Calculating Total Warp Length

It can be really scary to make the leap from pattern to original design- and not because you do not have wonderful ideas bursting forth but more because it is sometimes difficult to know where to begin.  One of the biggest stumbling blocks can be the amount of materials to gather.  Materials can be very expensive and/or difficult to get.  A lot of times they need to be ordered or special trips need to be made to stores outside our normal routines. And what could be worse than getting near the end of a project only to discover your warp is too short! (Trust me, it's awful. It happens and it is awful.)  However, with a little planning (and yes, a little math) you can move forward in the creative process with confidence.  This post will be part of a multi-post series about planning for a project.  This post is going to focus on calculating warp length.  To follow this post will be small tutorials on weft length, color choices, sampling and fiber content considerations.

Step 1:  What are you weaving?  How big would you like it?
Knowing what you are going to weave is really important in deciding the width and length of your project.  A scarf is different from a wrap which is different from a curtain which is different from a placemat.  How about if you want more than one item on a single warp?  There are resources that list the standard sizes for most garments, accessories and kitchen items, or you can decide a custom size that you, the artist would enjoy. 
Once you know what you are weaving and what size you would like the finished item to be, you're ready to start writing things down, gathering information and really getting into the planning of your project. 

Step 2: Width in the Reed
When you weave, you must make considerations for draw-in and shrinkage.  This can change drastically based on your weave structure, fiber choice and finishing techniques.  The surest way to know what shrinkage you can expect is to sample. Not only does this help you to decide width in your reed and warp length, but it also gives you crucial information for your weft, which we will discuss in a later post.  However, I do understand that sampling on a loom is not always easy or practical, especially if you need to order materials in advance of the project. The good news is you can make some good estimates and achieve a close approximation to your desired final project size. (And make sure to take notes, that way if you ever plan a project with similar structure and materials, you'll have sample information in your previous work.) Perhaps I am getting ahead of myself though; why do we care about shrinkage?  
If you want your project to be a certain width, let's say a scarf that is 10 inches across, you need to warp your loom wider than this final measurement so that there is room to shrink when all of the draw-in and finishing is complete.  So for that 10-inch scarf you may need to have a 12-inch weaving width, giving two inches for draw-in and shrinkage.  Like I said, sampling is the best way to know what kind of shrinkage to expect from your materials.  However, you need not try to reinvent the wheel, you can always look at reference materials of similar structures and fiber to see how much shrinkage those projects experienced and model your plans from there.  If exact length and width is not critical (but this can be risky) you can assume a percentage.  This approximation must consider your weave structure and finishing techniques, as well. If I go this route, I typically assume a 10-20% shrinkage depending on the fiber, structure and finishing. Silk shrinks very little and would be closer to 10% in my calculations but wool typically shrinks a lot and would likely get closer to 20% in my calculations.  To use a percentage, whether determined from sampling or close estimation, you would use an equation that looks something like this:

Desired Final Width + (Desired Final Width x Percentage of Expected Shrinkage Converted to Decimal)= Weaving Width In Reed

In real numbers for our example, that might look something like this:

10 inches + (10 inches x 0.2) = 12 inches

So, for the example scarf, I would need to center for a 12-inch weaving width in my reed.

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Step 3: Dent Size
The sett of your project is determined by your reed.  Sett refers to the number of threads or ends per inch (epi) there are for your cloth.  If you are using a 12-dent reed and warp one thread per dent, then you will finish with 12 threads in an inch or, more formally, 12 epi.  There are other scenarios that can affect the calculation of your epi, such as if you wish to sley two or more ends in a single dent. There are charts available that give recommended ranges of epi for various warp threads.  The most complete I have found is the "Master Yarn Chart" by Handwoven of Long Thread Media.  These recommended ranges allow you to plan for what would be an appropriate dent size for your desired thread.  Included are ranges for lace, plain and twill weaves and the chart is organized by fiber type.  If you wish to weave a denser than average plain weave, you would need to move to a higher than recommended epi count. Likewise, if wish to weave a looser than average plain weave, you would need to move to a smaller epi count than the recommended number.  
Once you have your appropriate dent-size chosen and you know how many ends you would like to sley per dent, you can begin to calculate how many threads in total you will need for your project.  

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Step 4: Put Steps 2 and 3 Together
Now that you know how wide you want your project in the reed and what dent-size you are using, you can calculate how many ends you will need to measure out for your project.  This calculation is for both direct and indirect warping methods as it is simply the number of warp threads for your project independent of your warping style.  However, when using a direct warping method, it is important to remember that each journey out to your peg contains 2 warp ends just as the journey to and fro on a warping board also contains two warp ends.  The equation you will use is as follows:

Ends per inch (epi) x total width= number of warp ends

The ends per inch is determined by your dent size.  Remember to double this number if, for example, you are using two threads in every dent.  The total width is the how wide you want your final cloth making considerations for shrinkage and draw-in.  The number of warp ends will be the total number of ends you will need to measure out for the warp of your project.  In my example, the equation would look like this:

​12 epi x 12 inches = 144 ends

Step 5:  You Know How Many, Now How Long?
Once again you need to account for shrinkage.  When you weave on a loom, your warp threads are under a lot of tension.  When that tension is released, your cloth will relax and shrink.  This shrinkage happens before you apply any kind of finishing to the project, which can cause even more shrinking. The means the shrinkage in the vertical direction is often greater than the horizontal shrinkage we calculated earlier.  In addition, you have to make room for the portions of your warp you simply cannot weave on.  There are some tricks to minimizing this loom waste, but the fact remains there will be some waste coming off your loom and you need to plan for this.  The best way to plan for loom waste is to look at similar projects from looms that are closely related to your own.  Rigid heddle looms, tapestry looms and floor looms typically produce different amounts of waste. The range, however, is typically 15-38".  When uncertain, always aim high.  You do not want to run short on warp after weaving an entire project. Also, if you are planning for a fringe, you need to leave space on your warp for this.  Lastly, if you are planning more than one project on your warp, like if you want two kitchen towels or four placemats, you need to plan for any spaces needed between the different cloths.  Whew, that's a lot to consider.  Luckily they are all numbers that can easily be plugged into an equation such as this:

(Desired Final Length for 1 Project x Total Number of Projects) + Shrinkage + Loom Waste + (Fringe Hem Length x 2 for Each Project) + Space Between Projects = Warp Length 

In our example, I was making a scarf.  I want this scarf to be 70 inches long with a 5-inch fringe.  I will assume 20% shrinkage and 20" of loom waste.  I am only making one scarf on this warp.  My total warp length would then be:

​(70 inches x 1 Project) + (70 inches x .20) + 20 inches + (5 inches x 2 x 1 Project) + 0 inches = 114 inches

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Step 6: Put Together Steps 4 and 5
Now that you now the number of warp ends and the length of your warp, you can figure out exactly how much thread you will need to complete your project.  This is done by multiplying the number of ends by the total length, since each end needs to be the full length of the warp.  

Number of Warp Ends x Total Length of Warp= Total Amount of Warp Needed

So, in our example, the total amount of thread I would need is:

144 ends x 114 inches= 16,416 inches.  This is not a super useful number in inches, as most cones and skeins are NOT listed in total inches.  However, it is easy to convert to yards by dividing my number by 36 inches because there are 36 inches in a yard.

16,416 inches / (36 inches/yard) = 456 yards

And, to get to meter, which is very useful, you simply divide your total yards by 1.09 yards/meter and round to the nearest meter.

456 yards/ (1.09 yards/meter)= 419 meters

In conclusion, I would need 456 yards (419 meters) of thread to create my desired 144 ends, each 114 inches long for my scarf.  With the correct amount of material calculated, I can move forward and order the supplies I need to get warping!
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I hope this was helpful.  Stay tuned for more project planning tutorials!