Here is a simple method of measuring the outside leg for use in buying or tailoring pants or jeans.
Stand up or lie down with your legs completely straight.
Take a tape measure and place the top on your waist at the point where you want to wear the top of the pants. Some people want their pants to ride high up on their waist, while some want hip-huggers that ride low.
Run the tape measure down to your ankle or lower at the point where you want the bottom of the pants leg to end. Some people want their pants leg to reach just above their ankles, while some want it to cover the top of their shoes.