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Guide

How to use a hydrometer and calculate ABV

How to use a hydrometer: take OG and FG readings, read the meniscus, correct for temperature, know when fermentation is done and calculate ABV.

Updated November 2026 · 7 min read

A glass hydrometer with scale on a white background
Photo: Jacek Halicki, CC BY-SA 4.0, via Wikimedia Commons
The short version
  • A hydrometer floats higher in sugary wort and lower as yeast turns sugar into alcohol. The number on the stem is specific gravity (SG).
  • Take an original gravity (OG) reading before pitching and final gravity (FG) readings at the end.
  • Read at the bottom of the meniscus, at eye level, with the hydrometer floating freely.
  • Most hydrometers read true at 60°F (15.6°C); some at 68°F. Correct warmer or cooler samples.
  • Standard ABV = (OG − FG) × 131.25. The ABV calculator also shows standard drinks per serving.

Learning how to use a hydrometer takes five minutes and answers the three questions every brewer has: did I hit the recipe, is fermentation finished, and how strong is this beer? A hydrometer is a weighted glass float with a scale. In dense, sugary wort it rides high; as fermentation turns sugar into alcohol, which is lighter than water, it sinks lower.

This guide follows the American Homebrewers Association’s method for accurate readings, the temperature-correction polynomial printed on our ABV calculator, and Palmer’s How to Brew. A hydrometer is the only tool that can safely tell you when beer is ready to bottle.

Hydrometer scales: SG, Plato and potential alcohol

Most brewing hydrometers are triple-scale. Specific gravity is what recipes and calculators use in the US. Plato (or Brix) expresses the same thing as percent sugar by weight. Potential alcohol is a rough guide intended for wine; brewers use the SG difference instead.

What a reading tells you

ReadingScaleMeaning
1.000SGPure water at calibration temperature (check yours)
1.040–1.060SGTypical OG for everyday beers (about 10–15°P)
1.065 and upSGHigher-gravity beer; needs more yeast and oxygen
1.008–1.015SGTypical FG for many ales
Below 1.000SGPossible in very dry beers, cider and wine (alcohol is lighter than water)
°P ≈ 259 − 259 / SGPlatoConversion used by the pitch-rate calculator

Source: AHA; BJCP 2021 vital statistics; GarageBrewer formulas

How to use a hydrometer, step by step

  1. Sanitize the sampler

    Sanitize a wine thief, turkey baster or the auto-siphon. The AHA notes that every time you open the fermenter you add contamination risk, so sample only as often as you need.

  2. Fill the test jar

    Use a test jar wide enough that the hydrometer floats without touching the sides or bottom. Don’t use the plastic shipping tube unless the maker says it doubles as a test jar.

  3. Float and spin

    Lower the hydrometer in, give it a gentle spin to knock off bubbles, and wait until it settles. Make sure it isn’t clinging to the wall.

  4. Read at the meniscus bottom

    Get your eye level with the liquid surface and read the line at the bottom of the curve, not where the liquid creeps up the stem. Reading the top of the meniscus gives a value a few points too high, per the AHA.

  5. Take the temperature

    Measure the sample temperature and correct the reading if it isn’t at the calibration temperature.

  6. Record, taste, discard

    Write the reading in your log, taste the sample (it tells you a lot) and throw it away. Never pour it back into the fermenter.

    ThermoWorks ThermoPop 2 Instant-Read Thermometer
Two glass hydrometers lying on a dark cloth
Hydrometers with their printed scales.Photo: Agne27, CC BY-SA 3.0, via Wikimedia Commons
Printable brew-day log and recipe sheetThe log has OG, pre-boil and FG fields and calculates ABV from them.

Correcting for temperature

Liquids expand as they warm, so warm samples read lower than they should. Check the paper inside your hydrometer for its calibration temperature: older models are usually 59–60°F and newer ones up to 68–70°F, according to the AHA. A quick calibration check: distilled water at that temperature should read exactly 1.000.

True gravity of a sample that reads 1.050

Hydrometer calibrated at 60°F; correction polynomial from the ABV calculator

50°F: 1.01.050°F60°F: 1.11.160°F70°F: 1.11.170°F80°F: 1.11.180°F90°F: 1.11.190°F100°F: 1.11.1100°F
Source: GarageBrewer ABV calculator (hydrometer correction polynomial); Craft Beer & Brewing tools

Small differences don’t matter much, but a hot wort sample is off by several points. The cleanest fix is to cool the sample to near calibration temperature (set the jar in cold water) and then read. Never put a glass hydrometer into boiling wort.

ABV calculatorIncludes the temperature correction for 60°F and 68°F hydrometers, plus a refractometer mode.

Original gravity: did you hit the recipe?

Take the OG after the wort is chilled, topped up and well mixed, right before you pitch yeast. With extract partial boils, top-up water that hasn’t mixed fully is the most common source of a misleading reading; stir hard first.

Compare it with the recipe and with the style range. A few points low means a slightly lighter beer; it isn’t a problem. For all-grain, a big miss points to efficiency or volume, which the dilution and boil-off calculator can help correct next time.

All-grain brewers take one more reading: pre-boil gravity, from a cooled sample of the wort collected in the kettle. It tells you before the boil whether you are on track. Because the boil concentrates sugar, the gravity points multiply up as volume drops: 6.5 gal at 1.044 boiled down to 5.5 gal ends near 1.052. If the pre-boil number is low, a longer boil or a little dry malt extract brings it up; if high, a measured top-up of water brings it down.

Beer styles by the numbers (BJCP 2021 vital statistics)

StyleOGFGIBUColor (SRM)ABV
18A Blonde Ale1.038–1.0541.008–1.01315–283–63.8–5.5%
18B American Pale Ale1.045–1.0601.010–1.01530–505–104.5–6.2%
19A American Amber Ale1.045–1.0601.010–1.01525–4010–174.5–6.2%
15B Irish Stout1.036–1.0441.007–1.01125–4525–403.8–5.0%
21A American IPA1.056–1.0701.008–1.01440–706–145.5–7.5%

Source: BJCP 2021 Beer Style Guidelines. Ranges are for judging; your recipe can sit anywhere inside them. Color swatches are an on-screen approximation.

Final gravity: is fermentation done?

Fermentation is finished when the gravity stops changing, not when the airlock stops. Take a reading, wait 2–3 days, take another. If both match and sit near the expected FG, you can bottle or keg. If the beer is still well above the expected FG, it may be stuck: warm it a few degrees and rouse the yeast.

Expected FG comes from the yeast’s attenuation (percent of sugar it eats) and the recipe’s fermentability. Fermentis lists 78–82% apparent attenuation for US-05, which turns an OG of 1.050 into an FG around 1.009–1.011. Mash temperature, crystal malt and unfermentable extract can push the real number higher.

Avoid bottle bombsBottle only after the gravity reads the same 2–3 days in a row, measure priming sugar with a calculator, use standard bottles only to about 3 volumes, store them in a closed box at room temperature and refrigerate once carbonated.Source: BYO carbonation priming chart

How to calculate ABV

The standard homebrew formula is ABV = (OG − FG) × 131.25. It is accurate enough for most beers. Above about 6% ABV, the alternate formula (also in the calculator) reads a little higher and is often preferred for strong beers.

Worked ABV examples (standard formula)

OGFGABV
1.0401.0103.9%
1.0501.0125.0%
1.0601.0146.0%
1.0751.0157.9% (alternate formula about 8.3%)

Source: Brewer’s Friend ABV calculator (standard and alternate formulas)

Bottle conditioning adds a small amount of alcohol from the priming sugar, typically a few tenths of a percent.

Strength and standard drinksA US standard drink contains 14 g (0.6 fl oz) of pure alcohol, the amount in 12 oz of 5% beer. A 16 oz pour of 7.9% homebrew is about two standard drinks. Homebrew is for adults 21+; never drink and drive.Source: NIAAA: What is a standard drink?

Hydrometer vs refractometer vs digital hydrometer

A refractometer needs only a few drops, which is handy on brew day, but once alcohol is present its reading is skewed and needs a correction formula (the calculator includes Sean Terrill’s). Digital floating hydrometers such as the Tilt and RAPT Pill log gravity and temperature continuously; they are convenient trend trackers, and makers list their own accuracy (Tilt claims ±0.002 SG). Confirm the final reading with a glass hydrometer before bottling.

A handheld optical refractometer with rubber grip on a white background
A refractometer needs only a few drops of wort.Photo: Jacek Halicki, CC BY-SA 4.0, via Wikimedia Commons

Mistakes to avoid

  • Reading the top of the meniscus. Read the bottom; the top reads high.
  • Ignoring temperature. Hot samples read low; cool them or correct.
  • Bubbles on the stem. CO2 in fermenting beer lifts the hydrometer. Spin it, or pour the sample back and forth to degas.
  • A reading with the float touching the jar. It must float freely.
  • Unmixed top-up water. Stir before the OG reading.
  • Returning the sample to the fermenter. Discard it.
  • Bottling on a single reading. Two matching readings, days apart.

Which tool should you buy?

  • Every brewer: a glass triple-scale hydrometer and a test jar. Buy a spare; they break.
  • All-grain brewers: add a refractometer for quick pre-boil checks.
  • Data lovers or busy schedules: a digital hydrometer to watch the curve without opening the fermenter.
  • Everyone: an instant-read thermometer for temperature corrections.

Next, put the readings to work: how to bottle homebrew uses the FG check, and fermentation temperature explains why gravity sometimes stalls.

Gear picks for this guide: best hydrometers for homebrewing and best refractometers for brewing.

VerdictRead at eye level at the bottom of the meniscus, correct for temperature, and trust two matching final readings over any airlock. With OG and FG in hand, the standard formula or the ABV calculator gives you the strength, and the NIAAA standard drink tells you what a pour really contains.
ABV calculatorWork out ABV from OG and FG with the standard and high-gravity formulas, hydrometer temperature correction,…Printable brew-day log and recipe sheetFree brew-day log: record recipe, water and salts, mash, gravities, yeast, fermentation temperatures,…

Questions

How do you read a hydrometer?
Float it freely in a test jar, spin it to remove bubbles, then read at eye level where the flat liquid surface (the bottom of the meniscus) crosses the scale. Correct for temperature if the sample isn’t at the calibration temperature.
What should my hydrometer read in water?
1.000 in distilled water at its calibration temperature, usually 60°F (15.6°C) and on some models 68°F (20°C). If it reads differently, note the offset and apply it to every reading.
How do I know when fermentation is done?
When two readings taken 2–3 days apart are the same and close to the expected final gravity.
How do you calculate ABV from gravity?
ABV = (OG − FG) × 131.25. For example, OG 1.050 and FG 1.012 give about 5.0%.
Can I take a hydrometer reading of hot wort?
Not accurately, and you risk cracking the glass. Cool the sample to near the calibration temperature first, or apply a temperature correction.
Is a refractometer accurate after fermentation?
Not without a correction. Alcohol bends light differently from sugar, so post-fermentation refractometer readings need a formula such as Terrill’s and remain approximate.
Why is my final gravity high?
Common causes are a high mash temperature, lots of crystal malt or unfermentable extract, underpitching, low oxygen or a fermentation that got too cold. Warm the beer slightly and rouse the yeast before assuming it’s done.

Sources

  1. AHA: How to take an accurate hydrometer reading
  2. Brewer’s Friend: ABV calculator
  3. Craft Beer & Brewing: Brewing tools (hydrometer correction)
  4. Sean Terrill: Refractometer FG results
  5. Fermentis: SafAle US-05 product page (attenuation)
  6. BJCP 2021 Beer Style Guidelines
  7. NIAAA: What is a standard drink?