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Informational

IRS CP521 Notice

Your Installment Agreement Payment Is Due

A monthly reminder on an existing agreement. Missing it is how an agreement defaults.

What is CP521? CP521 reminds you that an installment agreement payment is due.

What This Notice Means

CP521 reminds you that an installment agreement payment is due. The IRS states it is issued as CP521, CP521 (SP) or CP621.

It is the least alarming notice in this family and the one whose consequence is easiest to underestimate. The IRS states what happens if the payment does not arrive: interest and penalties increase, and you could default on the agreement. CP523 is the notice that follows a default.

Why You Received This Notice

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    The IRS states your monthly installment agreement payment is due by a specific date.

Important Deadline

The IRS says to pay by the due date, and that the notice tells you the specific date your monthly payment is due. It publishes no separate number of days.

What You Should Do

  • Read the notice. The IRS describes it as a reminder to send in your monthly payment.
  • Pay the amount due shown on the notice by the due date.
  • If you mail it, the IRS says to use the envelope it sent and include the bottom part of the notice so the payment is credited correctly.
  • If you have questions, the IRS says to call the number at the top of the notice.

Resolution Options That May Help

Payment Plan

Taxpayers with steady income who can afford monthly payments

CNC Status

Taxpayers experiencing severe financial hardship

OIC

Taxpayers who cannot pay their full tax liability

What NOT to Do

  • Do NOT skip a payment on the assumption that one will not matter. The IRS states you could default on the agreement.
  • Do NOT mail a payment without the notice stub. The IRS asks for it so the payment is credited to the right account.

Frequently Asked Questions

What happens if I do not pay?
The IRS states: "If you don't send us a payment, the interest and penalties you'll have to pay will increase and you could default on your installment agreement."
How do I make sure the payment is credited correctly?
The IRS says to mail your payment in the envelope it sent, and to include the bottom part of the notice.

Need Help With This Notice?

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Related Notices

CP522The IRS Is Reviewing Your Installment AgreementCP523Intent to Terminate Your Installment Agreement and LevyCP166Your Direct Debit Payment Could Not Be TakenLT27Your Payment Plan Application Was Not ReceivedCP504Notice of Intent to Levy (IRC §6331(d))

Recommended Next Steps

  • Check the due date and the amount on the notice.
  • If the amount is no longer affordable, CP522 is the notice under which the IRS reviews an agreement, and calling before a default is the cheaper path.

Sources and last reviewed

Sourced to primary IRS materials and editorially reviewed on 2026-08-21. Not reviewed by a tax professional. Not tax advice. This page explains a notice in general terms and is not tax or legal advice about your situation. Always use the dates and instructions printed on the notice you actually received. Report a correction.