Now that I'm on a computer I can type this out.

I asked the internet, and it says that sweet cherries have a brix ~20 (SG = 1.083), tart cherries are ~13 (SG = 1.053), and that 3lbs of cherries contain 1.25 quarts of juice. So you can see that sweet cherries will probably raise OG of most beers, though probably only a little. However, tart cherries added to a beer with OG > 1.05 will actually lower OG.

Calculate a 'revised' OG from this formula.

(Wort OG * Wort Volume) + (Juice OG * Juice Volume)

------------------------------------------------------------- = New OG of Wort + Juice

(Wort Volume + Juice Volume)

For example - tart cherry juice added to 5 gallons of 1.070 OG beer.

(1.070 * 20 quarts) + (1.053 * 1.25 quarts) 21.4 + 1.32

------------------------------------------------- = -------------- = 1.069

21.25 quarts 21.25