Brewing decisions, explained
Hydrometer vs refractometer for final gravity
A hydrometer measures density; a refractometer measures refraction. After fermentation starts, raw refractometer Brix is not a direct final-gravity reading because alcohol changes the optical response.
BrewMyBeer editorial · Sources checked 24 September 2026 · Calculated examples, not laboratory measurements. How we create content
What each instrument actually measures
A floating hydrometer indicates density relative to water. Its useful accuracy depends on scale resolution, calibration, sample temperature and reading technique. Bubbles attached to the instrument can disturb the reading.
A refractometer uses a small sample and can be convenient for following wort before fermentation. Its Brix scale represents a reference sugar solution; wort composition and the instrument’s correction factor also matter. Neither device automatically supplies a laboratory-grade alcohol measurement.
Why final gravity needs a different treatment
Fermented beer contains alcohol as well as remaining extract. Converting final Brix directly with a sugar-only SG table ignores that change. Use a documented fermentation correction with the original and current readings, and keep the model name with the result.
Without a dependable original reading, avoid reverse-engineering a convenient value from the recipe target. A hydrometer cross-check can be more useful than a confident calculation from missing inputs.
A repeatable measurement checklist
Follow the instrument’s instructions for calibration solution, temperature range, cleaning and reading the scale. Let a hot sample reach an appropriate measurement temperature; automatic temperature compensation does not make every sample temperature acceptable.
For a hydrometer, use enough liquid for it to float freely, remove bubbles and read at the specified meniscus. For a refractometer, use a clean prism and a representative sample. Record repeat readings before interpreting a small difference as fermentation progress.
Resolve disagreement before changing the recipe
Check units first: SG, °Bx and °Plato are not interchangeable inputs. Then check temperature, calibration and whether an alcohol correction was applied. Compare repeatability rather than assuming one device always wins.
There is no universal accuracy figure for every hydrometer or refractometer. Use the model’s specifications and your measurement method. Do not treat extra decimal places from software as extra measurement precision.
Before you use the result
- Raw fermented Brix is not FG.
- Read the calibration temperature on your specific instrument.
- A predicted recipe OG is not a measured original sample.
A quick learning check
Does automatic temperature compensation remove the need for alcohol correction?
Try a craft decision game →Sources & further reading
These references explain the methods and limitations. The worked examples and learning questions on this page are BrewMyBeer calculations, not claims of hands-on testing.