Packing underwear for a short trip is easy. You just pack a pair for each day you're away. However, longer trips require extra planning. Pack a pair for every day, and you get a bag that's too heavy. Pack too few and you have to launder your dirties more often.
Simply select your trip length on the top, and then move down to find your ideal underwear count. The numbers inside the grid cells indicate how many times you have to launder. Gold numbers indicate a perfect remainder of zero pairs of clean underwear by the time you get home.
Note: This chart assumes you do not turn your underwear inside out for another wearing. Not that'd I've ever done that.
See the full post for further dirty underwear details.