The band on your bra rides up at the back
Short answer: The band rides up because it is too loose, not because the straps are too long. Tighten the band, loosen the straps and measure yourself again.
The band carries the weight
Most of the support comes from the band around the body and not from the straps. If the band is too loose, it travels upwards to the narrowest part of the ribcage and the straps end up carrying everything. Then you tighten the straps and the band rides higher still.
How to check
The band should sit level all the way round and be tight enough that two fingers just fit underneath. Raise your arms above your head. If the band lifts with them, it is too big. See bamboo bras without underwire.
Try the sister size
Going down one band size means going up one cup size to keep the same volume. Many people find a far better fit that way without really changing cup volume.
When the bra is worn out
The elastic in the band gives over time. If you are always on the tightest hook, it is a sign the bra needs replacing. See bamboo underwear for women and bamboo sports bra.
Frequently asked questions
Which hook should a new bra fit on
Buy it so it fits on the loosest hook, then you have room to tighten over time.
Does tightening the straps help
Only briefly, and it makes your shoulders ache. The band has to solve the problem.
How often should you measure
After a weight change and otherwise once a year. The body changes more than most people expect.