Update:  Janet Dawson came up with a much better solution to this problem. She provided a calculator! Visit Weaving with Janet Dawson and plug in your numbers without having to internalize all the math. 12/03/2022

For weavers who make their own yarn or other weft material, the possibility of running out of weft causes anxiety. That anxiety feels irritable and nervous. When you actually run out, you feel it in your heart sinking and then seizing. Try to estimate weft needs while you still feel nervous. Anxiety motivates.

Concept

If you know how much weft it takes to fill a linear inch of your warp, you can figure out how much weft you will need to get to the end of the piece. To find this value, we measure weft by weight rather than length. (Note: I measure weight in grams and textile lengths in inches. Sorry, everybody. Substitute your own units freely.) Like knitting samples where gauge is counted from a 10 cm X 10 cm area, we use several inches of weaving to calculate how much weft fills one inch. Equation 1. Calculating Weft Fill

Weft Used (g) / Filled Warp (in) = Weft Fill in Grams per Woven Inch of Fabric (g/in)

For a rag rug,

433 g Rags / 14 in = 30.9 g/in

Using the length of filled warp, is a shortcut that works for rectangular shapes where the selvedges or weft edges of the weaving remain parallel. If you find you are using weft at a rate of 30.9 g/in and you know you have 26.3 inches to go, then you can calculate how many grams weft you need to finish. Equation 2. Finding the Weft Estimate

Weft Fill (g/in) X Warp to be Filled (in) = Weft Estimate (g)

30.9 g/in X 26.3 in = 813.4 g Weft Estimate

Add a little extra to spare yourself that sinking feeling. Calculate five percent of the total weft expected in the piece, the original weft that you measured plus the Weft Estimate.

5% of (433 g + 813.4 g) = 1,246.4 g X .05 = 62.3 g

Adding five percent error to the Weft Estimate

62.3 g + 813.4 g = 875.7 g Weft to Finish

you could go to 876 g and finish the rug.

Materials

• weft: a good bit, shuttles-ful or some bobbins-ful
• measuring tape in inches (in) or centimeters (cm) according to your preference
• a scale that reads to 1 gram (g) (I use an old MyWeigh kitchen scale with 1g resolution.)
• the weaving in progress, a few shots in at least
• notebook, spreadsheet, or a slip of paper

Method

Measure the Weft to be Woven Weigh the weft. If it’s on a bobbin, weigh the weft and bobbin because you can subtract the bobbin weight later. Write the weight in your notes — you keep a notebook for important records like this. Prepare a space in the notebook for the weight of the bobbin after weaving.

Mark the Weaving Where you Start Before weaving with the measured weft, mark the last shot or row of prior weaving. Fold some short bits of yarn or sewing thread around a few warp yarns at the fell to create visible tails on the surface. This should give you a clear starting point to measure how much weft you are about to weave.

Weave with the Measured Weft Weave in the measured weft. Empty the bobbin or shuttle.

Calculate the Weight of the Weft Used Weigh the empty bobbin. Calculate the weight of the weft by subtracting the weight of the empty bobbin from the weight of the full bobbin. Equation 3. Finding the Weft Used by Weighing the Bobbin

Full Bobbin (g) – Empty Bobbin (g) = Weft Used(g)

You need not empty a bobbin to calculate how much weft you have used.

Full Bobbin (g) – Partially Empty Bobbin (g) = Weft Used(g)

Repeat For larger pieces, you will create a better estimate by measuring the weft that fills a longer length of warp. Fill the bobbin again, weigh it, weave it off, subtract the bobbin weight. If using multiple bobbins, write the weight of each bobbin on the bobbin for accurate calculation of the weft.

Measure the Filled Warp After weaving for several inches, use the measuring tape to measure the distance from the weft shot where you started weaving to the fell where you have just finished. This is your Filled Warp for use in Equation 1.

Calculate the Weft Used to Fill that Distance This will be your Weft Used (WU) measurement for Equation 1. above. If you used one bobbin, it is Weft Used  from Equation 3. If you used several bobbins, the Weft Used will be the total of all the Full Bobbin – Empty Bobbin calculations that you made. Equation 4.

WU1 + WU2 + WU3 … = Weft Used

Calculate the Weft Fill in g/in Using Equation 1. and your Filled Warp and Weft Used values, calculate how much weft it takes to fill a linear inch of your fabric.

Weft Used (g) / Filled Warp (in) = Weft Fill (g/in)

Find the Weft Estimate Using Equation 2. and the length of warp that remains to be woven to make your estimate.

Weft Fill (g/in) X Warp to be Filled (in) = Weft Estimate (g)

Decide on Your Comfort Level Make corrections in your weft supply to fill the warp. If you have a firm ending point, then you may want to boost your weft supply a little past the value that you found for the Weft Estimate. If you like doing calculations and being able to explain to yourself why you did what you did, I showed above how to accommodate 5% error in the direction of coming up short. Otherwise lay in a little bit extra weft and get to work.