Trouser length depending on your shoes
Short answer: With flat shoes the trousers should just touch the shoe. With boots they can be a fraction longer, and with sandals shorter.
Why the shoe decides
Trouser length is measured to the floor, but the shoe decides how high the foot lifts the trouser. A three centimetre heel needs three centimetres more trouser to land in the same place on the shoe.
The three common cases
With trainers and flat shoes the hem should rest lightly on the shoe without folding. With boots it can sit a couple of centimetres longer so it does not ride up as you walk. With sandals it should finish above the ankle bone. See bamboo trousers for women.
Joggers are a different matter
Trousers with a cuff are chosen by how high you want the cuff, usually just above the ankle bone. Too long and the fabric gathers and bunches at the ankle. See bamboo sweatpants for men.
With two shoe types, choose two lengths
If you wear both trainers and boots daily, the simplest answer is one pair of trousers for each. A single length rarely suits both. See bamboo jogging sets for women.
Frequently asked questions
How much shorter for sandals
Two to three centimetres above the ankle bone so the trousers do not drag.
Should trousers break over the shoe
A light touch is enough. A deep break looks accidental and wears the hem.
Can I turn them up instead
Yes on firm fabric. On knitted trousers the turn up gets bulky and falls down.