OG SG and FG

Can someone please provide the overview of the relevance and importance of checking and having accurate OG, SG, and FG? Also, is there any way to do a standard conversion to get the OG from the pre-boil gravity reading for a one hour boil so the sanitized wort does not have to be pulled before it goes in the fermenter? Much thanks.

OG and SG are the same thing. You need the SG and FG to figure out your ABV.

If you are doing extract batches and hit your water amounts, the SG should be VERY close if not dead on. Adding too muvh water will drop the SG. Boiling too long or not adding enough water will raise the SG.

A refractometer is a way to save some wort. If you use it post fermentation you will need to use a table to adjust your FG as alcohol throws the reading off.

Having accurate readings also helps you know when fermentation is complete.

To calculate the OG of preboil wort, you need to know your boil off amount.

So, if you start with 6 gallons at 1.050 and end up with 5 gallons, you can figure it out.

SG is Specific Gravity

OG is Original Gravity

FG is Final Gravity.

OG does not equal FG

This site has a calculator that will tell you what your SG will be when you reduce the volume. Also many more useful calculators.


Just a few clarifications:

specific gravity (SG) is a way of describing density of a liquid by comparing it to water. An SG of 1 has the same density of water. SG is a generic term that can be applied to any gravity measurement at any time, it doesn’t refer to any particular point in the brew process. You measure it with a hydrometer.

OG and FG are original (before fermentation starts) and final (after fermentation finishes) readings. You care because: OG tells you your efficiency (if you do all grain), a stable FG (same reading over multiple days) tells you fermentation is complete, and the difference between the two can be used to compute % alcohol and % attenuation.

If you brew extract you don’t necessarily need to take an OG reading, and people frequently get it wrong because of inadequate mixing. If your final water volume (AFTER boil and top up) is correct, your OG will be correct.

Pretty much it.

Technically you don’t HAVE to take a gravity reading ever. If you basically know your efficiency and calculate everyting propery you can get it in the ballpark every time. If you want to know you are hitting your exact targets yes.

I spent a year where everytime I went to go take a reading I realized I had broken my hydrometer and had forgotten to pick up a new one. Beer was fine and I had a good idea of what the ABV would have been.

I certainly wouldn’y say no to a refractometer if I had the money.

Thanks for the education