Estimating pre-boil measurements

I was wondering if there were ways to calculate pre-boil wort volume after you have mashed and sparged depending on how much grain you use. I am asking because I only have room for 3 gallons in my brew pot and wanted to do a small AG test run of about 2.5 gallons to see if I want to go down that road instead of just doing partial mashes. But I don't know what my size limit would be as far as a grain bill. Is there any way to calculate the mash and sparge run off volume? Thanks.

Get ProMash or Brewsmith and set your batch size to 2.5 gallons. They both have calculators that will tell you what your pre-boil volumes should be.

People typically mash with between 1 and 1.5 quarts of water per lb of grain

Grain will absorb roughly 0.1 gallons per lb. You'll also lose some "other" volume in the bottom of your MLT (about 1 quart stays behind beneath my false bottom)

The amount you get in your kettle will be

Mash + Sparge - Absorb - "Other" = to kettle

Lets say you want to use 4 lbs of grain, at 1.25 quarts per lb for the mash.

Mash = 4 lb x 1.25 qt/lb = 5 qt = 1.25 gal
Absorb = 4 lb x 0.1 gal/lb = 0.4 gal
Other (guess) = 0.25 gal

So if you want 2.5 gal in your kettle, you'll need to sparge with about 1.9 gallons

1.25 + 1.9 - 0.4 - 0.25 = 2.5

For maximum efficency, you should try to shoot for 0.5 gal of sparge water per lb of grain (at 1.9 gal for 4 lb, you're pretty close).

It will really depend on your system, ie. how much deadspace, absorption, ect...

Based on some rough estimation in ProMash I would guess you are looking at 5 to 6 lbs of grain.

Thanks for all of the replys, it really helps.

Bike N Brew thanks for the calculations. That was exactly what I was looking for.

Can't wait to try out a mini AG batch. Now I just have to figure out what to brew.

promash was made for this kind of math. i'm a math-tard, so without promash I doubt I could mash without complications.