Refractometer final gravity calculator

A refractometer measures how much light bends in your sample, and calibrated on sugar water it turns that into °Brix. Before fermentation that is a good proxy for gravity. Once yeast has started turning sugar into alcohol it isn't: alcohol bends light differently from sugar, so the same °Brix means a higher gravity than a plain conversion would tell you. Convert a finished beer's reading the ordinary way and you can be a long way out — a 14 °Brix wort that finishes at 7 °Brix is about 1.013, not the 1.028 the plain conversion claims. The Refractometer Final Gravity calculator applies a published correction for exactly that case.

It's free, needs no account, and computes in your browser.

Open the calculator

• On the public site: in the top navigation choose Tools, then on the Refractometer Final Gravity card choose Open calculator.
• Inside the app: in the navigation choose Tools, then on the Refractometer Final Gravity card choose Open calculator.

Estimate a final gravity

1. In Original reading (°Brix, before fermentation), enter what the refractometer showed on the unfermented wort — for example 14.0.
2. In Current reading (°Brix, now), enter what it shows now — for example 7.0. It can't be higher than the original; the field says so if it is.
3. Leave Wort correction factor at 1.04, or change it if you've worked out your own. Both readings are divided by this factor before the correction is applied. It exists because the refractometer is calibrated on sugar water and wort is not quite sugar water; 1.04 suits most instruments. To find yours, compare a refractometer reading of a finished beer against a hydrometer reading of the same sample and adjust until they agree.
4. Estimated final gravity appears as soon as the two readings and the factor are valid. Underneath it you'll see the simpler linear form of the same correction, the original gravity worked out from your first reading, and an approximate ABV.

If a value is out of range the field says so rather than showing a made-up answer.

Worked example

A wort reads 14.0 °Brix before fermentation and 7.0 °Brix now, with the default factor of 1.04. The calculator shows an estimated final gravity of 1.013, an original gravity of about 1.055, and about 5.5% ABV.

When the answer gets shaky

The correction was fitted to a set of real beers with original gravity from 1.036 to 1.106, final gravity from 1.007 to 1.022 and apparent attenuation between 73% and 91%. Outside any of those, the calculator adds a note saying which limit you crossed. The number is still the best published estimate, but it's an extrapolation — confirm with a hydrometer if it matters.

What it assumes

• Beer. The correction was developed on beer worts. It hasn't been tested on wine, and its author expects it to do worse for dry wines. It isn't offered for spirits.
• A reading taken after fermentation has started. For a reading of unfermented wort, the Brix to Potential Alcohol calculator and the ordinary conversion are the right tools.
• An estimate. The formula is Sean Terrill's refractometer final-gravity correlation (seanterrill.com, 2011). It's a reference estimate with no government standard behind it, and a calibrated hydrometer remains the better instrument when you need the final gravity precisely. For a hydrometer read on a warm sample, use the Gravity Temperature Correction calculator.