Partial boil and topping off

I normally do partial boils and top off to 5 gallons. I don’t normally check the OG before pitching. Is there a way to check the gravity of the concentrated wort and calculate how much top off water is needed to hit the target OG?

How big are your boils? I would rather be a few points off my target OG than low on my target volume.
There is a formula, but I can’t think of it right now. Sorry. I tried to work some numbers, nothing came out right.

Normally 2.5 - 3 gallons pre boil volume. I couldn’t tell you the post boil volume. I usually pour the wort on top of a gallon or so of water already in the fermenter after cooling. I agree on missing the gravity vs losing volume.

The reason I ask was because it seems a lot of people ask about missing the target OG of a recipe and most responses are about not fully mixing the wort and top off water. I was wondering if you could measure before topping it off to be more accurate.

That makes sense. There is a way to figure it out. With a full boil you do the opposite,
knowing your SG and boil off will give you target OG and volume.

Someone should be able to give us a method.

This formula is used in all grain brewing to calculate an estimated OG. It could be reverse engineered for extract brewing, but would rely on an accurate measurement of your post boil volume.
preboil og × preboil volume = X
X ÷ post boil volume = OG

I don’t take a SG reading after topping off because of the possible inaccuracies. I use the recipe estimated OG if my beer was made from a kit. As long as all the fermentables were used and the fermentor volume is correct the estimated OG should be right.

I use a OG calculator like this one when a recipe is not from a kit to get close enough for ABV calculation.
Link: Brewers Friend OG calculator

I think this is what your looking for:

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I usually take gravity readings on the fly but never actually calculated out the exact amount of top off water. I was doing partial boils on the stove, had a 5-gallon pot. I was trying to start my boil close to 4 gallons (partial mash brewing, so I had less extract to add, or if it was something that required a lot of extract, I’d start around 3.5 gallons). I’d slowly add some water if I had to in an effort to keep up the volume. After chilling, I’d take a temp reading and drop a sanitized hydrometer in the kettle to see where I was at. Dump it in the fermenter and get close to 5 gallons then stir the heck out of it for a bit and take another reading. If I was too high, I’d add enough to bring the volume up to 5.5 gallons max. If I was close, then I’d put the volume at 5 gallons and leave it.

A lot has to do with yer system. Also, how much in a gallon or how much to the gallon… Seems I do gain from 6-8 points post boil…. take notes and find the curve. You should be able to make tests of your DME, and syrup extract in a gallon… Part of brewing, then you can determine how much TO the gallon to get pre volume so you can get close to yer final gravity… Oh yeah, how do you boil? Fast er slow… Sneezles61