How do I calculate the length of the beginning chain for a scarf?

If getting a particular finished length of the scarf is important, make a gauge swatch. Chain a multiple of your pattern repeat that is at least six inches (15 cm) long, and work until the piece is at least six inches tall. Measure the length of the piece from side to side. Then, do the math: the length you want the scarf to be should be divided by the width of the gauge switch and multiplied by the number in the original chain. If you don't need to be so exact, just chain until the chain itself is about as long as you want the scarf to be.