
Light lager: 8-15. American pale ale: 30-50. IPA: 40-70. Imperial IPA: 60-100. Stout: 25-45. Pilsner: 25-40.
Why does gravity matter?
Higher gravity (more sugars) reduces hop use. The Tinseth bigness factor accounts for this.
First wort hops?
Treat as 60-min addition. Boil time approximation is fine for most calculators.
Dry hopping IBU?
Negligible. Dry hops don't isomerize without boiling. They add aroma + flavor, not bitterness.
Why is my homebrew flat?
Most common reason: not enough priming sugar at bottling. Standard is 4-5 oz dextrose for 5 gallons (5 vols CO2). Cold conditioning for 2 weeks helps carbonation absorb. If still flat after 3 weeks at room temp, the yeast didn't survive the bottling - re-pitch fresh yeast.
How accurate is the ABV reading from a hydrometer?
Within ±0.2% if both OG and FG are taken at the calibration temperature (usually 60°F / 15°C). Temperature corrections matter: a hydrometer reads ~0.001 too low for every 10°F above calibration. Use a refractometer for OG only (it's inaccurate post-fermentation due to alcohol).
What's the ideal fermentation temperature?
Ale yeasts: 65-72°F (18-22°C). Lager yeasts: 48-55°F (9-13°C). Higher temperatures speed fermentation but produce more fusel alcohols and esters - usually bad except for Belgian styles. Use a temperature controller for consistent results.
How long should beer condition?
Most ales: 2-4 weeks in the bottle/keg. Lagers: 4-8 weeks at 32-40°F. Big stouts and barleywines: 6-12 months improves them significantly. IPAs are the exception - drink them young, hop aroma drops fast.
Can I dry hop in primary?
Yes, but the active yeast scrubs hop oils. Better: transfer to a secondary or wait until primary fermentation is mostly done (gravity stable for 2-3 days). Dry hop 3-7 days at fermentation temperature.
How accurate is the IBU Calculator?
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
Is the IBU Calculator free to use?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads around the tool but not inside the calculation flow.
Are my inputs saved anywhere?
No. All inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but the analytics never see what you type into the form.
Can I use the IBU Calculator on my phone?
Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum.
Does the IBU Calculator work offline?
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
How do I report a bug or suggest improvement to the IBU Calculator?
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours.
Can I share results from the IBU Calculator?
Take a screenshot or copy the output. The page doesn't generate shareable URLs for specific calculations - inputs stay in your browser only.
Why are the results different from another ibu tool?
Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.