You have yarn. You do not know how much. This is a solvable problem.
Weigh your skein, ball, or whatever sad remnant is left on the cake. Enter the weight from your label and the yardage it claims to contain. Enter what your scale actually says. The calculator does the rest.
It works on partial skeins, mystery yarn with no label, and that thing you wound six months ago and forgot about. If you know the original weight and yardage, you can find out what you have left.
Tip: If you don’t have the label, but know what type of yarn it is, look it up on the yarn brand’s website.
Bonus: Select your yarn weight category, and it will tell you what you can reasonably make with what you’ve got.