A hydrometer is the simpler choice for checking finished beer. A refractometer is convenient for small samples on brew day, but alcohol changes its reading once fermentation starts. Neither instrument is automatically more accurate: calibration, temperature and the right calculation matter.
What each tool measures
| Feature | Hydrometer | Refractometer |
|---|---|---|
| How it works | Floats in liquid; buoyancy indicates density | Measures how light bends through a sample |
| Sample | Enough liquid to float freely in a test cylinder | A few drops on the prism |
| Before fermentation | Measures original gravity | Useful with a wort correction factor |
| After fermentation | Useful for final gravity | Needs alcohol correction and the original reading |
In denser liquid a hydrometer floats higher, rather than sinking farther. A refractometer does not float at all. Its Brix scale is based on sugar solutions, so a raw reading is not a direct alcohol measurement.
Get a dependable hydrometer reading
Check the instrument in distilled water at its stated calibration temperature. Take a sanitized sample, let the hydrometer float clear of the cylinder walls, and dislodge clinging bubbles. Read the bottom of the meniscus unless your instrument specifies otherwise. Cool the sample to the calibration temperature or use an appropriate temperature correction. Do not pour the sample back into the fermenter. The American Homebrewers Association explains the reading procedure.
Use a refractometer after fermentation correctly
Record the original reading before pitching yeast. Once alcohol is present, use that original value, the current reading and your wort correction factor in an alcohol-corrected refractometer calculator. A printed specific-gravity scale alone does not make this correction. Automatic temperature compensation also has an operating range; it does not mean a hot sample can be measured without care.
Estimate alcohol from gravity
A common homebrew approximation is ABV (%) = (OG − FG) × 131.25, using specific-gravity values such as 1.050. For OG 1.050 and FG 1.010, the estimate is 5.25% ABV. This is an estimate, with greater limitations for strong beers. See Brewer’s Friend’s explanation of ABV equations.
Which should you buy first?
For a beginner brewing occasional batches, a hydrometer and test cylinder make fermentation checks straightforward. Add a refractometer when small, frequent brew-day samples become useful. Before bottling, check that gravity is stable over several days and appropriate for the recipe; a stalled fermentation can also appear stable.
Next: how long brewing takes and how to pitch yeast.