The one standard factor needed to convert fibers into yarn is twist. Twist is added to the fibers by the spinning wheel (or spindle) and the amount added is determined by the ratio of the wheel and the length of the draft. But how do you determine the amount of twist needed to spin a good yarn? A good place to start is by looking at the amount of crimp in the fleece.

Here’s how it works. For this example, let’s say we have a grease Romney fleece and want to spin a 2 ply yarn. The first thing we need to do is count the crimps per inch (or centimeter) in a lock of the fleece. Take a thin lock and lay it on a ruler and count the crimp bumps along one side of the lock only, like this:
((
)) 1
((
)) 2
((
))3
((
)) 4
We’ll call that one inch (2.5 cm) and say we have 4 crimps per inch (CPI). This is the number of twists per inch (cm) that we want to have in the finished yarn. If you have already prepared fibers, you will have to estimate the CPI from a couple strands of the wool or rely on printed information for the breed. That’ s OK, spinning to the crimp is only a starting point for determining how much twist should be in the finished yarn.

Now, multiply the CPI times 1.5 which equals 6 TPI ( twists per inch)or 2.54 (twists per cm). This adjusts for the amount of twist that will be removed from the yarn when it is plied.

But we must also adjust the twist for the bulk of the fleece.
Subtract:
1 for low bulk wools
2 for medium bulky wools
3 for high bulk wools.

Washing a lock of fleece will help you determine how much your fleece will bulk up when it’s washed.

Romney is not bulky so we'll subtract 1 which gives us a goal of 5 TPI for the singles yarn.

The next step is figuring out how many wraps per inch (WPI) to spin to get the correct grist: To do this,
multiply the crimps per inch by 2. 4 x 2 = 8. Then add 2 for low bulk wools, 3 for medium bulk, or 4 for high bulk.
Our Romney is low bulk so we’ll add 2 for a total of 10 WPI..

So - our singles will be spun with 5 twist per inch (2.54 per cm) and will have 10 wraps per inch (2.54 per cm)

To figure the plying twist needed, take the number of crimps per inch (cm) and subtract for shrinkage during wet finishing. Higher bulk wools have mores shrinkage so subtract 3 for high bulk, 1 for medium bulk, and 0 for low bulk. Our Romney his low bulk so nothing is subtracted and this gives us 4 plying twist per inch (2.54 cm).

Our plied finished yarn will have 4 TPI (cm) which equals the crimp per inch of the grease fleece.

Note that spinning to the crimp is a starting point only. You may want more or less twist in your yarn depending on the planned end use and on your personal preference.
Here it is in a much simpler format:

Figuring twists per inch (cm):
1. Measure the crimps per inch (cm).
2. Multiply by 1.5.
3. Subtract for bulk: 1 for low bulk; 2 for medium bulk; 3 for high bulk.
The results is the number of TPI (cm) for singles.

Figuring Wraps per inch (cm):
1. Multiply CPI (cm) by 1.5.
2. Multiply the answer to the above by 2.
3. Add for bulk: 2 for low; 3 for medium; 4 for high.
The results is the number of WPI (cm) for singles.

Figuring plying TPI (cm):
From the number of crimps per inch, subtract: 0 for low bulk, 1 for medium bulk; 3 for high bulk.
The answer is the number of plying twists per inch (cm) needed ans will match or be only 1 or 2 TPI (cm) off from the number of crimps per inch (cm).

Many thanks to Pamela Abell for these simple formulas.