I am assuming a lot here.  Have you knitted your swatch in your stitch pattern?  Have you measured your swatch?  Did you record your stitches and rows per inch or 10 cm?  I knew you did.  You’re the best students ever!

What in the world is the conversion factor?  It is a simple mathematical calculation which will give you all the freedom in the world.  Once you master this you can adapt your own yarn choice to your patterns.  First you must choose an appropriate yarn substitution, or you will risk ending up with a 5-pound sweater with no drape that grows longer forever until you finally put it in the back of the closet because you spent a lot of money on the yarn.  Sounds familiar?

As I mentioned in a previous post, this technique works for hand knitting, too.  Our primary focus will be machine knitting.  Brand and gauge of machine does not matter, as you will soon see.  Exciting, right!?!
Pick a pattern you want to knit.  I will be walking you through a basic pull-over/jumper but feel free to choose your own.  Every pattern will have a suggested yarn.  Often the yarn label will tell you the best needle size for that yarn.  This will give you an idea of which knitting machine will be most appropriate.  Decide your tension and make your swatch.

• Pro Tip:  When I am trying to figure out the hand of a knitted fabric, cast on for a tension swatch with WY.  Knit several rows then mark tension with eyelets.  Switch to your fashion yarn.  Knit several rows at that tension.  Switch to WY knit several rows mark again with eyelets.  Switch back to fashion yarn.  Continue doing that unit you exhausted all the possible tension options.  Scrap off the machine with WY.  Determine which sample you like best.
  

How to Calculate

Measure your swatch and record your stitches and rows per inch.  (See post Reading the Swatch for details) 
Stitches:
Take your recorded stitches/inch (A) and divide it by the pattern’s stitches/inch (B).  This equals your conversion factor (C).
Formula:  A ÷ B = C (Conversion Factor)
Example:  Your gauge is 5.5 stitches/inch, the given gauge is 5 stitches/inch, therefore the conversion factor is 1.1.   (5.5 ÷ 5 = 1.1)

How to Use It
You will multiply every stitch number in the pattern by the conversion factor. 

• Pro Tip:  Start calculating stitches at the widest part of your garment.
  

This will adjust your stitch gauge.  Stop! Does your machine have enough needles for the widest part of the garment?

Example:  Pattern states CO 100 stitches.  Conversion factor is 1.1 so you should CO 110 for your substituted yarn.  (100 × 1.1 = 110)
Convert decreases, increases and all stitch related numbers with your conversion factor.  Write it on your pattern before beginning to knit as a reminder.
Now do this for the rows.
It is just that simple.  You are now free to choose whatever yarn you want to use with whatever pattern you want.  Empowering, huh? 

We will next discuss all the buttons, levers, dials and switches.

I am sure you are all caught up and have made your tension swatch.  If not, stop reading, go make a swatch then continue.
Your swatch should look something like this:

I use this technique when I am using expensive fine yarns or rare yarns, think handspun.  Another reason to use this technique is if you think you may only have enough yarn to finish the project.

Measuring the Swatch
The manual which comes with your machine will have instructions on how to measure the swatch.  Here is a brief overview.  Remember:

Green – Standard Machine
Yellow – Midguage Machine
Blue – Bulky Machine
Passap – Use a standard ruler

Lay the ruler with the ‘S’ horizontally with the ‘S’ against the left marked stitch.  Record the number at the right marked stitch.  Turn the ruler over displaying the ‘R.’ Lay it vertically with the ‘R’ at the top of the fashion yarn (or below the WY).  Record the number at the bottom of the fashion yarn (or above the WY).
Remember the number will give you stitches/rows per 10 cm. 

Example:   You recorded 28 stitches and 40 rows to 10 cm., to figure out your stitches per inch divide the numbers by 4.  If you are comfortable working in metric, simply multiply 24 x 2.8 to get the number of stitches to cast on and 19 x 4 to get the number of rows to knit.

• Pro Tip:  Roll the tension swatch and give it a gentle pull lengthwise. This will cause the stitches to “move” into place.

Ideally you want to allow your swatch to rest a few hours before measuring.  Wash the swatch however you plan to wash the final garment.  Many people skip this step because they want to get knitting right away.  This is an important step.  It could show you this yarn is not appropriate for the project intended.  For example, I thought a yarn was firm enough for an outer garment.  After washing, it softened.  It would have been a disaster as an outer garment.  Washing the swatch saved me from using a yarn which would have created disappointing results.

• Pro Tip:  Measure your swatch before and after washing.  This is a good indication of how much a yarn will shrink (or not).  I may have to knit my sleeves an inch longer so that once it is washed it will fit appropriately. 
  

You may say, “I have the preshrunk measurement.  That’s all I need.”  You are right, technically that is all you need.  How do you check your pieces without the before measurement?  Would you not prefer to know you lost count on the sleeve before its washed?

So, what do we do with it and how does it give us flexibility in our pattern choices?  We will discuss the Conversion Factor next time.

The answer is you.  Machine knitters know the importance of a tension swatch.  Did you know it is just as important for hand knitting?  Once you master the tension swatch, you will realize a whole world of design possibilities are open to you. 
Although everything I am about to say applies to hand knitters, too, I am going to address machine knitters.  I am not blowing off hand knitting but, you can find tons of information on all type of subjects.  For machine knitting, on the other hand, the information is fragmented.  It is my goal to provide you with classic information you can use for all your machine knitting pleasure.
Have you ever wished a pattern were written for a bulky machine or vice versa?  Once you master the tension swatch you can knit any pattern written for any gauge machine.  Well, as long as you have enough needles . . .  
 Flat Bed Machines
A tension swatch should be made for all stitch patterns used in the garment, ribbing, tuck, etc.  The tension swatch is the same for all the machines except the Passap.    Here is how I knit my tension swatches:

• Cast on the number of stitches listed below with waste yarn (WY)
• Knit 30 rows. 
  • This step, I knit enough so the swatch can run into the working yarn.  Usually, 20-30 on a standard and 10-15 on a mid-gauge or bulky.  If you want to ensure the swatch will not run, do a finished cast on.
  • Create eyelets for the tension number.  I put whole number on the right and the between marks on the left.
• Switch to the working yarn (WY) and knit the number of rows below.
• Place markers on the needle listed below
  • I prefer to place WY scraps.  Some people prefer to drop that stitch allowing it to run the length of the swatch.  I have found both give accurate measurements.
• Knit the number of rows below.
• Switch to WY. Knit 30 rows.  Bind off.
  

The chart is listed below for your convenience.