Making my own Boil kettle: Formula for pre-boil vs post boil anyone?

I'm one of those lucky guys with a small machine shop. I shopped around for equipment and figured out it would be cheaper and more fun to fabricate my own equipment. What I can't seem to find is an actual formula to figure a pre-boil volume for a given post boil volume. I know that the diameter of the BK factors in as well as the boil time. Anyone have info or thoughts on this? I'm not going to buy software to make beer and I doubt Beersmith gets this in depth anyway.

Not sure if you really need a forumla...... How much beer do you want to make per batch? Whats the pot made of? Theres a lot of other factors that come into play too like ambient temperature, dead space in the bottom, etc. Blichmann has a guide for pot sizes at this LINK, click on sizing guide tab

Thanks for the reply. The reason this maters is I plan to make a matched set. HLT, MLT,BK, and conical fermenter. I'm thinking I want about 25 gallons post boil miniumum, but also considering using 55 gallon plastic barrel fermenters with which I would want about 40 gallons post boil. So this problem factors in to the size of every other piece of equipment and what size sheets of metal I will be ordering. As far as I can tell so far, air temp, humidity, and surface area are the biggest factors. From what I've read Beersmith doesn't account for these factors accuratlly. It makes a big difference at $200 - $300 a sheet for 18 gauge 304 stainless.

Thats quite a bit of beer! Sorry I dont think Ill be much help, but maybe someone else will chime in with some more info.

I've got alot of freinds

brewjedi: I'm jealous. How about just copying a Blichmann or similar. They have already done the R&D.

Any reason not to just err on the side of caution? Assuming your kettle is very roughly as tall as it is wide, an extra couple inches of height will get you an extra five gallons of capacity. Make that sucker 35 gallons, content in the knowledge that (1) you'll probably never have to fill it all the way, because 10 gallons/hour is an insane amount of boil-off, and, (2), you'll have extra space if you ever decide to boil down more wort for a high-gravity batch/ferment an experimental 5-gallon batch with the same wort from your main batch/whatever.

For smaller batch brewing. a rule of thumb is to have your boil kettle be double your batch size.

A height to diameter aspect ration of 1.2 is a popular number for a kettle.

Have you looked at this?

http://usedstainlesssteelbarrels.com/