Question on mash water/sparge ratio

Sparge water volumes are not even universal between mash tun brewers/batches. Yes, the total water needed will be very similar. But the water to grist ratio is variable, so the sparge water needed to make up the total is variable. With mash tuns, ratios between 1-2 quarts per pound are typical. With BIAB, some people mash in the entire volume of water, with no sparge at all. I'm pretty sure that's how the original BIAB brewing was done, with people later adding a sparge step in pursuit of higher mash efficiency. IMO, if you're going to sparge with BIAB, you've removed one of the main reasons (simplicity) for doing BIAB.

 

You need to account for grain absorption, hop absorption, boil off, kettle deadspace (if applicable), and siphon transfer loss (if applicable). Software makes this easier. BrewCipher is a free integrated brewing excel workbook. And many people use the Beersmith program (not free). Add these volumes to the amount of wort you want going into the fermenter.

If you're not ready to use software and want to compute manually, here are BrewCipher's defaults, which might work for you until you have determined the values you get in your system...

Grain Absorption: 0.12 gallons per pound
Pellet Hop Absorption: 0.025 gallons per pound
Boil Off: 1.24 gallons per hour

 

Boi-loff only applies to the boil time. You can safely ignore evaporation that might happen during the mash.

 

FWIW I lost closer to .15-.18 gal/lb when I did BIAB. This piece will depend on how long you're willing to let the bag drain after the mash. I was not very patient.

 

I usually see 1 gallon boil off per hour on my system. Point to the OP being don't freak out if your results aren't 1.24 gallons per hour. They easily could be but ultimately the goal is to boil off at least 8% (about 0.6 gallons on your batch) of your total boil volume over the course of your entire boil. More is OK but less is negative from a flavor and aroma standpoint.

Some general numbers (I don't know your system specifics)

1.5 quarts of water x 11.75 pounds of grain = 4.4 gallons of strike water (make it 4.5)
11.75 pounds of grain x 0.125 absorption per pound = 1.47 gallons (assume 1.5)
= 3 gallons of 1st runnings collected

1 gallon per hour boil off x 1 hour = 1 gallon
maybe 0.25 gallons of dead space in the kettle
maybe 0.5 gallons of transfer / hop absorption loss
maybe 0.25 gallons of loss for cooling expansion
= 2 gallons of loss on the boil and transfer side

If you are looking to brew a 5 gallon batch size, this is a rough estimate of volumes...

4.5 gallons of strike water
- 1.5 gallons grain absorption
= 3 gallons of 1st runnings

sparge with 4 gallons of water
= 4 gallons of 2nd runnings collected

7 gallons of total runnings collected
- 1 gallon boil off
- 0.25 gallons (4%) cooling expansion loss
- 0.25 gallons dead space
- 0.5 gallons transfer loss
= 5 gallons into the fermenter

You will lose anywhere between say 0.25 - 1 gallon between the fermenter and final packaging (depending on dry hopping amounts and how good you are at transferring) so that leaves you with anywhere between 4 - 4.75 gallons of final packaged product.

Now what I'd do is use a 6.5 gallon fermenter, a 5.625 gallon batch size (simply up the sparge water volume from 4 gallons to 4.625 gallons), and end up with 5 gallons of final product in a keg or in bottles.