Mashing water amounts

I'm planning on doing my first all grain batch this weekend and I'm trying to make sure I've got my volumes worked out...

Recipe has 17 lbs of grain (yowsa!) and I'm planning a final post boil volume of around 5.75 gallons doing a single infusion mash with batch sparging. With a ratio of 1.25 quarts per lb of grain, that gives me around 5.25 gallons of water for the mash. I'm calling my losses as around 4 gallons, which is around 3.5 gallons lost in the spent grain and .5 gallons lost in boil evaporation. Sound reasonable?

That gives me a total of around 10 gallons of water needed (5.75 final volume + ~4 gallons in losses). So, if I need 5.25 for the mash, then 4.75 for the sparge?

Does this sound reasonable? Is there an easy way to calculate your water needs? And since I have so much grain, anything special I need to take into account?

You won't lose that much water in the spent grain. IIRC, the rule of thumb is about 0.1 gallons per pound, so you might lose one and half gallons or so.

With 17 pounds of grain I would use about 5 1/2 gallons for the mash. You will lose about 1 3/4 gallons to grain absorption.
For the sparge you will need about 8 1/2 gallons of water. If you loose 1 gallon trapped in your tun you will have over 11 gallons of wort in your brew pot. It will take a long time to boil this down to get 5 3/4 gallons in the fermenter.
For your first mash I would recommend doing a smaller beer. Maybe 9 or 10 pounds of grain just to get your feet wet.
No matter what you try good luck and enjoy.

I agree with a smaller beer, unless you want to boil for several hours. If you want to boost the abv then just add some DME.

I have the ingredients all set. I figure, regardless I'll end up with beer...

So if I mash with 5 gallons and sparge with 5 gallons, what's the worst thing that could happen? Lower than expected gravity? Other problems?

I realize my efficiency will probably suck, but I'm ok with that. I'm considering this one sort of an experiment...

Assuming you're using a cooler mash tun, make sure your strike liquor is ~20F higher than your mash temp. The grain is going to drop the temp more than you'd figure.

Everyone's system is different. In my system with your grain bill I'd mash in with about 5.25 gallons, but in my friends system he'd be closer to 4.75. The thing you want to work towards is collecting the same amount of wort from your first runnings as from your sparge, so about 3+ gallons each. 5 gallons is a good round number and if you get 3 gallons out of your runnings, you're in business.

Take very good notes and make sure you have an accurate thermometer.

In order to get good efficiency, make sure that your sparge water is very hot. It, too, will lose about 15-20 degrees when you pour it onto your grains. I heat my sparge water to 195F and it ends up soaking at about 180F for 20 minutes with no ill effects.

But most of all, have fun! And learn! The better your notes, the better you'll be able to fine-tune your system on subsequent brewing sessions. Here's to beer!

Well all done I started out with 5 gallons to mash, but apparently putting 1 gallon of boiling water in the cooler to pre-heat it wasn't enough, and after 30 minutes it was down to 138 degrees. I had to add probably another gallon of boiling water to bring the temp back up, but after that it dropped maybe 1 degree. I kept it in the mash tun for 90 minutes total to make up for the temp drop. I sparged with 3 gallons and ended up with around 7 gallons total, which I boiled for 90 minutes to reduce a bit. Ended up with around 6 gallons after a few boilovers

I was also mashing with just a grain bag in a cooler, which is far from ideal. I ended up with an efficiency of 65% which isn't too shabby considering the circumstances.

Congrats on your first AG!