Understanding Gravity in Beer Brewing, OG, FG and SG – What ?

This bear has brewed many batches of beer for private use and has totally neglected to do any sort of gravity measurements at all.  The beer came out really good, definitely had alcohol in it (trust me) – but because it was for private consumption and this bear can get sometimes kind of lazy (yawn), this bear didn’t even bother or even care to take the measurements – bad bear!  just bad!

In reality you should take these reading and records them, learn – keep a log, even if you re-do the same batch over and over, because its important for a variety of reasons, which I am about to explain.

A hydrometer is used to read the specific gravity of un-fermented (wort) or fermented (beer).  http://en.wikipedia.org/wiki/Hydrometer

This device measures the relative density of wort/beer or SP (Specific Gravity)… there is also:

OG – Original Gravity (gravity taken after brewing beer finished, but before fermentation starts)

FG – Final Gravity (gravity taken after fermentation is done)

O.G. 1.056 (minus) F.G. 1.012 = 0.044 then multiply that by 131 to get 4.192% alcohol by volume approximate (example).

The hydrometer rating should be taken at a specific temperature of 59F / 15 C, and rarely people when taking readings get it that right temp, so that’s why you take a temperature reading.  In addition you can use a calculator that also has temperature adjustments for the formula, so record the temperature too for both OG and FG readings in your log.


The less dense the liquid is (after fermentation), the deeper the hydrometer will sink into the liquid.  The more dense it is (before fermentation), the less deeper it will sink into the liquid.

also – look at your hydrometer, it will say at what temperature to take the perfect reading, and it was calibrated at, on mine it reads 60 F and it should tell you the alcohol by volume right on the scale inside the glass.  Some are made differently, so look at yours.

If you pick up a recipe that someone wrote or it came with a kit that you have purchased, it will have printed the expected OG range that you should get after finishing to brew beer.

Usually it is very difficult to get it spot on, so a Range is provided, and as long as you are within this range, you should be ok, example OG Range: 1.056 ~ 1.061.

You probably have noticed that the gravity reading number goes down after the fermentation is over, that is because the yeast will convert sugars into alcohol and alcohol is less dense and the gravity meter measures what ? density, super simple at the high-level.

To give you an idea, the hydrometer is also used in other applications, not just beer; for example in the salt water aquarium hobby, you start with fresh water, then add salt until you reach a certain safe and acceptable range.

If you are going to brew beer for commercial consumption, than this is super important, because if you claim that the beer is 7.2% on the beer label, but it is 5.1%; than you have a problem, don’t you ?

So it is a good idea from day one to get used to doing this and it will help you to understand another critical aspect of brewing and that is the efficiency method that you are employing during the mash phase to extract the sugar from the grain.

How will you know if the mash is doing well, if you never take a reading and you assume this ? you won’t know!

So even if you are not a commercial brewer, but want to confirm that you are doing the mashing correctly and your method or setup is working as intended, there is only one way to know and that is to take a gravity reading!  Or what if you employ a new method and you want to compare to see which one does better ?

There is also another type of measuring device called a Brix Refractometer; but it needs to be calibrated, the hydrometer, not so much.

There is a benefit to this tool!  While you are mashing, you can put a drop of the wort into the Brix and it will tell you the specific gravity, so that’s a plus for this tool and a lot of people use it for that reason, before you begin the process of actually brewing beer, so that if the reading isn’t right, you maybe have time to make some adjustment to improve the mashing process.

Its best to own both if you are a serious home brewer :- )



and they come in different shapes and sizes, including laboratory grade equipment that is tested and certified.  We recommend the old fashioned way of using the hydrometer – cheap, reliable and did I mention its cheap ? and it don’t require batteries ?





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