Formula for grain displacement?

[agentbud]

I have a 9.25 gallon mash tun. I want to do as close to a full volume mash as possible. (ie - total mash volume = 9 gallons). If I know the total weight of grains going in, what is the formula to determine the amount of water to add so that the water and grains together = 9 gallons?

 

[DBhomebrew]

I target a mash volume of 4.4 in a 5 gallon pot and find it to be fairly full.

Take a look at the lid and see if it sits above or inside the kettle volume. The surface of my All-Clad kettle's lid sits a few mm into the kettle volume, 10oz or so of volume.

Strike water will expand ~2.7% at mash temp.

You'll need space to mix.

You may want to undershoot you're goal, a quart of space in a 9.25G tun isn't much. It's a game of shuffleboard. Better score a 2 or 3 than 0. In this case, 0's a big ol' mess. Been there.

There are a few online calculators, including Can I Mash It?

https://web.archive.org/web/20201111195637/https://www.rackers.org/calcs.shtml

 

[agentbud]

most mash calculators I have seen compute a total mash volume base on your input of mash thickness, Lbs of grain, temps, etc. I want to back into it. I want to input what I want my total mash volume to be and the lbs of grain and have it spit out the amount of water i need to add to get to that volume.
ie. 15 lbs grain + x gallons water = 9 gallon total volume . Compute x

 

[ScrewyBrewer]

The formula in column B will display the MLT volume needed in gallons. Enter your required grain weight, absorption rate, and strike water volume to calculate the MLT volume accurately. Column F will display the mash thickness based on the volumes entered.

=(E4*1.03)+(C4*D4)

 

[DBhomebrew]

http://howtobrew.com/book/appendices/appendix-d/sizing-the-tun
10oz = .078gal

9 - (.078*15) = x

 

[DBhomebrew]

Further, temp adjustment.

2.7% of 9 gallons is roughly a quart. Substract that from x.