Modat22 wrote:
I'm cobbling together a DIY band saw and I'm trying to size Pulleys.

Band wheels are 15 inch diameter.

I need around 75 fpm speed on the band (I'm guessing on this, waiting
on band specifications for real number)

Hokay.
15" * 3.1415926538 = 48" (nearly)
Now, this is 4'.
The band and the triving pulley run at the same speed, so 75'/4' = 19RPM.

I will have a double pulley directly connected to one of the band
wheels probably being 12 inch diameter

Motor will be 1800 rpm 10 hp gas No idea on the motor pulley size.

Ok, so you need to go down a factor of a hundred in RPM terms.

I could really use a formula that I could play with editing the sizes
of the wheels and pulleys.

Belts are easy.

Unless you care about the last couple of percent accuracy, then you can
live with the approximation that all belts run at the same speed as the
driver/driven pulleys.
This means that the RPM of a pulley connected to a belt depends on its
diameter.
If one is twice as big as the other, then it turns half as fast as the
other.
If you're using a double pulley, the one on the motor needs to be 12"/100,
or .12".
So, you're going to need several.
Even with 2, you're looking at 10 speed reduction with each, which is a
bit much normally, unless you are willing to go for very large pulleys.
3 sounds likely.
3 stages of 4.5 reduction sounds in the ballpark, which would give you
(for example) the 12" pulley on the driver wheel, then a 2.5" -12" double
pulley (which turns at about 90RPM), and another similar one, which turns
at 420RPM, then another 2.5" pulley on the motor, to the large side, which
turns at 1800RPM.
(figures done in my head, and are approx)

I have doubts that a single belt will give you enough torque on the
driver pulley.