Also called: LEP
If you go without Part B when you could have had it, and had no coverage from current employment, your premium rises 10% for every full 12 months of delay, for as long as you have Part B.
The penalty is a percentage of the standard premium, so its dollar amount grows as the premium rises. Months covered by a group health plan based on your own or your spouse's current employment do not count toward it.
Signing up during a Special Enrollment Period avoids the penalty.