When you’re getting into brewing and distilling there’s a log of jargon, and a lot of science to get your head around. Don’t worry. It’s not too scary. And we’re here to help!
In simple terms, an alcoholmeter and a hydrometer are tools that look almost identical and operate using the same principles of physics. The only difference is an alcoholmeter is calibrated to measure the amount of ethanol (alcohol) in a liquid that is otherwise water, while a hydrometer is calibrated to measure the amount of sugar in water.
Alcoholmeters should be used to measure distilled spirits. Hydrometers should be used to measure fermented products like beer, wine, and cider.
In this article, we’ll explain what a hydrometer is, and what to use one for. We’ll also discuss the difference between these and alcoholmeters, as well as compare the two instruments side-by-side.
So without further ado, let’s just jump straight into it:
Table of Contents
First Things First, What Is A Hydrometer?
A hydrometer is a measuring instrument that is typically used by beer and winemakers to determine the alcohol content of their beverages. It consists of a tube (that looks a bit like a thermometer) that has a bulbous bottom which means it floats in water or other liquids.
It measures the density of a liquid, based on how deep it floats in the liquid. This is known as the Specific Gravity (SG).
The hydrometer is marked with a scale, and as it is placed in the liquid, will rise up or down based on displacement. It will float higher in denser liquids and lower in less dense liquids, allowing you to read the density of your liquid by looking at the reading on the scale next to where the hydrometer is floating.
How Does A Hydrometer Work?
The heavier (or denser) the liquid is, the higher the hydrometer will sit in the liquid surrounding it.
The lighter (less dense) the liquid is, the lower it will sit.
This is based on the scientific principle of displacement. Imagine a ship floating in the ocean. The weight of the boat will push away the same weight of water in order to find equilibrium. If the water is denser, like in the Dead Sea, it will be displaced less by the same ship and therefore the ship will float higher!
The reason beer/wine makers use them is because of fermentation. Before fermentation begins the wort/wash can be assumed to be a mix of water and sugar. The more sugar, the higher the density and higher the hydrometer will float. This reading is generally referred to as the OG or Original Gravity.
As fermentation progresses, the sugar in the fluid is consumed by yeast and turns into alcohol. Alcohol has a slightly lower density than water. We measure the gravity again with the hydrometer to record the Final Gravity or FG.
By measuring the before and after we can work out how much sugar has been converted to alcohol and then infer what percentage ABV the final fluid contains.
A calculator like this one will help you determine the alcohol by volume (ABV) from these two readings.
What’s An Alcoholmeter?
An alcoholmeter is used to measure the alcohol content of your spirits. To be more specific, it measures the ratio of ethanol to water in a mixture.
To the untrained eye it looks the same as a hydrometer. It also works on the same principles. As the hydrometer, as gravity pulls the alcoholmeter down into the mixture, taking its reading from where it floats.
It does this again based on density. Ethanol has about a 21% lower density than water.
The alcoholmeter is very similar to the hydrometer. It’s a long tube that floats in an alcohol/water mixture, with a scale next to it… The difference is that it has a line on it for every “percent” of alcohol. That’s right; You can read your spirits and liqueurs by measuring them for alcohol content without ever worrying about plugging them into a calculator.
A 0-100% Alcoholmeter is generally less sensitive than a typical hydrometer which ranges from 0.99 to 1.17. Just like a hydrometer, it’s heavily affected by temperature. Most alcoholmeters are calibrated at 20 degrees celsius. This means that if the product is hotter than this, you’ll read a higher alcohol percentage. If the product is cooler than 20C, your reading will be too low.
All good alcoholmeters provide a correction table that you can use to adjust your reading for temperature.
To estimate the actual % abv without a temperature correction table you just need to remember one general rule.
For every 1 °C over 20°C subtract off 0.33% from your reading.
In a nutshell, this is really all there is to both instruments. They both measure the density of a liquid and give you an accurate reading that you can use to measure alcohol content.
Is An Alcoholmeter Just A type of Hydrometer?
Yes. Technically yes, an alcoholmeter is just another type of hydrometer. It serves the same purpose and can be used in the same way as any other hydrometer.
A Hydrometer Doesn’t Have An Alcohol Scale On It
No, unfortunately, it doesn’t.
To determine the alcohol content of a liquid using a hydrometer you need to take a reading before and after fermentation and put these two numbers (the original gravity and final gravity) into a calculation program.
An Alcoholmeter Is More Accurate Than A Hydrometer
This one isn’t exactly true. Both instruments can be very accurate if they are calibrated correctly and designed to be used in the correct range.
It’s also important to understand the difference between precision, accuracy, resolution, and sensitivity.. which we won’t get into here.
Your typical ‘off-the-shelf’ hydrometer is more sensitive. That’s because the difference between a fluid that’s 100% water and 100% ethanol is 21%, while a sugar hydrometer usually ranges between a SG of 0.99 and 1.17 which is a difference of 18%.
Of course, if making accurate measurements is important to you, then using a precision hydrometer or alcoholmeter that is calibrated to a smaller range (such as 70-100%) is the way to go
The main difference between an alcoholmeter and a hydrometer is that they both serve different purposes, despite operating on the same principle and looking very similar.
The alcoholmeter will give you a precise reading of the alcohol by volume percentage of a distilled spirit.
A hydrometer is needed to work out the alcohol content of a fermented product, like a beer, wine or wash. You’ll need to take two readings, one before and one after fermentation, and then plug these numbers into a calculator to work out the ABV percentage.
Frequently Asked Questions:
Q. Can I use a hydrometer to measure spirits?
No. This is the job of an alcoholmeter.
Q. Can I use an alcoholmeter for beer and wine?
No, to work out the alcohol content of a fermented product you need to use a hydrometer and work out the difference between the original gravity (OG) and final gravity (FG)
Q. What about digital hydrometers?
Yes, these do exist and are really nifty. The only problem is that they are super expensive, and certainly outside the budget of a hobbyist.
The good news is you really don’t need one. The manual method of using an alcoholmeter is quick, easy, and accurate once you get the hang of it.
Q. What is the best alcoholmeter?
We’ve written a whole article dedicated to finding the best alcoholmeter for your application which you can read up on here. TLDR; if we had to buy one alcoholmeter it would be the
Q. What is the best hydrometer?
Our favorite all-purpose hydrometer is the Brewing America Thermo-Hydrometer.
Our favorite all-purpose refractometer is this handheld refractometer which costs less than $20!
Q. How can I be more accurate with my alcohol readings?
It’s most important however that you take the reading at the correct temperature. Every hydrometer is calibrated at a certain temperature (usually 20 degrees Celsius).
A warmer liquid will be less dense and cause the hydrometer/alcoholmeter to sit slightly lower. A cooler liquid is denser and will displace the meter more.
Fortunately, there are correction factors that can be applied if you need them.
Q. Do I take the reading from the top or bottom of the meniscus?
All readings should be made from the top of the meniscus line. Taking the reading from the bottom of the meniscus will introduce a 1-2% error into your measurement.