How to accurately use a refractometer to check specific gravity (ABV)

Thursday, November 6, 2025


Using a Refractometer in Homebrewing

What it is, why it matters, how to use it well, and how to run the numbers with confidence

What is a refractometer

A refractometer is a tool used for measuring concentrations of aqueous solutions. It has many applications across food, agricultural, chemical, and manufacturing industries. 

A refractometer can be used to measure things like the total plasma protein in a blood sample, the salinity of water and even the amount of water content in honey.

They work by measuring how much a beam of light bends when it moves from air into your sample. 

That bend is refraction. 

The device views this bend through a prism and projects it onto a scale you can read through the eyepiece.

The Brix scale is the common readout. Brix indicates the apparent percentage of sucrose by mass in water. Wort is not pure sucrose, it is mostly maltose, maltotriose, and dextrins, so brewers apply a wort correction to make Brix more honest for beer.

 

beer brewing refractometerA handheld Brix refractometer used by homebrewers

Why brewers use refractometers

  • Speed and tiny samples. A drop or two is enough. Great for small batch brewing.
  • Instant feedback during the mash and boil. You can track gravity rise during a boil or confirm pre-boil gravity quickly.
  • No fragile hydrometer cylinders to fill. Less risk and less cleanup.

The tradeoff is calibration and correction. Alcohol changes how light bends, so post-fermentation readings need math. 

Some brewers prefer hydrometers to calculate ABV, especially for final gravity. 

Either tool works if you understand what it is telling you.

The science in plain language

Refraction follows a physical rule called Snell’s Law. Different solutions bend light by different amounts because dissolved sugars change the solution’s optical density. 

A prism inside the refractometer spreads the light and gives a crisp boundary line across the internal scale. Where that boundary falls is your Brix. For brewing we interpret Brix as a proxy for sugar concentration, then convert to specific gravity with corrections for wort composition and, if applicable, alcohol.

What kind of refractometer to buy?

Use a unit built for sugar solutions with a Brix scale. Fruit-grower models work well. 

A model with ATC, automatic temperature compensation, is worth it because it normalizes readings to a reference temperature. Digital handhelds are convenient, analog optical units are rugged and inexpensive. Check out some options on Amazon.

Calibrating the refractometer

Just like when you use a pH meter, refractometers need to be calibrated.

  1. Clean the prism glass and cover plate, then dry with a lint-free tissue.
  2. Add a drop of distilled water to the prism. Close the cover so the drop spreads fully without dry spots or bubbles.
  3. Wait about 30 seconds so sample and instrument equalize. This matters because readings are temperature dependent, even with ATC.
  4. Point the window toward a bright natural light. Look through the eyepiece and focus the scale if needed.
  5. Use the calibration screw to set the boundary line to exactly zero Brix. You are ready to measure wort.

How to take a beer wort reading

  1. Rinse and dry the prism. Place a small sample of wort on the glass, enough to fully wet the surface.
  2. Close the cover plate gently. Check for full coverage and no trapped air. Give it half a minute to reach instrument temperature.
  3. Point the refractometer toward a bright light. Hold it level. Read the Brix value at the sharp boundary line.
  4. Record the value and the measurement point, mash, pre-boil, post-boil, or post-fermentation. Repeat once to confirm.

A reminder about safety is obvious but important. Do not stare at the sun.

From Brix to specific gravity before fermentation

Because the Brix scale is defined on sucrose, wort readings need a wort correction factor. Many brewers find their factor falls around 1.04, but you should determine yours for best accuracy. 

The typical workflow is simple.

  1. Measure Brix of wort, call it Braw.
  2. Apply your wort correction factor, WCF, to get Bcorr = Braw ÷ WCF.
  3. Convert corrected Brix to specific gravity, SG.

A commonly used conversion from Brix to SG for pre-fermentation wort is shown below. It is an empirical fit widely used in brewing software.

SG ≈ 1 + Bcorr / (258.6 − 227.1 × Bcorr / 258.2)

Worked example. You read 14.6 Brix on hot-side wort. Your WCF is 1.04. Bcorr = 14.6 ÷ 1.04 = 14.04. Plugging into the formula gives SG ≈ 1.057. That is your pre-fermentation gravity.

You can determine your personal WCF by taking parallel hydrometer and refractometer readings on several pre-fermentation worts and solving WCF = Braw ÷ Bhydro-equivalent. Average across batches for stability.

Or follow this guide to learn how to apply a wort ‘correction factor’.

Measuring during and after fermentation

Alcohol lowers refractive index, which pushes refractometer readings in the opposite direction to sugar. A straight Brix to SG conversion will be wrong once fermentation starts. 

Use a post-fermentation correction that combines your original refractometer reading with your current reading.

Let Ob = corrected original Brix before fermentation
Let Fb = current raw Brix reading during or after fermentation
Compute an alcohol-corrected final gravity, SGf, using an empirical fit:

SGf ≈ 1.001843 − 0.002318474·Fb − 0.000007775·Fb2 − 0.000000034·Fb3 + 0.00574·Ob + 0.00003344·Ob2 + 0.000000086·Ob3

This polynomial is the basis of many brewing calculators. It is not perfect for every wort, but it is reliable across typical gravities.

ABV estimate can then be calculated as 131.25 × (SGo − SGf) for a quick estimate, where SGo is your original gravity from the pre-fermentation conversion above.

Example fermentation math

You recorded Ob = 14.0 Brix corrected pre-fermentation, which was SGo ≈ 1.057. A week later your refractometer reads Fb = 7.2 Brix. Plug those into the polynomial to get SGf ≈ 1.012. Estimated ABV ≈ 131.25 × (1.057 − 1.012) ≈ 5.9 percent.

Practical checkpoints on brew day

  • Mash progress. Track Brix every 10 to 15 minutes. A plateau suggests conversion is mostly complete. Stir before sampling to avoid stratification.
  • Pre-boil gravity. Confirms your mash efficiency. Adjust boil length to hit target gravity.
  • Post-boil gravity. Confirms the final concentration. Cool a small sample or at least allow it to sit on the prism to approach instrument temperature so ATC does not work too hard.

Care, cleaning, and accuracy

  • Rinse immediately after use with warm water. Sugars dry sticky and can scratch if wiped aggressively.
  • Use a soft lens tissue. Paper towels can score the prism surface.
  • Cap the instrument and store it dry. Do not leave sticky wort under the cover plate hinge.
  • Re-check zero at the start of each brew day. Temperature, travel, and time can drift calibration.

Common mistakes to avoid

  • Hot samples. Very hot drops can give skewed results. Let a teaspoon of wort cool briefly in a small dish before the reading.
  • Bubbles and hop oils. Skim foam and avoid oily hop bits. They distort the boundary line.
  • Skipping wort correction. Always apply WCF before converting to SG on pre-fermentation readings.
  • Ignoring alcohol correction. Post-fermentation Brix must be corrected with the polynomial method.

Quick reference conversion samples

Raw Brix Corrected Brix, WCF 1.04 Approx. SG pre-ferm
12.011.541.046
14.614.041.057
18.017.311.071
20.019.231.079

Frequently asked questions

Can I use a refractometer for final gravity? Yes, with a correction. Take your current Brix, combine it with your original corrected Brix, and use the post-fermentation formula above to get SGf.

Do I still need a hydrometer? It helps for spot checks and for calibrating your personal wort correction factor. Many brewers keep both tools on hand.

How often should I calibrate? Check zero before every brew day and after any knocks or big temperature swings.

History note

The first refractometer was invented by Enst Abbe. It was a complex device with built-in thermometers and a circulating water jacket to control temperature. Modern handhelds are simpler and digitised, the principle is the same. Light bends in a predictable way, you read where the bend lands.

Leave a Comment , ,

0 comments:

Post a Comment

Powered by Blogger.

Tags

absorption caps abv acetaldehyde acid adjuncts advice about beer brewing aeration aeration kit aging air lock alcohol alcohol poisoning ale ale beer kits alkaline alkaline brewery wash all grain american amylase apera apples attenuation autolysis automatic temperature compensation bacteria baker's yeast baking yeast ball lock ball valve bar keepers friend barley batch prime beer brewing beer capper beer dispenser beer filtration kit system beer gushers beer kit beer kit review beer kits beer lines beer salt beer taps beerstone best brewing equipment biotin bittering BKF black rock bleach blichmann blow off tubing bluelab bohemian pilsner boil in a bag boil over boneface bottle cap bottle caps bottle conditioning bottling bottling beer bottling spigot bourbon brettanomyces brew and review brew day brewing beer guide brewing salts brewing spoon brewing sugar brewing thermostat british thermal unit brix brix scale BTU budvar buffer buffer solution burton snatch buyer's guide calcium chloride calcium sulphate calibration calibration probe calibration solution campden tablets capping carbon dioxide carbonation carbonation drops carboy cascade caustic soda cherry wine chinook chlorine christmas chronicle cider clarity cleaning your equipment clear beer clone recipe cloudy beer cold crashing coldbreak conditioning tablets conductivity conical fermenter contamination coopers copper tun corn sugar cornelius corny keg craft beer creamy beer crown cryo hops cubes danstar nottingham demijohn dextrose distilation DIY DME dopplebock draught dry hopping dry malt extract edelmetall brü burner eisbock ekuanot electrode enhancer enzyme equipment ester ethanol experiments in beer making faucet fermcap-s fermentables fermentation fermenter fermentis fermentor final gravity finings five star flat beer floccing foam inhibitor force carbonation french fresh wort pack fridge fruit fusel alchohol garage project gas burners gelatin gift and present ideas gin ginger beer glucose golden ale golden syrup goldings gose grain grain mill green bullet grist guinness gypsum hach hacks hallertauer heat mat heat pad heat wrap home brew honey hop schedule hops hops spider how not to brew beer how to brew that first beer how to brew with a beer kit how to grow hops how to make a hop tea how to wash yeast hydrated layer hydrogen sulfide hydrometer IBU ideas idophor infection inkbird instruments isoamyl acetate jelly beans jockey box john palmer juniper keezer keg cooler keg regulators kegco kegerator kegging kegs kettle kombucha krausen lactic acid lager lagering lauter lion brown liquid malt extract litmus LME lupulin lupulin powder lupuLN2 making beer malic acid malt malt mill maltodextrin mangrove jack's maple syrup mash mash paddle mash tun mccashins mead methanol micro brewing milling milwaukee MW102 mistakes mixing instructions moa mouth feel muntons must nano brewing New Zealand Brewer's Series no rinse nut brown ale oak oak wood chips off flavors original gravity oxygen pacific gem palaeo water pale ale panhead parsnip PBW pear pectine pectolase perlick ph levels ph meter ph pen pH strips ph tester pico brewing pilsner pitching yeast plastic drum poppet valve pot powdered brewing wash ppm precipitated chalk pressure relief valve priming prison hooch probe problem solving propane and propane accessories pruno pump system purity law radler re-using yeast recipe record keeping reddit refractometer reinheitsgebot removing beer labels from bottles review rice hulls riwaka rotten eggs saaz saccharomyces cerevisiae salt sanitization secondary regulator sediment seltzer session beer silicon simple tricks for brewing siphon site glass skunked beer small batch brewing soda soda ash soda stream sodium carbonate sodium carbonate peroxyhydrate sodium hydroxide sodium metasilicate sodium percarbonate sour beer sparge spigot spirals spirits spoon spraymalt star san starch STC-1000 steinlager steralisation sterilisation sterilization sterliization still stoke storage solution stout sucrose sugar supercharger tannins temperature temperature controller therminator thermometer tips for beginners tri-sodium phopsphate tricks and tips trub tubing tui turkey vodka infused gin vorlauf water water testing wet cardboard taste wet hopping weta whirlfloc tablets white claw williamswarn wine winter brewing wood wort wort chiller yeast yeast energizer yeast nutrient yeast rafts yeast starter yeast traps zinc

About the author Jimmy Jangles


My name is Jimmy Jangles, the founder of The Astromech. I have always been fascinated by the world of science fiction, especially the Star Wars universe, and I created this website to share my love for it with fellow fans.

At The Astromech, you can expect to find a variety of articles, reviews, and analysis related to science fiction, including books, movies, TV, and games.
From exploring the latest news and theories to discussing the classics, I aim to provide entertaining and informative content for all fans of the genre.

Whether you are a die-hard Star Trek fan or simply curious about the world of science fiction, The Astromech has something for everyone. So, sit back, relax, and join me on this journey through the stars!

Copyright How to Home Brew Beers.

Back to Top