Online Fret Calculator
To calculate fret spacing for any instrument just enter in the scale length (also known as the vibrating length). For most fretted instruments this is the distance between the nut and the bridge.
You can enter the length in inches or millimeters.
Because mountain dulcimers are not chromatic they don't have every fret. The numbers with an asterik represent the frets on a mountain dulcimer. So fret # 2 on this calculator would be fret #1 on a dulcimer. Fret #11 on this calculator would be the 6 1/2th fret on a dulcimer.
This fret calculator is courtesy of R.M. Mottola. His Lutherie Information Website is well worth a visit!