If you file on time, but pay late, the penalty is 0.5% of the late amount, per month.
If you file late and pay late, then the penalty is 5% of the late amount, per month.
You don't have to calculate or pay the penalty right now; the IRS will send you a bill.
You can request a "first time" abatement of the penalty if you've paid and filed, and if you haven't had any other penalties in the last three years. Instructions are here:
https://www.irs.gov/payments/penalty-relief-due-to-first-time-abate-or-other-administrative-waiver
Hope that helps.
