Hello from Germany!
I’m searching for a book or a website which helps to calculate the yarn needed. I did my last weaving 30 years ago and would like to restart. Thanks for answering and best regards.
I have answered questions similar to this before, but this might be a slightly different slant. The first bit of information you need to know for weaving project yarn calculation is your loom waste. That means the length of warp that is left over after you finish weaving —the amount used in tying onto or looped around the back apron rod plus the length from the back apron rod to the fell of the cloth where you cut the woven fabric from the loom. This distance can be anywhere from 15″ to 30″ depending on your loom’s construction. Then (this is important and not usually considered as part of loom waste) you have to add to that total the amount of warp you used to tie onto the front apron rod and that you wasted spreading the warp before you actually started weaving the real cloth. This could be about 10″. In Handwoven magazine projects, we allow at least 27″ for total loom waste. This is a bit tight, so I usually allow 1 yd for total loom waste (you are probably metric, so will have to convert that to a bit less than a meter).
Next, you have to consider take-up and shrinkage for the yarns and weave structure you have selected. I usually allow 10% for cottons and other spun yarns that do not shrink very much when woven in plain weave or similar structures. If the yarns shrink a lot or you use a weave structure that collapses a lot, you have to add to that number. But let’s start with 10%.
So say you are going to weave a scarf that you want to be 65″ long when finished, with 6″ fringe at each end. You can consider the amount needed for fringe at one end to ne the part of the warp you used for tying on and spreading the warp (the 10″ mentioned above). To determine the length of warp needed for the scarf itself, divide 65″ by .9 (100% – the 10% shrinkage), for 73″ warp length (rounding up). So 10″ + 73″ + 26″ (remaining loom waste, which will include the fringe needed at the other end) equals 109″ or 3 yd plus 1″ (a 3 yd warp length would be enough).
(I usually operate a bit differently from this kind of planning in that I never really like to warp for one item. As you weave the first one, you will have ideas about how you could vary the weft colors or the treadling order for something even better. So I always put on enough warp for two or more items.)
Beyond that, of course, you have to know the number of threads in your warp. You’ll multiply that number by the warp length (for an 8″, 3-yd warp at 20 ends per inch: 8 x 20 x 3 yd = 480 yd).
To calculate the amount of weft, you need to know warp width, the number of picks per inch, and the length of the weaving. I usually add ten percent to that number for weft take-up. (So for an 8″ wide warp woven at 20 picks per inch for 65″: 8″ x 20 x 65″ = 10,400″ divided by 36″/yd = 288 yd plus 10% = 317 yd.
So to sum up, here’s your weaving project yarn calculation:
Warp: Loom Waste + ((Desired Length / (100% — Take-Up) x Desired Width x EPI)
Weft: (Desired Width x PPI x Desired Length) / (100% — Take-Up)
Hope this helps! I realize that you will have to convert ends per inch into ends per centimeter, etc., but the process will be the same.
For a really helpful book that covers this and much more, consider The Weaver’s Companion!