Calculating warp and weft

I have a couple of friends who may soon be trying weaving out for the first time. The first thing they need to do is plan out their project, which may seem daunting with all the formulae and math involved, but its really all not that bad if you break the steps down bit by bit.

So I thought I’d walk my friends through an example of how I personally calculate the various parameters of my weaving project, with an example so you can see the math in action. There is some emphasis on rigid heddle looms, since that sometimes the loom people start with, but most of it applies to other looms as well.

  1. Decide on a desired width and length of your project: Your width is contrained by the size of your loom, the length less so. There is a point (that I haven’t reached), where you wind on so much cloth on the front beam that you can’t really weave any more. This will happen quicker if you’re making thick cloth. (bulky yarns, doubleweave). For the length, include desired fringes.
    Our example: I want a scarf that will be 8″ wide. The scarf itself should be 55″ long, with a 5″ fringe on each end.
    width: 8″
    length: 55″ + 5″ + 5″ = 65″
  2. Calculate the actual width to warp accounting for draw in: Yarns in a woven project actually go over and under themselves many times, and this interlacing takes up yarn all on its own – the yarn does not go straight across from one side to another. For this reason, if you measure out exactly the length and width you need and dress your loom for those measurements, you’ll find that the project is narrower and shorter when its taken off the loom. This is known as draw in. I think the actual amount a piece draws in varies based on the material used, but I usually factor 10% in each direction.
    Our example: Our desired width is 8″. if we add on another 10%, we have the following:
    width accounting for draw in: 9″ (8.8 actually but I just chose to round up for convenience, you don’t have to)
  3. Determine sett (epi): Now we want to figure out how closely we want to put our warp yarns next to each other in the final piece. sett, or epi (ends per inch) is exactly that – the number of warp ends (yarns) in one inch of fabric. The following calculation will give you an ideal sett for plain weave – what you usually weave on rigid heddle looms – but there’s no sett (pun!) rule: If you want a drapier fabric, you can put less warp ends in an inch, if you want a firm fabric, you’d put more ends per inch.I take my intended yarn, and determine the wraps per inch using a wpi tool or ruler. I put wrap the yarns around the tool or ruler nice and snugly, the way I’d expect them to lie against each other in the finished piece, and measure how many wraps in one inch. I divide that # by 2 to get the sett.
    Our example: I choose some random yarn from my stash that will look nice. I find that I have about 19 wraps per inch. Since that doesn’t divide evenly by two, I can just say the following:
    sett: 9-10 epiIf you’re not weaving plain weave, you would need to adjust the above number. Different weave structures require different setts for a similarly dense fabric.
  4. If you’re rigid heddling, Choose a rigid heddle!  Heddles commonly come in 5,8,10,or 12 dent sizes. A dent size is related to epi. An 8 dent heddle means 8 threads can be threaded every inch. If you’re somewhere between two numbers, you can decide now if you want a looser or denser fabric.
    Our example : I can choose an 8 or 10 dent heddle. I want a dense fabric
    10 dent heddle chosen
  5.  Determine number of warp ends needed: So we know how many warp yarns are in 1″, and we know the width that we want to warp, so this part is simple:
    Our example: 10 epi x 9″ = 90 warp endsNow you know how many times you need to go around the warping board, or if you’re direct warping, you now know you need to go through 90 slots and holes, or 45 slots and 45 holes.
  6.  We now want to find out how many yards of warp yarn you will need. We will start by figuring out how long 1 warp yarn should be. Start with your intended length and account for draw in, just like we did for the width:
    Our example: 65″ length (with fringes) * 1.10 = 72″ inchesNB: Some material shrinks after being finished/washed. After you know how much this might be, you might want to tack on additional %s for your warp and weft to account for shrinkage when finishing your piece.
  7. Add loom waste: Some of your yarn will always be unweavable. You use up some yarn to tie the warp to the front and back aprons. (On my rigid heddle loom, I can minimize the yarn at the back apron rod by not cutting one end of the warp putting the loops directly on the apron rod). In addition, it will be harder to form a clear shed as the back apron rod gets closer and closer to the heddles. The yarn that remains from the woven piece that goes through the heddles and to the rod at the end of the weaving is also waste. This varies from loom to loom. Usually, the bigger the loom, the larger the waste. Rigid heddle looms have little waste. I have been putting 24″ in my calculations. That’s probably more than necessary – I think my table loom is closer to 24″ of waste than my rigid heddle – but you can adjust after you’ve done some projects and get a feel for your amount.
    Our example: 72″ + 24″ (loom waste) = 96″
  8. Add sampling space: if you want to try a new pattern, or practice some techniques at the beginning, you can add some here so that you can do so at the beginning. 6″ can be enough if you’re just trying out the pattern to figure out how much to beat and if you like it / want to adjust it. If I’m doing plainweave or I have already done the same kind of weave, I leave it out
    Our example: 96″ + 6″ (sampling) = 102″8b. If weaving multiple pieces, you may want to add some inches between each piece if you want to actually seperate them while weaving (you may simply want to cut the cloth and sew hems though)
  9.  We have arrived at our next important piece of info:
    Our example: 102″ (or 2.83 yards) is the length of our warp. How many yards do we need? Multiply by the # of warp ends:Our example: 2.83 yards in one warp end  times 90 warp ends = 254.7 yards of warp total
  10.  Almost there! We need to calculate our weft. I am going to assume we’re aiming for a balanced plain weave(Balanced plain weave is simply a fabric that has the same number of warp ends per inch as it does weft ends per inch). If your goal is weave a balanced plain weave, then you can just use the same amount for your weft yarn:
    Our example: 254.7 yards of weft total.
    Total: 500 yards needed for entire project. You can add 10% if you want to make sure you have a comfortable amount: buy 550 yards of material.When you have more weft ends per inch (or less) than the warp, you need to make a similar calculation as you did for the warp. Calculate how many weft threads you’ll have per inch, how long one weft thread is (more than the width of the piece because of draw in), and multiply by the entire width of the project.
And that’s pretty much it! You now have: yardage needed, # of ends to warp, and the length of your warp. You are ready to start winding!
You can design your own calculating spreadsheet based on the above steps. There may be plenty out there but if you make your own, you can walk through the steps and organize your sheet any way you like. I use Numbers for the Mac. It allows you to put several spreadsheets of various sizes on one blank canvas.
A final note, do something which I always forget to do: In a notebook, write the length of the warp you calculated and the width you will be warping on. When its off the loom, measure and record those widths. Once you’ve washed the piece, measure again and record.
If you do this consistantly, you can start to adjust your weft and warp draw in, and you’ll know how much washing will affect the dimensions.