Calculating Total Weft Length

As promised, this is the next iteration of a multi-post series to help you design your own weaving projects.  This post is going to focus on calculating the total amount of weft you will need for a project.  This calculation is less direct, in my mind, than the calculation of warp because there are more variables that are dependent on the weaver and less dependent on the equipment used.  When we calculated the warp needs, the ends per inch were not determined by us, they were determined by the reed we chose to use.  While there is some variability in the total length and width due to take-up and shrinkage, there are reliable calculations to accurately estimate these measurements as percentages of your total on-the-loom warp.  While estimations can be made with weft, there is a lot more that is dependent on the weaver's own style.  How tightly do you beat? Do you achieve a balanced weave or do you have more/less picks per inch than ends per inch in your weaving?  (Picks per inch is how you measure the number of passes the weft makes in a given inch of weaving.  A balanced weave would have the same number of picks as ends per inch.) In addition to this variability, you also have to account for the take-up that occurs as your weft passes through your warp threads.

The best policy is to sample.  This way you can very accurately calculate your picks per inch as well as take measurements for take-up and shrinkage with a chosen finishing.  If you sample, you would really just need to whip out a ruler and count the passes of weft that occur in an inch of weaving to get your picks per inch.  This can be highly dependent on the weave structure you choose.  For example, twills tend to pack in a little tighter and have more take-up than plain weave.  However, I am very well aware that sampling is not always a possibility.  So, I will take you through making estimations.  With this type of semi-blind estimation, I would caution to always err on the side of rounding up.  However, the luxury of estimating with weft opposed to the riskier warp threads is that if you run out, you can always buy more and keep weaving.   

I thought we could get organized by first answering three basic questions about your design project. 

Question 1: What is Your Weaving Width?
What is the width in your reed or rigid heddle that your warp threads occupy?  You can take a ruler and measure this exactly or calculate it by taking the number of warp ends divided by the dent size.  But this is the distance required to get from one selvage to the other of your cloth, a distance your weft thread must traverse.
Question 2: What is Your Take-Up?
This can be difficult to measure because it is a question about a single pass or pick of your weft.  The take-up occurs because you weave in three-dimensional space.  While you measure the width of your project flat across the reed, the weft thread actually has to make small journeys over and under the warp thread, adding to the distance it must travel.  Take-up can differ depending on the weave structure.  However, because we are working with estimations, you can assume a 5-10% take up for a plain weave.  If you are using a complicated weave structure or a special weft thread, you may want to consult references and other published patterns for the take-up calculations they use.  
Question 3: How Long Do You Want Your Finished Cloth?
What is the total length on the loom that you are aiming for?  You made this calculation with the warp considerations, so make sure you have aligned your goals on this one. You can also break things down further if you plan on using more than one weft color.  You simply need to keep track of how many total inches of each color will be in the entire project and make each weft calculation separately.  

Once you have answered these important questions, you can calculate your total weft.  Let's break it down into small chunks.

Step 1: Calculate the Amount of Weft Needed for One Inch of Weaving
This equation assumes 10% take-up.
Width of Warp in Reed + (Width of Warp in Reed x .1)= Length of 1 pick or pass of weaving
Length of 1 pick x Number of PPI = Weft length needed for 1 inch of weaving

This is a useful number because it will be the same no matter how many colors you plan to use.  It is simply the amount of weft needed for one inch of weaving.

Step 2: Calculate the Amount of Weft Needed for the Whole Project

Weft length needed for 1 inch x Total number of inches in finished cloth= Total amount of weft

You will need to repeat this calculation for each individual color if you are switching weft threads throughout your project.  If you are using one color of weft, you're done! Well, almost. 
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Step 3: Convert To A More Useful Number
Up to this point, we have been dealing in inches.  It is not very useful to know that your need 1256 inches of weft thread because that is not typically how yarn is sold...by the inch.  It is easy to convert, though.  

If you are looking for the total number of feet, simply divide by 12 inches/foot.
If you are looking for the total number of yards, simply divide by 36 inches/yard. 

You can also convert to meters by first converting to yards, then multiplying by 0.914 meters/yard.

I hope you found this a useful aid in calculating your weft needs!  You now have all the number you need to make some yarn purchases, and who doesn't love that? 
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