A helpful way to choose the right shoelace length for your new pair is to measure against a shoelace you already use. Start by removing one full lace from your shoe and laying it completely flat and straight on a table or other flat surface. Using a measuring tape, measure from tip to tip, including the aglets (those small plastic or metal tips on each end).
Use this measurement as your baseline when buying replacements. For example, if your current laces feel perfect, buy the same length. If they're a little short, go longer, and vice versa.
If your old laces are lost or chewed up by a pet, don't just guess what size lace you need — test it first. Grab a piece of sturdy string, yarn or even a spare shoelace from another pair that you think is longer than the lace that you'll need. Lace up the shoe using your normal style and tie the string or laces like usual. Once you're satisfied, mark where you want your shoelaces to end, unlace and measure from the end to your marking.
This method gives you a more flexible way to tell what size shoelace you need, and is ideal for shoes with unusual eyelet spacing. Don't be afraid to test out a few different lacing techniques and see how the amount of lace you need varies between styles.