The other posters had the math right at about 100:1 reduction. A few
comments:

V belt pulley systems do not like to transmit heavy power loads at much
over 5:1 per stage so you would need at least 3 stages. You can go
higher but the drive pulley gets so small that it cannot handle much
torque.

You mention a 10 hp gas motor at 1800 rpm. this would calc to about 28
ft pounds of torque. Do a 100:1 speed reduction and you have 2800 ft
pounds, enough to do an 18 wheeler justice.

Consider using a purchased gear reduction box. This one is 20:1, would
work well with a 5:1 V belt off the motor. Bearings are probably good
enough to use it for the band saw wheel.
http://www.surpluscenter.com/item.as...ame=powerTrans
Note that this is only rated to 1/2 hp, not the 10 you are talking about.

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)

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.

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

Thanks folks.

P.S. on a different topic I going to try a few welding classes. It is
cheaper in the long run and in instruction.