How to Read a Hydrometer: Brewing Gravity Measurements Explained

What a Hydrometer Actually Measures

A hydrometer measures the specific gravity (SG) of a liquid — that is, its density relative to pure water. Pure water at standard conditions has a specific gravity of exactly 1.000. When you dissolve sugar in water, as you do when you make wort from malt, the liquid becomes denser than pure water, and the SG rises above 1.000.

The hydrometer works on Archimedes' principle: it floats higher in denser liquids and lower in less dense ones. The weighted bulb at the bottom keeps it upright, and you read the gravity value where the liquid surface intersects the scale on the stem.

During fermentation, yeast consumes the dissolved sugars and converts them to alcohol (ethanol) and carbon dioxide. Since ethanol is less dense than water (about 0.794 g/mL compared to water's 1.000 g/mL), the liquid becomes less dense as fermentation progresses, and the hydrometer sinks lower. The final gravity reading, taken after fermentation, reflects the residual unfermented sugars, proteins, and dextrins remaining in the beer.

This is also why specific gravity readings alone don't directly measure alcohol — you need to know both where you started (OG) and where you ended (FG) to calculate how much sugar was fermented and how much alcohol was produced.

The Three Scales on a Brewing Hydrometer

Most brewing hydrometers include three scales printed on the paper insert inside the glass tube. Understanding all three helps you get more out of a single tool:

1. Specific Gravity (SG) — The Primary Scale

This is the main scale, ranging from approximately 0.990 to 1.120 on most brewing hydrometers (some go higher for winemaking). Readings are expressed as four-decimal numbers like 1.048 or 1.012. This is the scale used for all standard brewing calculations including ABV. Key reference points:

• 1.000 = pure water
• 1.030–1.040 = session beer / light lager range
• 1.040–1.060 = typical ale OG range
• 1.060–1.075 = strong ale / IPA range
• 1.075–1.100+ = high-gravity: barleywine, imperial stout, strong cider
• Below 1.015 = most ales are considered fully fermented
• Below 1.010 = well-attenuated, dry finish

2. Potential Alcohol Scale

This secondary scale, running from 0 to 15% (sometimes 0–20%), is a rough guide to what ABV the wort could produce if fermented completely to 1.000. It's not a precise calculation — it assumes 100% attenuation to 1.000, which never happens in practice. Use it as a ballpark only; always calculate ABV properly using (OG − FG) × 131.25.

3. Brix Scale

The Brix scale, running from approximately 0 to 30°Bx, measures sugar concentration in degrees Brix — a unit where 1°Bx = 1 gram of sucrose per 100 grams of solution. It's more commonly used in winemaking and cidermaking where the starting ingredient (grape juice, apple juice) is often described in Brix. The conversion is approximately: SG ≈ 1 + (Brix / 258.6) for brewing purposes, or more precisely using the formula SG = 1 + 0.004 × Brix for ranges typical in brewing.

How to Take an Accurate Hydrometer Reading

Technique matters — a sloppy reading can be off by 2–5 gravity points (0.002–0.005 SG), which translates to meaningful error in your ABV calculation. Follow these steps every time:

1. Sanitize everything. Rinse your hydrometer and trial jar with Star San or your chosen no-rinse sanitizer. Never introduce unsanitized equipment into your wort or fermenting beer.
2. Pull a representative sample. Use a wine thief or turkey baster to draw at least 150–200 mL of liquid into a tall, narrow trial jar or test tube. The sample needs to be deep enough that the hydrometer floats freely without the bottom touching.
3. Degas the sample. Gently stir or spin the hydrometer to dislodge CO₂ bubbles clinging to the glass. Bubbles make the hydrometer float higher than it should, giving a falsely high reading. You can also pour the sample between two containers a few times to degas it.
4. Read at eye level. Hold the trial jar at eye level — not looking down at an angle. You'll see a curved surface called the meniscus where the liquid meets the hydrometer stem. Read the scale at the bottom of the meniscus curve, where the flat surface of the liquid would be if it weren't curving up at the glass.
5. Note the temperature. Record the temperature of the sample at the time of reading. You'll need this for the temperature correction below.
6. Record immediately. Don't trust your memory — write it down in your brew log.

Common mistakes: reading at the top of the meniscus instead of the bottom (gives a reading that's 1–3 points too high), not degassing (same effect), and reading at an angle from above or below.

Temperature Correction: Getting Accurate Readings at Any Temperature

All hydrometers are designed and calibrated to give accurate readings at a specific temperature — almost universally 60°F (15.5°C) for brewing hydrometers sold in North America (some European models are calibrated at 68°F/20°C — check your instrument's documentation).

Liquid is more dense when cold and less dense when hot, so the same wort will give a different gravity reading depending on its temperature. If you read your freshly boiled, chilled-but-still-warm wort at 75°F, the reading will be slightly lower than the true gravity because the wort is still a bit warmer than calibration.

Quick Correction Rule

As a fast approximation: add 0.001 for every 10°F above 60°F. So if you read 1.050 at 80°F, the corrected gravity is approximately 1.052.

More Precise Correction Table

• 50°F (10°C): subtract 0.001
• 60°F (15.5°C): no correction needed
• 70°F (21°C): add 0.001
• 77°F (25°C): add 0.002
• 86°F (30°C): add 0.003
• 95°F (35°C): add 0.005
• 104°F (40°C): add 0.007

For even more precision, use a dedicated temperature correction calculator — these use the full polynomial correction formula. At typical homebrewing temperatures (65–80°F), the error without correction is small (1–3 points) but worth correcting for accurate ABV calculations.

Practical tip: It's often easiest to cool your sample in a cold water bath until it reaches room temperature (68–72°F) before reading. This minimizes the correction needed and reduces the risk of error.

When to Take Your OG and FG Readings

Original Gravity (OG) — Before Pitching

Take your OG reading after you've finished brewing, chilled the wort to your pitching temperature (usually 65–75°F for most ales), and transferred it to your fermentor. At this point, the wort should be well mixed — if you added late-boil adjuncts or partial-boil extract topping water, stir thoroughly before pulling a sample to ensure the gravity is uniform throughout the vessel.

If you brew all-grain, your OG reading tells you how efficient your mash and sparge were. If you hit lower than target OG, you can add dry malt extract to bring it up. If you're significantly over OG, you can dilute with pre-boiled or filtered water.

Final Gravity (FG) — After Fermentation Completes

Patience is essential here. Most homebrewers make the mistake of taking an FG reading too early. Wait until you see no visible signs of fermentation activity — airlock bubbling has stopped, krausen has dropped — then wait at least 2–3 more days. Take a reading, record it, then take another reading exactly 48 hours later.

If both readings are identical, fermentation is complete. If the gravity has dropped even slightly between the two readings, wait another 48 hours and check again. A reading dropping from 1.013 to 1.012 between checks means fermentation is still happening, however slowly — package that beer prematurely and you risk over-carbonation or worse, bottle bombs.

Refractometer vs. Hydrometer: A Practical Comparison

Both tools measure sugar concentration but use different physical principles. Here's how to decide when to use each:

Refractometer Advantages

• Only needs 2–3 drops — great for pre-boil and mid-boil gravity checks during the brew day without pulling large samples
• Reads in seconds, no waiting for temperature equilibration if using an ATC (Automatic Temperature Compensation) model
• No breakable glass tube to worry about dropping in the brew pot
• Great for checking gravity at multiple points in your process

Refractometer Limitations

• Cannot be used accurately on fermented beer without correction. Alcohol (ethanol) has a different refractive index than water and sugar, causing significant reading errors — typically showing 3–8 gravity points higher than actual FG on a normally-fermented beer
• Requires calibration with distilled water before each use
• More expensive ($20–$60 vs $5–$15 for a hydrometer)

Best Practice

Use a refractometer for convenient OG measurements during the brew day. Switch to a hydrometer (sanitized) for FG readings once fermentation is complete. If you want to use only a refractometer for everything, use a refractometer-to-hydrometer correction calculator (also known as a Novotny or Sean Terrill correction formula calculator) for your FG readings.

Cleaning and Sanitation of Your Hydrometer

A hydrometer that isn't properly cleaned and sanitized is a contamination risk. Wild yeast and bacteria can be introduced into your fermentor on an unsanitized instrument and ruin the batch.

• Before use: Rinse with Star San solution (1 oz per 5 gallons water, no-rinse) or a similar sanitizer. Let it drain but don't rinse with water.
• After use: Rinse with clean water, then clean with warm water and a small amount of dish soap if there's visible residue or dried wort. Rinse thoroughly with clean water.
• Storage: Keep your hydrometer in its protective plastic tube when not in use. These instruments are fragile — a single drop onto a tile floor typically shatters them. Keep a spare on hand.
• Checking calibration: Periodically test your hydrometer in a sample of distilled water at 60°F. It should read exactly 1.000. If it reads 0.998 or 1.002, note the offset and apply it as a correction to all future readings.

Frequently Asked Questions

How do I read a hydrometer for homebrewing?

Fill a sanitized trial jar with your sample, lower the hydrometer in and spin it to release CO₂ bubbles, then hold the jar at eye level. Read the scale at the bottom of the meniscus — the lowest point of the curved liquid surface where it meets the hydrometer stem. Record the temperature alongside the reading. If the sample isn't at 60°F (the calibration temperature), apply a temperature correction to get an accurate gravity value.

What is a normal final gravity for beer?

For most ales, a normal final gravity falls between 1.008 and 1.016. Highly attenuative yeasts like Chico strain (US-05, WLP001) can finish ales as low as 1.005–1.008. Belgian and wheat beer yeasts often finish 1.008–1.012. Malt-forward styles like Scottish 80/- or oatmeal stout might finish 1.014–1.020. A reading above 1.020 on a standard session ale suggests stuck fermentation.

How do I correct hydrometer readings for temperature?

If your hydrometer is calibrated at 60°F (most North American brewing hydrometers are), add approximately 0.001 to your reading for every 10°F above 60°F. A sample at 80°F reads 1.050 on the hydrometer? The corrected gravity is approximately 1.052. For the most accuracy, use a temperature correction calculator, or simply cool your sample to room temperature before reading.

Can I use a refractometer instead of a hydrometer?

For OG measurement, a refractometer is excellent — it only needs a couple of drops and gives instant readings. However, for final gravity, a refractometer gives inaccurate results on fermented beer without applying a special correction formula, because alcohol distorts the light differently than sugar. The safest practice is to use a properly sanitized hydrometer for FG readings. If you insist on using a refractometer for FG, you must use a refractometer correction calculator to account for the alcohol present.

Why is my beer still showing a high gravity?

A stuck fermentation — where gravity stops dropping before reaching the expected FG — usually has one of these causes: the fermentor temperature dropped below the yeast's minimum range, the yeast was underpitched, high osmotic stress from very high OG, or the wort has an unusually high percentage of unfermentable dextrins from a high mash temperature (above 158°F). Try moving the fermentor to a warmer location (2–4°F above current), gently swirling to rouse settled yeast, or pitching a fresh highly-attenuative yeast like Lallemand Nottingham with a small nutrient addition.