How do I calculate how much yarn I need when seaming? I’ve done one end but I now have to go to the opposite end so have to cut the required length of yarn but how do I know how much to cut? Like when casting off its 3 times the width of the project - so is there a similar calculation for seaming?
For mattress stitch, the yarn is pulled taut and mostly straight in the seam, so you need about 1x the length of the seamline plus a safety margin so there's enough yarn to weave in the end (and to account for extra yarn if it isn't pulled completely tight), that leaves you with about 1.5x the length of the seam.
Other seaming methods will be different. A simple whip stitch would probably also use about 1.5 to 2x the seam length, but for backstitch or blanket stitch it you'd need 3x or more because the thread doubles back more or takes a complicated path.