With your right arm at your side bend it at the elbow bringing your hand up 90 degrees (palm up), measure from your elbow crease to the last joint on your index finger. Should get you pretty dang close.
If you're a lefty...... well you know..... use the other hand


At 6'2" mine runs around 14.75, but I usually order 14.5 to allow for some heavier clothing in the cooler weather.
Chris