Sorry for the poorly worded subject line...can't find a way to edit it. Is there a method (or preferably and online calculator) that can predict what effects topping your wort off with water will have on your preboil gravity? For example: Today I collected 12.5 gallons of wort with a specific gravity of 1.056 My target was to collect 13 gallons of wort with a specific gravity of 1.051 If I would have added 0.5 gallons of RO water to my 12.5 gallons of 1.056 wort, what would my resulting specific gravity have been?

Again all of these volumes and gravities are preboil and I'm looking to know what effect adding water to my preboil wort would have had on my preboil gravity.

(0.5 x 0) + (12.5 x 56) ---------------------------- = 53.8 (0.5 + 12.5) So your pre-boil gravity would have been 1.0538 (1.054 rounded)

Not sure if I'm helping but a while ago some some asked a slightly different question and HB42 helped out. It was more gravity going in the other direction. From his formulas Imade a spreadsheet. I plugged you're numbers into my spreadsheet and got 13.73 gallons, which is the 1.23 POSSIBLE gallions that you might of wanted to add. If you're interested I could post the specifics.

A gravity of 1.056 means that you have 56 gravity points of sugar per gallon, which means that in total you have 700 gravity points of sugar (56 points per gallon X 12.5 gallons). If you added another half gallon of water then you'd have the same 700 point of sugar except they'd be spread out over 13 gallons instead of 12.5. In other words you would then have 53.8 points of sugar per gallon (700 points / 13 gallons), or a gravity of approximately 1.054.

Makes sense. Thanks everybody! If I follow that logic correctly, would that also mean that if I boiled the 13g of 1.054 wort down to the point where I had 1.068 SG, I'd have to have roughly 10.25 gallons of wort in the boil kettle at flame out (after cooling expansion loss, which means roughly 10.75g of boiling wort)? 700 points / 10.25 = 68.29

I don't know about the expansion/contraction rate (I don't worry about it in my system because I have dialed in the boiloff rate using cooled volumes). But your points math is sound.