Mash Tun Big Enough?

[zhopper]

I have a 40 quart (10 gallon) mash tun and I want to make a 5 gallon batch of Russian Imperial using 20 pounds of Grain. I usually use a ratio of 2 quarts per pound but could certainly use less if necessary. This is my second all grain and it would suck if I got to brew day and learned my mash tun is too small. Thanks.

 

[jescholler]

Mash Calculators

Use the "Can I Mash It?" calculation. Be careful though. I've heard that square coolers count the lid as volume, but you obviously can't use that space for mashing.