I brewed up a 5 gal of pale ale. 2.5 gal of wort give or take. Went into bucket. Added water till 5 gal mark. But my dumbass took a gravity reading of just the wort😂. So my question is, by adding the remainder with water, how much would my gravity change?

I’m not an arithmeticologist, so please take this with a grain of salt:

You had 2.5 gal of wort and diluted it to 5 gal with water.

By adding 2.5 gal of water you reduced your specific gravity to half the specific gravity of your initial wort. If the wort was 1.050, subtract 1 and multiply the remainder by 1000 to get the number of “points per gallon” for the wort. Multiply the “points per gallon” by the number of gallons to get the total points in the batch. 2.5 x 50 = 125 points in the batch before diluting it to 5 gallons. After diluting to 5 gallons, you have 125 points divided by 5 gallons = 25 points per gallon. Divide (unmultiply) 25 by 1000 to get 0.0025 and add (unsubtract) 1 to get your ending SG of 1.0025.

Was this a Northern Brewer extract recipe? I don’t take a OG reading anymore with NB recipes. The OG will be as the recipe states if you used all the fermentables and the volume in the fermentor is correct.

It was. But i like to check and see for myself since im new at this and wanna make sure im doing things correctly.

Take their word for what the OG is. When the SG sample is taken from the fermentor after the top off water is added the sample is usually part of the mixed wort low in sugars or part of the mixed wort high in sugars. It is very difficult to mix the wort from the boil kettle and the top of water even with the most vigorous aeration.

You can extrapolate from a temperature corrected SG sample from the boil kettle for OG. The exact volume in the boil kettle, that will make it into the fermentor, needs to be known.