The calculations were easy but were based upon one assumption that may or may not hold. I assumed that my soap recipes will turn out to be the same density (weight per volume) as River Soap's milled bars. In any case my soaps will probably be in the ball park.
I just multiplied height x width x length to get volume in cubic inches.
River Soap ( 4.5 oz) 3-1/4 x 2-3/16 x 1-3/16 = 8.4 cu in
Paul's dimension: 3-1/2 x 2-1/2 x 1-1/4 = 10.9 cu in
proportion: 10.9 / 8.4 = 1.3 (130 percent)
Compare this with a fairly popular part time soaper size:
hand bar: 3 x 2 x 1-1/4 = 7.5 cu in (which was too small for me)
In the end I decided to just go with the river Soap size (8.4 cu in) and find out what it weighs when I get some. I could calculate from the weight of oils and lye, then add part of the weight of the water (to allow for drying) but the best answer is to just make some bars and weigh them.
Actually I plan to try Paul's size for my next molds. What I've discovered is that there's no exact answer for what size to make the bar.
I'm nearing retirement age and thinking that soaping might make a fun part time business to supplement my retirement income, and might also be a nice social outlet to meet people and get me out of my house. I'll already have a separate income so I figure that the soap might be a nice way to earn money for luxuries and walking around money.
Actually, here in L.A. I bet I could have a yard sale every weekend or every Saturday and sell soap along side getting rid of excess stuff too, pick up some money for both.
Ultimately I figured on going to swap meets and craft shows and the like, and combining that with Internet sales. I've already sold stuff related to my career online (electronics) so the Internet stuff is easy, and in any case has gotten easier with sites such as etsy and ecrater.
I'm glad though that I don't intend on supporting myself on selling soap. I think that would be difficult to do and would be at least a 40 hour a week job, probably more.