Do you measure wort gravity?

I always have to top off with water after adding my wort to the fermenter. Is this however when I would measure the wort SG or before I add water? If I measure after adding water, would that still give me an accurate mash efficiency reading?

Thanks!

You are brewing a partial boil all grain?

You only measure mash efficiency right after mashing (only if you're doing an all-grain mash). It helps figure out your mashing process and how good/bad/consistent it is. Brewhouse efficiency is also only when doing all-grain, but it measures efficiency after the boil. I don't use mash efficiency numbers since I really only want to know what will be happening after the boil anyway.

If you're using extract only, there is no such thing as efficiency. Malt extract has a specific points/pound/gallon number that you can easily compute your gravity numbers with. If you know how much your boiloff will be, you can crunch the numbers ahead of time and figure out exactly what your gravity will be in the end.

Since adding water is going to dilute a thick wort, or make a regular wort more thin depending on your perspective, I would recommend taking a gravity reading before and after. Only dilute if your wort has higher gravity than you want it to, or if your only mission is to get to a certain volume (if doing a partial boil, or if you have less wort than you intended to ferment) regardless of gravity.

if you top off, measure it at flame out (cooled) and then just do this math .. take the pre top off gravity's last 2 numbers times the amount of pre top off wort and then divide that number by the final volume for your OG .. example

4 gallons of pre topped of wort @1060

60 times 4 divided by 5 = 48 or an OG of 1048

that's all much easier than trying to get the water mixed evenly .. trust me it really is hard to get the water mixed for a good reading.

Well my next batch will be my first all-grain brew in about a month so consider it all-grain.

So after I do my mash, and then sparge, I measure the gravity while in the brewpot before boiling gets started?

If you want to calculate mash efficiency, then yes- measure the volume of wort into your brew kettle, and the gravity of that (make sure you chill the wort to ~60F before measuring it!). If you want to calculate brewhouse efficiency then measure the volume of wort going into the fermenter, and the gravity of that.

One takes into account your entire brewing process, and the other only takes the mash itself into account.