You want to wear the shirt untucked, so you are going for a shorter shirt length, but somehow it just feels too short.
How it should fit
Generally speaking we think the shirt tail of an untucked shirt should just come to the center of the butt. However, if you wear your pants particularly low around your waist this may end up being too short. The shirt tails curve up two inches at the sides of the shirt, and you don’t want the top of your pants to be below this line. In this case, you may want to lengthen the shirt so that this is covered. Also see how long should a dress shirt be.
Long sleeve vs short sleeve shirts
Interestingly, we find that short sleeve style shirts being worn untucked have a shorter optimal shirt length than long sleeve shirts. Short sleeve shirts can be made to fit almost like a t-shirt, while long sleeve shirts being worn untucked can seem too short if, when you have your arms at your side, the sleeves come down lower than the shirt tails.
Other factors to consider
You may also want to take a second look at the bottom width of your shirt. If you like to wear your shirt untucked, but it gets stuck around your belt the issue may be a function of bottom width rather than length. Consider adding 1″ to the bottom circumference of an untucked shirt so it doesn’t catch around your waist or hips.