Forgot to take gravity reading after boil

[J-stone]

I did my first AG brew the other day and have a question. By the time I was done with my boil, I forgot to take my OG reading. I did however take a reading before boil. My before boil gravity was 1.038 at 6.75-7 US gal. After boil volume was 4.5 gal. (I got a little carried away and did not have a marked stick for my new unmarked kettle). Any help would be great.

Thankyou HomeBrewTalk.com for being the best resorce for begining brewers. I don't post much, but read alot on here, thankyou
Oh, not that it matters. . . but I only brew pilsners. . . just thought you should know.

 

[dbsmith]

Have no fear, those crazy chemists have made us a concentration/dilution formula:
c1v1=c2v2

(38)*(6.88 gallons) = c2 * (4.5 gallons)
c2 = 58.1

So your OG (post-boil) should be something around 1.058 (+/- .001)

 

[J-stone]

Sweet! Thankyou sir. Now, off to find an efficiency formula