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A hydrometer and a refractometer both measure the sugar content of wort or beer, but they work on different physical principles, have different practical strengths, and serve different moments in the brewing process. I use both: the refractometer for quick readings during the mash and boil, and the hydrometer for final gravity readings after fermentation. Understanding which tool does what and why helps you spend money on the right instrument (or instruments) for your brewing style.
How each instrument works
Hydrometer
A hydrometer is a sealed glass tube weighted at the bottom, it floats in liquid, and the depth to which it sinks indicates the liquid’s density. Denser liquid (more dissolved sugar) holds it higher; less dense liquid lets it sink lower. Read the specific gravity where the liquid surface intersects the hydrometer scale. Requires a sample volume of 100–150 mL in a tall graduated cylinder for accurate reading. Must be temperature-corrected if reading at temperatures other than calibration temperature (usually 60°F or 68°F), use a hydrometer correction calculator or a combined hydrometer/thermometer.
Refractometer
A refractometer measures the bending (refraction) of light passing through a liquid sample. Higher dissolved sugar concentration bends light more sharply. The instrument has a prism and eyepiece, place 2–3 drops of wort on the prism, close the cover, and read the scale through the eyepiece. Results in Brix (a sugar concentration unit) which converts directly to specific gravity. Requires almost no sample and works on hot wort, just dip a stainless spoon, let the sample cool for 10 seconds on the spoon, apply to the prism.
The critical limitation of refractometers: alcohol
Refractometers are calibrated for sugar solutions. Alcohol has a different refractive index than sugar and water, it bends light differently. Once fermentation begins and alcohol is present in the beer, refractometer readings are inaccurate and cannot be read as specific gravity directly. A refractometer reading on fermenting or finished beer requires a correction formula (the Brix to SG correction for alcohol-containing solutions) to get the real gravity. For final gravity readings on finished beer, a hydrometer is more accurate and requires no correction formula.
Where each excels
- Refractometer strengths: Pre-fermentation only. Measuring mash gravity to track conversion; measuring pre-boil gravity; tracking boil concentration; checking OG of wort going into the fermenter. The 2-drop sample means no wasted wort and no waiting for large samples to cool. This is where the refractometer genuinely shines, during active brewing when speed and convenience matter.
- Hydrometer strengths: Final gravity during and after fermentation. Checking fermentation progress (is FG stable over 48 hours?). Taking OG if you don’t have a refractometer. Measuring anything where absolute accuracy matters more than convenience.
Which to buy first
If you’re buying one instrument: buy a hydrometer. It works accurately at all stages of brewing without correction and costs $6–12. A hydrometer is the more fundamental and universally applicable measurement tool.
If you can buy two: add a refractometer. The ATC (automatic temperature compensation) refractometer models ($20–40) compensate for temperature automatically up to 86°F, useful for reading hot wort samples. The Hanna Instruments HI 96811 or any ABBE-style ATC refractometer from homebrew suppliers works well. Calibrate with distilled water before first use (should read 0 Brix with distilled water; adjust the calibration screw if not).
Common Questions
How do I convert Brix to specific gravity?
For pre-fermentation wort (no alcohol present): SG = 1 + (Brix / (258.6 – (Brix/258.2 × 227.1))). For practical homebrewing purposes, the simpler approximation SG ≈ 1 + (Brix × 0.004) is accurate enough for Brix values under 20 (corresponding to SG under 1.080). A 12 Brix reading ≈ 1.048 SG; a 16 Brix reading ≈ 1.064 SG. Most brewing software (Beersmith, Brewfather, Brewer’s Friend) includes a Brix to SG converter, use these for reliable conversion rather than mental math. For post-fermentation readings with alcohol present, use the Sean Terrill correction formula (available as a calculator online) for accurate final gravity from refractometer readings.
