# Calculating original gravity

### Help Support Homebrew Talk:

#### ExCathedra

##### Member
I took a gravity reading before pitching the yeast of my latest batch and it read 1.097 -- way to high. The fermenter was sealed and stored before I realized I took my reading before adding 2.5 gallons of water to "top it off". Is there a way to calculate my "actual" O.G. based on my reading before topping off (it was a 2.5 gallon boil, by the way)? I'd rather not mess with the fermenter at this point.

Thanks.

##### Supporting Member
HBT Supporter
[(volume 1 x OG 1)+(volume 2 x OG2)]/(volume 1 +volume 2)

##### Supporting Member
HBT Supporter
[(2.5 x 1.097)+(2.5x1)]/5=1.049

#### jbambuti

##### Well-Known Member
If your gravity was 1.097 and that was half of your volume, then your actual gravity would be half that, or 1.048-49. That doesn't account for boil loss, however. If you started with 2.5 gallons and boiled for an hour, you lost some of your starting volume, which would change your OG. 1.048 or 49 is just a ballpark figure.

OP
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#### ExCathedra

##### Member
Thanks for the replies guys. They were just what I was looking for.