• Each year is figured separately, using that year's due date, extension, payments and filing date, then added together.
• Failure to file: 5% of the unpaid tax for each month or part of a month the return is late (counted from the extended due date if you had a valid extension), up to 25%. In months when the failure-to-pay penalty also applies, the late-filing penalty is reduced by that amount. Returns filed more than 60 days late have a minimum penalty (the smaller of an annual dollar amount or 100% of the unpaid tax).
• Failure to pay: 0.5% of the unpaid tax for each month or part of a month after the original due date, up to 25%. With a valid extension, if at least 90% of the tax was paid by the original due date and the rest is paid by the extended due date, no late-payment penalty is estimated.
• Payment plans: one plan covers all years. Payments are applied to the oldest year first, and within a year to tax, then penalties, then interest. The IRS decides the actual order, which can change which years are paid and which age out. While a plan is in place, the late-payment penalty drops to 0.25% a month for years filed on time.
• Collection deadline (CSED): the IRS generally has 10 years from the date a year's tax is assessed to collect it. Each year has its own CSED, estimated here from the later of the due date and filing date. Your actual dates are on your IRS account transcripts and can be extended by events like bankruptcy, an offer in compromise or a collection appeal.
• Plan types: $10,000 or less in tax can usually get a guaranteed plan over up to 3 years; $50,000 or less in total (all years, including penalties and interest) can usually get a streamlined plan over up to 72 months without a financial statement. Larger balances or longer terms usually need a financial statement (Form 433-F or 433-A).
• Interest: charged on unpaid tax from the original due date, and on the late-filing penalty from the return due date, compounded daily at the IRS quarterly rate.

Not included: disaster postponements, estimated-tax penalties (Form 2210), the increased 1% rate after a levy notice, interest on the late-payment penalty, accuracy-related or fraud penalties, payments made after the due date (other than plan payments), IRS plan setup fees, and penalty relief such as First-Time Abatement or reasonable cause. Many of these can lower or raise what you actually owe, which is why a professional review is worthwhile.