What Kind of Yarn?: I usually use 100% wool for the weft, as the weft does not have to be as strong as the warp. You can also use mercerized cotton, or silk yarn, if available. Shoot for durability and tensile strength in the yarn you choose.

How Many Warp Strings?:
What is Wraps per inch?:

The number of warp strings depends on your warp yarn's WPI, or wraps-per inch. The sample belt is woven across 7 warp strings, with a WPI of (about) 18 wraps-per-inch. This is to achieve a belt of about 1 inch across.

Yarn in wraps per inch (wpi):
This chart is from Interweave Press.
However, as the yarn I am using is NOT laceweight, I'm not sure how far to trust this wpi chart.

18 or more wpi	lace weight
16 wpi	fingering weight
14 wpi	sport weight
12 wpi	worsted weight
10 wpi	bulky
8 or less wpi	very bulky

To determine how wide to make your belt:

• Determine the wraps-per-inch of your warp yarn (see picture above) by wrapping the yarn around a ruler. The number of times the yarn wraps around the ruler within an inch (without gapping or overlapping) is your wraps-per-inch (wpi) number.
• Consider your gap between warp strings (when strung for weaving) will be the width of two strings. Remember there will be one less 'gap' than there will be strings. If there are 5 warp strings, there will be 4 gaps, 7 warp strings = 6 gaps etc..
• The horizontal stripe pattern shown requires an odd number of warp strings.

My layout for 18 wpi (rounded to 19).

The resulting fabric is greater than one inch, and has seven strings (black lines). The numbers count the gaps as being two strings wide.