Back to Shopping at

Hydrometers You need one, here is how to use one

Hydrometers are based on Archimedes Principal. “Archimedes’ principle indicates that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces and it acts in the upward direction at the center of mass of the displaced fluid.”
What that means to us as brewers is when the sweet wort is measured with your hydrometer the thicker it is, the higher your hydrometer floats. The more malt extract you use or grain you mash, the higher the reading. After the yeast works its magic and converts the sugars into alcohol, the reading will be lower. These two readings are known as OG, original gravity and FG, final gravity.

So what can we use these reading for? First if you are using a known recipe it may have an expected OG and FG. You can see if your beer matches the expected reading. Second you can figure out the alcohol content after the beer is done fermenting. Alcohol by volume or ABV can be figured out by subtracting the FG from the OG then multiply that by 131.25. So a beer with an OG of 1.050 that finishes at 1.010 would be 1.050-1.010=.04x131.25=5.25%ABV. There are also a lot of online calculators to do the math for you. Lastly you can determine if your beer is done fermenting or stuck. Simply measure the gravity each day for about three days. If it does not change, it is either done or you have a stuck fermentation. A typical FG is around 1.010 however some really big (high alcohol) beers may not reach that low.

How do we use a hydrometer? First when you get your hydrometer you may want to calibrate it. Don’t use the plastic tube it came in. They are not wide enough so the hydrometer may stick to the side and give a false reading. Buy yourself a hydrometer flask. They are cheap and will stand up by themselves. Place the hydrometer in it and fill it with 60° preferably distilled water. It should read 1.0. If not you will know how far off your reading is and adjust accordingly. If it is way off, return or exchange it for a new one. When you read your wort it should also be at 60° but there will be a temp adjustment chart with it. EDIT: Some hydrometers may have a different calibration temp.Check the card that comes with your new one.

When you take a reading of your wort, the easiest way to get a sample is to use a wine thief. They are just dunked in and have purge type valve that allows the wort in but not out until you press the valve against the inside of your hydrometer flask. Next spin the hydrometer or pour the sample back and forth between two glasses to remove any bubbles that may float the hydrometer too high. Then read the gravity at the meniscus. Wort will cling to the hydrometer and climb up a little. The meniscus is where the flat line is, not the top where the wort clings up higher. Don’t return the wort to the fermenter. It is not worth the risk of infection. Drink it or just toss it. Sanitize all of the equipment first!


Specific Gravity (SG) is what the hydrometer measures, it is a measure of the ratio of the wort’s, or beer’s, or whatever’s density compared to pure water at calibration temp; an SG of 1.050 means the wort is 1.050 times as dense as water at the calibration temp. As you say, OG is the Original specific Gravity reading, FG is the final. It worth emphasizing because I occasionally see posts with people using “SG,” without clarifying when in the process they took the reading, while not wrong, it is better to specify which SG reading posters are citing. SG is the measurement; OG and FG are the measurement + when the measurement was taken.

Wort loses density as it ferments to beer because dissolved sugar is heavier than alcohol. The ABV calculation works because we presume the yeast has removed heavy sugar and replaced it with lighter alcohol. Once we measure the change, we can compute how much alcohol was made. The 131.25 number is the result of over-my-head science. It’s a good enough number, but there are different formulae out there, so don’t be surprised if an online calculator gives a different answer.

You can take the SG reading at whatever temperature you like; if you measure 1.050 at 80F that wort really is 1.050 times as dense as water at the hydrometer’s calibration temp. But, liquids change density with temperature, and the 131.25 trick presumes all samples were at the calibration temperature. To workaround this, there are also online calculators that will help convert SG readings at whatever temp to the reading you would actually get at 60. Plugging 1.050 at 80F into a calculator will result in a higher number at 60F; That’s often referred to as a “calibrated” SG. I find it way easier to use an online calculator than to get my sample to precisely 60F as implied. Again, the ABV calculation assumes calibrated measurements, so don’t forget that step.


Thanks for adding that jmck. I tried to keep it short so I may have unintentionally left out important info. Had a little trouble explaining the meniscus. Hope I got the message across.

If found that a little research will provide more that the average brewer needs to know but wanted say that a hydrometer is a must have piece of brewing equipment.

1 Like

I’m one of those jack a**es that calls my OG, SG. Its just as I’ve done since I started. Also, one other note, say your starting gravity is 1.050, and you finish at 1.010, simple to subtract 10 from 50 and then read the ABV on yer hydrometer, just to keep some of this process simple.Sneezles61

1 Like

Good Stuff… Answered some questions I wanted to ask…Thanks for the info!

So dumb question here.
I’m on my 4th Brew and haven’t used a hydrometer before.

I know. I know. Save it. I have one coming tomorrow from Amazon :slight_smile:

So here’s my Q. I’m about to bottle bit will save some to test tomorrow.

Do I take a reading before or after priming sugar solution is added? I could see it going either way, but would assume id get a different reading each way.

What would be proper?


Before, ideally. But, if you forget and end up taking a reading after taking a sip of the weird 1/4 filled last bottle, you can do some math. Figure out how much sugar you added and your volume, plug it into a calculator, and subtract those gravity points from your primed sample. I’ve done this more often than I’d care to admit. The difference is minor (one or two points usually).

Back to Shopping at