My next brew day, I plan giving BIAG a try. Looking on web sites and researching calculators, I found ones that were way too complicated or too simple than I wanted. After some research, I came up with a simple spreadsheet that I like.
[side bar] You can use this for batch sparging as well, but it does not account for heat lost due to the mash tun.
Grain = Total recipe to be mashed
Pre-Boil = Desired volume when the boil starts (or after you first batch sparge)
Grain Temp = Temperature of the grain
Target Temp = Desired mash temperature
Absorption = Input your allowance for water to be absorbed by the mash. Entering zero here is an option.
Qts/Lb = Calculated for an FYI
Strike Volume = Total water required with absorption included
Strike Temp = Calculated temperature of your strike water
Mash Volume = Calculated volume of mash in your mash tun
It is based on John Palmer's latest "How To Brew" book. In the latest version, Palmer accounts for the difference between volume and weight.
Latest formula is:
Tw = ( ( 0.4 / R ) * ( T2 - T1 ) ) - T2
where R = Qts / Lb * 2.055
The previous versions:
Tw = ( ( 0.2 / R ) * ( T2 - T1 ) ) - T2
Where R = Qts / Lb
Palmer changes the numerator from 0.20 to 0.40 and then divides it by 2.055. The update returns a slightly different Strike Temp value. Between temperature reading errors and rounding up I doubt it makes a real world difference, but might as well be as accurate as we can.
The second tab is a chart that calculates mash volume with gran absorption factored in. This is an issue for those of us doing BIAG using a 10 gallon mash tun/boil kettle.