How long a t-shirt should be
Short answer: It should still cover the waistband by a couple of centimetres when you raise your arms. Shorter and it lets go of your trousers, longer and it looks borrowed.
The practical test
Put the t-shirt on and raise both arms above your head. The hem should still cover the waistband. If your stomach shows, the garment is too short for everyday use however well the rest fits.
What length does to proportions
A short t-shirt makes the legs look longer and the upper body shorter. A long one does the opposite. If it goes inside your trousers, an extra long style is easiest because it stays put. See bamboo t-shirts for men.
The difference between front and back
Many styles are cut slightly longer at the back. That is deliberate, so the back stays covered when you bend. A marked difference, on the other hand, signals a cut made for another body type. See bamboo t-shirts for women.
Length after washing
A t-shirt that is only just long enough becomes too short after two hot washes. Wash at thirty degrees if length is critical. See bamboo clothing for men.
Frequently asked questions
How much shorter does it get in the wash
At thirty degrees almost nothing. At sixty degrees a couple of centimetres.
Should a t-shirt cover the seat
Only if you want that look. For everyday wear covering the waistband is enough.
What if it is too long
An alterations shop can shorten it at the hem. That is a cheap and quick change.