Homebrewing is full of lore and dogma: you soak it in and just take it for granted. Most things make sense as you learn the hobby, but other things are more arbitrary. Look at any recipe, from beginner to more expert, and two numbers jump out as guideposts: original gravity (OG) and final gravity (FG). The numbers themselves are anything but random, but the measurement system we use is a different story. You might have noticed a clue with your first homebrewing kit. Almost every hydrometer is marked with three scales: specific gravity, degrees Plato or Brix, and potential alcohol, but homebrewers are steered toward specific gravity as the obvious choice. Professional brewers, on the other hand, usually use degrees Plato.
Even though you can convert between the two with varying accuracy, these two systems are not quite equivalent. Let’s take a closer look and then figure out whether we should care.
Three Degrees: Balling, Brix, and Plato
From the perspective of brewing, the Balling, Brix, and Plato scales are functionally equivalent scales that were derived by measuring the density of various concentrations of sucrose solutions (the Balling and Brix scales are used more in winemaking while the Plato scale is used in brewing). In each case, the measurements got a little more precise, but the differences among them are tinier than we can detect at home. If your wort or beer has a gravity of 12° Plato, that means that the density is the same as if it were a solution that was 12 percent sucrose by weight. Even though your beer is not a mix of simple sugar and pure water, this works because we tend to focus on comparing relative density measurements.
Specific Gravity
The specific gravity scale that most homebrewers use is based on comparing the density of your beer against that of pure water. So, a wort with an original gravity of 1.048 is 4.8 percent heavier than the same volume of pure water. It’s important to know that a floating hydrometer is affected by temperature; for accuracy, the measurement should be made at the reference temperature (usually 60°F/15.5°C). If the temperature is higher, your hydrometer reading will be lower than the actual gravity.
Converting Measurements
The standard rule of thumb for converting from degrees Plato to specific gravity is to multiply the Plato value by 0.004 and add 1. So, 12° Plato = 1.048. This is relatively accurate until you hit about 13° Plato, then it creeps up slightly with, for example, 19° Plato being 1.079. It’s easiest to use a table or brewer’s calculator for the conversion, but it’s worthwhile to know the more complex polynomial formula:
°Plato = (-205.347 * (SG^2)) + (668.72 * SG) – 463.37
Of course, now that you know the formula, you can use the tables with a clear conscience. ☺
Does it Matter?
For homebrewers, either scale will work equally well. Some will argue that degrees Plato is more accurate, but on a typical hydrometer your finest precision is about halfway between marks. This is close enough to offer little benefit either way.
There are a couple of reasons you might want to get comfortable with the Plato scale. If you’re a homebrewer thinking of going pro, Plato is the scale that the industry favors. You might also have to deal with it if you have sugar refractometer; these usually read in Brix/Plato. Since you’ll still need to use a hydrometer for final gravity, you’ll either want to convert your original gravity measurement or start using the Plato scale on your hydrometer.
Even if you decide to stick with homebrewing tradition and measure using the specific gravity scale as you’ve always done, at least now you’ll know what that other scale on your hydrometer is for.