The DT Swiss spoke length calculator has an option for nipple length. What’s that about?
DT Swiss deals with the issue in a complicated way, which is the reason their calculator asks for nipple length. Standard DT nipples are 12mm long. When you use longer DT nipples, such as 14mm or 16mm, you need to shorten your spokes. This is because DT Swiss adds internal threads as nipples grow — so you need a shorter spoke to avoid bottoming out. As a result it’s best to pretend DT nipples longer than 12mm don’t exist, particularly in the case of aluminum nipples.
With Sapim nipples it’s easy. No matter the length of nipple, spoke length is constant. This is because longer Sapim nipples have longer tool flats but keep the same internal threading regardless of size. If asked, always select 12mm nipples for the purpose of calculating spoke length even though you’re probably using 14mm nipples on Sapim builds. You can’t go wrong.
There’s another protip in all of this. If you bought DT Swiss 12mm nipples and they turn out to be too short, do not buy longer DT Swiss nipples — remember you need shorter spokes to use longer DT nipples. Instead buy longer nipples from Sapim and carry on. (And start with Sapim next time!)