Your penalty is calculated when you first join a Medicare drug plan. To estimate your penalty, take 1% of the national average premium for the plan year. 1% of the 2018 average premium is $0.35. The 2018 average premium and penalty amounts are under What you pay for a Medicare Prescription Drug Plan in 2018 - Part D. This page also shows the average premium and penalty amounts since 2007.
You would multiply the number of full months that you were eligible to join Medicare Part D but were not enrolled in a plan by $0.35. This is your estimated penalty amount, which is added each month to your Medicare drug plan’s premium for as long as you have a plan. So if you were eligible but did not enroll for a 12 month period, your monthly penalty would be an additional $4.20 per month on top of your plan’s premium.