Calculating Yarn Needs

If I know that I need 400 of garter stitch in a worsted weight yarn, how can I figure out how many yards I need? I know I can do a swatch and see how much I use, but I was thinking there must be a calculator online or a rule of thumb that would help.

Sorry for the repetitive topic, I feel like people are always asking for help calculating yarn needs but somehow I didn’t retain what they were told and can’t find any old threads.


Here’s a link to give you general guidlines on the amount of yarn needed.