Reaching pre-boil gravity but missing original gravity

So after my mash/sparge is done I take a reading with my refractometer and see that I've reached my estimated pre-boil gravity of 1.050 right on the nose. I then boiled for 90 minutes adding my hops at 60, 15, and 5. Before cooling with my chiller I take another reading with my refractometer and notice I that I am much lower then the expected original gravity. Just curious exactly why this is? Is it something to do with the boiling process?

Did you evaporate off the amount of water you expected during the boil? The most obvious explanation I can think of is your post boil volume is higher than expected.

Yes I was a tad off on my post boil volume was shooting for 5.5 gallons and ended with like 6.25 just didnt think it would make such a large difference in my original gravity by only missing my post boil volume by .75 gallons.

~15% off of your intended volume is not insignificant. But now you have a better idea of how much water your setup boils off so you can adjust your mash/ sparge volumes accordingly for next time.

EDIT: just to be thorough, lets look at some numbers. you said your pre-boil gravity was 1.050, if you take the decimal portion of that, as the whole number 50 (this is the number of gravity points) and multiply that by your pre-boil volume in gallons (you didnt specify so lets just use 7) this gives you 350 point-gallons. now if you take this number and devide by your post-boil volume you should get your post-boil gravety in points. for example, 5.5 gallons will yield 64 points or 1.064 where 6.25 gallons will yield 56 points or a OG of 1.056. So you can see the difference is significant.

You obviously did not evaporate enough water to get to the concentration that you had planned on. But... there is an easy fix. Just add some DME at the end to boost the gravity to whatever you wanted.

And if you have the reverse situation. If for some reason you over shoot your gravity and finished on the high side, just add some water to thin it down.

OR if it's no big deal, just accept that you missed your target, RDWHAHB!

Oops, I was going to say something about temperature correction and now I can't delete my comment.

I'm working on a similar problem, in my case with PM batches. I tend to hit my mash gravity on the nose, or in some cases overshoot what I expected. However I have been coming out low on OG. In addition to things covered above, I think one error I'm making is not properly accounting for the trub/break material. My gravity works out where I expect, but my actual volume is lower than it looks, so I have less dissolved sugar than I computed.

This is probably only an issue if you're doing a BIAB type mash, since you probably leave most of this behind in more traditional procedures.

Let's say that your recipe is telling you that you should have an OG of 1.064 and 5.5 gallons of wort at the end of the brew day.

Drop the "1." off of the target OG and then multiply that number by your expected final volume.

64 times 5.5 gallons equals 352 gravity units (GU's). That's your target GU.

Let's say you evaporate a half of a gallon per half hour. For a 90-minute boil you'd boil off 1.5 gallons, so your target pre-boil volume should be 7 gallons. 352 divided by 7 is 50.2857, so your target pre-boil SG is around 1.050.

Don't forget, though, that when you measure your pre-boil volume it's going to be at around 150° F or so, and water expands around 2% at that temperature, so your actual target pre-boil volume would then be 7.14, but that amount is pretty negligible in this case.

Water expands about 4% at boiling temps, so your final measured volume before cooling should be around 5.75 gallons.