How to Measure Alcohol Content Without Original Gravity

by Dane Wilson | Last Updated: July 5, 2022

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You have gotten through the whole process of brewing up a new batch of beer when—oh no—you have forgotten about measuring the original gravity of the beer. Now how are you supposed to figure out how strong your beer has become? If you are curious, then you need to know how to measure alcohol content without original gravity. While getting the original gravity after the fact can be challenging, there are a couple of ways to get a close estimation.

Here is how to measure alcohol content without a hydrometer and get the original gravity.

Table of Contents

• What is Original Gravity?
• Step By Step: Measuring Alcohol Content Without Original Gravity
  • 1. Attenuation Rate and Final Gravity
  • 2. Fire Up The Refractometer
    • Calibrate Your Refractometer
    • Measure Specific Gravity
    • Use The Alcohol By Volume Formula
• Are There Any Other Methods For Calculating ABV?
• OG Can’t Hold You Down
• FAQs

What is Original Gravity?

Unless you read a whole book on the basics of homebrewing, the actual definition of original gravity (OG) may have eluded you. Original gravity is not the kind of gravity spoken about in physics class. No, this gravity is all about beer, as it measures the fermentable and non-fermentable substances present in wort. The substance in question is sugar.

Brewers use OG for a couple of reasons. First, to predict how much alcohol will be produced post-fermentation. After fermentation, the gravity is called Final Gravity—or the alcoholic strength of a beverage.

But if you forget to measure OG before popping your wort into the fermenting chamber, you may be at a loss. The good news is that you can measure alcohol content without original gravity.

Step By Step: Measuring Alcohol Content Without Original Gravity

There are a couple of routes you can use to measure the alcohol content of your final product without the OG. Sometimes the recipe you are using will tell you how much ABV to expect at the end. You may be able to go with your gut, but if you need to know the exact amount of alcohol, you are going to need a calculator and one of the methods below. Take a look:

1. Attenuation Rate and Final Gravity

Let’s say you missed out on measuring the original gravity but you manage to write down the final gravity. You are already halfway to finding out the ABV of your beer. Final gravity can be used to calculate the attenuation rate, which happens during the fermentation process. As sugars are converted into carbon dioxide and alcohol, the gravity of the beer decreases.

Typically, you need the OG to calculate the attenuation rate—but not always. Attenuation rate generally falls between 75-80%, meaning that the density of the beer will have decreased by that much since the beginning of fermentation. Thus, the final gravity would be about 25% of the original gravity.

Here is how the arithmetic works out: Make it easier on yourself and change the final gravity reading to a decimal—0.015. When you multiply 0.015 by 4, you get 0.06. Add back the 1 to get an original gravity of 1.060.

From there, you can plug those numbers into the equation for alcohol content:

Alcohol by Volume = [1.05/0.79 ((OG-FG)/FG)] x 100

ABV = [1.05/0.79 ((1.060-1.015)/1.015)] × 100

ABV = [1.05/0.79 (0.045/1.015)] × 100

= [1.05/0.79 (0.044)] × 100

ABV = [1.33×0.044] × 100

ABV = 0.058×100

Alcohol by Volume = 5.8%

The only downside to this method is that the ABV you come out with will not be entirely accurate, as you cannot predict the attenuation rate exactly.

2. Fire Up The Refractometer

Don’t have a hydrometer? No worries. To do this method, there are three steps, and none require the use of a hydrometer:

1. Calibrate your refractometer
2. Measure specific gravity
3. Use the alcohol by volume formula

Here are those steps in more detail.

Calibrate Your Refractometer

If you have one of these handy instruments, now is the time to put it to use. Refractometers are cylindrical implements that are used to measure how much sugar is in a liquid by seeing how much light is refracted by said solution. Digital refractometers are available—and make life a little easier. Also, look for a refractometer that uses Brix percentages, not specific gravity (SG).

In order to use a refractometer correctly, you need to calibrate it first. Open the flap, add a droplet or two of distilled water to the chamber, and then seal the flap. Some digital versions have a built-in light source, but if yours didn’t simply aim it at a light then look through the lens. You should see a scale. The line dividing blue from white should align with zero. If it doesn’t you need to adjust the calibration screw before proceeding.

Once you get the refractometer calibrated, remove the water, clean the glass, and then repeat the process with your beer. Jot down the reading on the scale, as you are going to need that.

This video describes how to use a refractometer correctly:

During the second or third week of fermentation, take another reading with your refractometer. Since these devices do have some error, divide both readings by 1.04, which is the standard value for correction. Write the initial Brix (IB) and final Brix (FB) numbers down, both rounded to two decimal places.

Measure Specific Gravity

The next part is a two step process, as you must find both the initial and final gravity using the IB and FB you recorded.

To find the final specific gravity (FG), use the following formula:

1.000 – (0.0044993 × IB) + (0.011774 × FB) + (0.00027581 x IB²) – (0.0012717 × FB²) – (0.0000072800 × IB³) + 0.000063293 × FB³)

So, for instance, if your IB is 12.5 and the FB is 8.65, you end up with a final specific gravity of 1.020.

Next, you calculate initial specific gravity (IG) with:

(IB / [258.6 – (IB/258.2) x 227.1)] + 1

Using the IB 12.5, the equation gives you 1.050 as the initial SG.

Use The Alcohol By Volume Formula

Having calculated both the initial and final specific gravity, you can now plug those numbers into a formula for alcohol by volume that was mentioned earlier:

ABV = [1.05/0.79 ((OG-FG)/FG)] x 100

Another formula that gives a slightly more accurate result is (76.08 x [IG – FG] / [1.775 – IG]) x (FG/0.794).

Using the information you have, the equation looks like this:

Alcohol Content or ABV = (76.08 x [1.050 – 1.020] / [1.775-1.050]) x (1.020/0.794)

= (76.08×0.03 / 0.725) x (1.285)

= 3.15×1.285

ABV = 4.0%

Are There Any Other Methods For Calculating ABV?

If you are thinking to yourself, “Holy s—t, that’s a lot of math,” and don’t want to play the numbers game, you have a couple of options. First, you can play around with an ABV calculator (provided you have some gravity readings) and come up with an estimate.

Second, you could try purchasing a density meter, also known as a pycnometer. Models designed for measuring alcohol content do exist, but they can be pricey. The upside is that you only have to wait a couple of seconds for an accurate result.

Another option is also kind of expensive and involves contacting the American Society of Brewing Chemists (ASBC) to see if any laboratories near you follow their standard practice. The ASBC has a method of distillation that allows them to calculate ABV accurately by using a small sample of your beer. Since distillation is still illegal in many places around the US, visiting the laboratory works in your favor.

OG Can’t Hold You Down

While most people will measure original gravity and final gravity with a hydrometer to figure out how much alcohol content is in their beer, there are other ways to go about it. By either extrapolating the attenuation rate or using a refractometer, you can calculate alcohol content without needing to know the original gravity of your brew.

FAQs