Understanding & Using Refractometer

Refractometers are widely used in the wine and beer industry to track the progress of fermentation, but they are less commonly used by homebrewers. However, if used properly a refractometer can be a great tool to track specific gravity in place of, or to supplement, your hydrometer. It is used primarily in brewing beer to determine the specific gravity readings of wort and beer.

The Advantages and Disadvantages of a Refractometer

Most homebrewers start out in the hobby measuring their original and final gravity using a simple hydrometer. The hydrometer is a weighted glass tube that you immerse in the wort or beer to directly read the gravity. A well-made hydrometer is fast and accurate, but it requires a fairly large liquid sample and also often needs temperature correction if working with hot or cold liquids.

A refractometer, in contrast, requires only a few drops of liquid, and also works very quickly. Because you can work with very small samples, which are easier to cool, temperature correction is a much smaller issue. The only major disadvantage with refractometers is that other than a relatively accurate pre-fermentation reading you can’t use the direct reading from the device to get an accurate specific gravity once the beer starts fermenting. Instead you typically have to type any reading containing alcohol into a brewing calculator available on many homebrew supplier websites or spreadsheet to convert it to an original or final gravity reading.

How it Works

A refractometer is a laboratory device that measures the index of refraction (RI) of a sample. The index of refraction is a dimensionless number that represents how light propagates through the sample. For those who remember their physics course it is simply the ratio of the speed of light in a vacuum c to the velocity of light in the sample v, and measures how light is bent or “refracted” in the sample. So the index of refraction is simply: RI = c/v

To put it in simpler terms, if you start with a clear glass of water, you will notice that the light passing through the water is bent in a certain way. The bending of the light is called refraction. Light bends to differing degrees as it passes through different substances because the light is slowed by the material.

This is the same effect that the lenses in a pair of eyeglasses are based upon — the light is bent to focus on the retina at the back of your eyes, compensating for any imperfections in the lenses in your eyes.

Now if we add sugar to the glass of water the light will actually bend more. Using a prism, the refractometer can measure these very small changes in the amount of bending (refractive index) giving us an indirect measurement of the amount of sugar in the sample.

Refractometers use a prism and light source to illuminate the sample. On inexpensive refractometers you typically just hold the instrument up to a natural light source, while more expensive models have a light source built in.

Given the refractive index and some knowledge of the material being measured you can actually use a variety of equations to determine various properties of the material. Because of this, refractometers are used in a wide variety of practical applications including measuring total plasma protein in blood, in gemology for identifying the type of stone, measuring salinity of sea and aquarium water, measuring coolants and oils used on automobiles, and hundreds of others.

In homebrewing, an equation is applied to determine the specific gravity of a sample — be it unfermented wort or fermenting beer. Most often it is used to measure original and final gravities.

Refractometer Needs Adjustment

You may be tempted to read the specific gravity or Brix scale on your new refractometer and use that as your original gravity and final gravity reading. For unfermented wort, you probably won’t be too far off, but for fermented beer you won’t get an accurate reading. To get an accurate original or final gravity, refractometer readings have to be corrected using software or a spreadsheet.

Refractometers are typically designed and built to measure the amount of table sugar (sucrose) in a solution of distilled water. Unfortunately the sugar in beer (maltose) is a different animal and has slightly different refractive properties. So there is a small error when we take an unadjusted Brix or gravity reading directly from the scale on a wort sample and use it.

The error gets much larger when we ferment our wort. After fermentation has started, the beer now has alcohol in it, which actually has a lower density and higher refractivity index than water. So if you are taking a final gravity reading, for instance, you will be significantly off if you use a raw reading.

To correct for both of these sources of error you need to calibrate your refractometer and use software, an online refractometer tool, or spreadsheet designed for beer brewing which can give you an accurate number for both unfermented and fermented liquids as described later in the section on using a refractometer. See the sidebar above for instructions on calibrating a refractomer.

Refractometer Types and Features

A quick search online reveals a wide range of refractometer features and prices. As with many purchases, a word of caution — you get what you pay for. Choosing a model that is a little more expensive will offer you more features, and will also probably not break as easily. Here are some of the various types of models and features to consider when you start shopping for a refractometer for
your homebrewery:

Types of Refractometers

There are several different types of refractometers sold for different applications. Here are a few of the common ones:

Handheld analog Refractometer

This is the least expensive and typical refractometer used for homebrewing. It is an analog device with a prism in it that has an angled sample slide on one side where you add a few drops of your wort or beer. On the other end is an eyepiece. They often have a focus ring (sometimes you rotate the entire eyepiece) to bring the scale into visual focus. To use it you add the sample and then hold it up to a light source and read from the analog scale by putting your eye up to the eyepiece. These are the least expensive option and suitable for home use. Price-wise they range from around $30 to $100.

Handheld digital Refractometer

These are handheld digital devices that have a built-in light source and digital sensor system. A few drops are added to a sample well and the system will illuminate and take the reading directly and show it on a digital display in a few seconds. The only downside is often these units require distilled or ionized water be added to the sample well to calibrate it for each use. They are slightly more accurate but also more expensive than the analog units — typically a few hundred dollars.

Laboratory Refractometers

Intended primarily for lab use, these units are large and typically sit on a desktop. They have their own light source and digital measuring sensors, and they provide greater accuracy than a typical handheld unit. They are also quite expensive! These units are typically several hundred to $1,000.


When you add your sample to be tested and look through a typical refractometer you will see a line formed along a scale in the device from which you take your reading (see the photo below). Older lab refractometers had only one scale, which usually measured the RI (Refractive Index) of the sample. Later a number of refractometers added a Brix scale, mainly for use in winemaking. Depending on the application the scale might also measure salinity or other readings. More recently we see refractometers with dual or even triple scales.

As refractometers have become more popular, manufacturers have started producing them with scales showing Brix, specific gravity, or degrees Plato. These are often sold as beer, wine, or brewing refractometers. While often these readings still require some calculation (read on for instructions for use), a refractometer with two scales in Brix and Specific gravity is probably the best choice
for brewers.

Note that Brix and Plato are nearly identical scales, and for practical purposes (such as in homebrewing) can be used interchangeably. There is a slight difference, but they differ only in the fourth decimal place for typical beer brewing ranges. For example 10 °Brix is equal to 10.001 Plato, which is close enough for me!

Automatic Temperature Compensation

Older refractometers did not have temperature compensation and users would calibrate to a fixed temperature of 68 °F (20 °C), so they were truly accurate only at that single temperature.

Newer refractometers typically have ATC or Automatic Temperature Compensation built in. This corrects for the temperature of the device itself and not the temperature of the sample (which is typically not a factor since it’s only a few drops). ATC is a useful feature, particularly if you are brewing on a hot or cold day and still want an accurate reading, and the cost has dropped to the point where ATC refractometers are very affordable. Note that ATC is typically only good within a limited temperature range, so you will need to operate within that range for accurate readings.

Using a Refractometer

Once your refractometer has been calibrated (see below for more on this), its zero is set properly, and we have our wort correction factor, let’s look at typical cases for measuring specific gravity with a refractometer:

Measuring Unfermented Wort (OG)

We treat unfermented wort the same as we did our calibration sample above. Clean your prism or sample well and add a few drops of the wort and then take your reading. You can use your software/spreadsheet now to adjust your unfermented wort reading based on the earlier calibration or calculate it by hand from the wort correction factor using this formula:

Adjusted gravity plato = refractometer reading brix / wort correction factor

Measuring Fermenting and Fermented Beer (FG)

Once fermentation has started, you can no longer use a single refractometer measurement to get an accurate gravity reading due to the effects of alcohol. This is because, as mentioned earlier, the sample has alcohol in it, which has a lower density and higher refractivity index than water. At this point, typically you need both an original gravity measurement taken earlier and also a current refractometer reading to get an accurate specific gravity reading for fermenting or fermented beer.

The equation to calculate fermenting beer gravity is a bit complex, so the quickest way to convert your raw refractometer reading to a specific gravity is to use some brewing software, an online calculator (many homebrew supply websites have free calculators available), using a spreadsheet, or using software like BeerSmith or Brewer’s Friend etc. There are many platforms available that will take your original gravity and current refractometer reading and convert it for accurate use.

a brewer peering down the scope of a refractometer

Using a Refractometer

Using a refractometer is simple:

  1. Before performing any samples with homebrew, check the calibrations of the instrument by placing a drop of distilled water on the glass prism (sample area), cover the lid with the daylight plate to spread the sample, making sure to avoid air bubbles, and wait 30 seconds to let the sample adjust to temperature. Point the sample end of the refractometer at a light source and look through the eye piece. You should see a graduated scale with the upper portion blue, the bottom portion white (see photo above). You might need to adjust the focus at this point. The refractomer, if calibrated properly, should read 0 °Brix. If it does not, see the sidebar on page 102 for instructions on calibrating.
  2. Using an eye dropper or other means of sampling small amounts of liquid, extract a few drops of your wort or homebrew to be sampled. Make sure that the glass sample area of your refractometer is clean and dry, then place a few drops of your sample onto the prism. Close the lid, again making sure to avoid air bubbles. Wait 30 seconds, then again aim the instrument toward a light source and look through the eye piece to see your reading. If you are taking a sample of wort before fermentation, you can use this reading directly. Once the beer starts fermenting, however, you will need to use either a brewing calculator, spreadsheet, or perform a wort calibration to get a proper reading as the sample will now contain alcohol, which has a lower density and higher refractivity index than water.

When you are not using your refractometer, wipe the sample area clean, dry it, and store it in a dry place (many models come with a zippered pouch or hard-shell case). Moisture can negatively affect the accuracy of the refractometer.

S.G. vs. Plato vs. Brix

Refractometers and hydrometers will provide you with readings for specific gravity, Plato, and Brix, depending on the model you choose. But what is the difference?

Specific gravity is the measure of the density of a liquid or solid as compared to that of water.

Plato is a measurement of the sugar in a solution as the weight of extract in a 100-gram solution at a temperature of 64 °F (17.5 °C).

Brix is a similar scale to Plato, but is based on a temperature of 59 °F (15 °C). It is used primarily by winemakers. Brix and Plato are nearly identical scales for practical purposes (like in homebrewing) and can be used interchangeably.

Most homebrewers in the US use a specific gravity for their measurements. However, many pro brewers use a combination of Plato and specific gravity.

To convert S.G. to Plato (up to about 1.050 SG), use the equation: °P = SG points/4

Calibrating a Refractometer

Before you use a handheld refractometer it needs to be calibrated. Generally two calibration steps are needed — one using distilled water to set the “zero” point on the refractometer, and a second test using some wort and a hydrometer to adjust for any variations due to maltose.

Calibration Dial or Screw

Virtually all modern refractometers have a calibration screw or dial that lets you adjust the zero line for the scale. Digital refractometers usually have a “zero” button for this purpose. You will use a sample of pure water to calibrate your refractometer.

Distilled Water Calibration

Clean and dry your sample slide or sample well first and then add a few drops of distilled or deionized water. If you are using a digital refractometer you can simple press the zero or calibrate button to calibrate it.

For a handheld refractometer, the unit should read 0 Brix or 1.000 SG on the scale when you hold it up to the light. If you have an adjustment screw or dial, adjust the zero on the scale until it properly reads 1.000 SG or zero Brix. If you have no adjustment dial, you’ll need to adjust the reading by manually subtracting the zero reading from each reading you take or use software or spreadsheet that lets you enter the zero calibration offset.

Wort Calibration and Wort Correction Factors

As mentioned earlier, most refractometers are calibrated to measure sucrose (table sugar) in water and not wort. A wort correction factor needs to be applied to adjust for this, even if you are measuring unfermented wort.

To determine the brix or wort correction factor you need to prepare a small sample of distilled water and dried malt extract (DME), typically 2 oz. (56 g) of DME in 8 ounces (227 g) of water, and then measure that sample using both a hydrometer and your refractometer. If you are using brewing software, you can usually enter the hydrometer and refractometer reading in the software and it will calculate and store the correction factor for you.

If you are not using software but you have a hydrometer that measures in Plato you can just divide the two: wort correction factor = refractometer reading brix / hydrometer reading plato

Take multiple readings and average them for more accuracy. Typically you will get a number around 1.04, but it could be different depending on the device used. Note: This assumes that you are using a calibration-grade hydrometer.

A typical handheld refractometer that can be found at most homebrew suppliers can cost anywhere from around $30 for a basic, inexpensive model, up to $100 or more for a more high quality and advanced model. Digital refractometers are also available, which cost a few hundred dollars.

Issue: December 2016