Way 1 · best default
I'm running a bit late
The normal way to warn someone. "A bit" keeps it small and calm.
Examples
Sorry, I'm running a bit late. Grab a table and I'll be there soon.
I'm running a bit late this morning, so can we start at quarter past?
In a conversation
A: Are you nearly here?
B: Sorry, I'm running a bit late. Ten minutes, tops.
Common mistake
Not: I will late a little.
Say: I'm running a bit late.
"Late" isn't a verb. Say "I'm running late" or "I'm going to be late".
