U.S. · IRS notices
IRS CP521 notice, installment payment reminder.
What the IRS says this notice is, what it asks, and where to respond.
CP521 (printed by the IRS as CP521, CP521 (SP) or CP621): The IRS is reminding you that you have an installment payment due and says to send your payment immediately. Deadline as printed by the IRS: Pay by the due date on the notice. The page says the monthly installment payment is due by a specific date shown there and prints no fixed number of days.
Look up a part of the CP521 page
Result
What the IRS says about the CP521 notice
| Topic | What the IRS page says |
|---|---|
| What it is | The IRS is reminding you that you have an installment payment due and says to send your payment immediately. |
| What it asks | Read the notice carefully; it is a reminder to send in your monthly payment. Pay the amount due shown on the notice by the due date. Mail the payment in the envelope the IRS sent and include the bottom part of the notice so the payment is credited correctly. |
| Deadline as printed | Pay by the due date on the notice. The page says the monthly installment payment is due by a specific date shown there and prints no fixed number of days. |
| Where to respond | Mail your payment in the envelope the IRS sent, with the bottom part of the notice. Questions: call the number at the top of the notice. |
| If you disagree | The page does not describe a dispute process; it covers questions only, directing you to call the number at the top of the notice. |
| If you do not respond | If you do not send a payment, interest and penalties increase and you could default on your installment agreement. |
| Payment options | The page points to the IRS payments page for payment options. |
| Need help | If you cannot resolve a penalty on your own, the page points to the Taxpayer Advocate Service, an independent organization within the IRS. |
Check your own notice
This page restates the IRS web page for the notice number printed as CP521, CP521 (SP) or CP621. Dates, amounts, tax years and the phone number or address that apply to you are printed on your notice, not on the IRS web page. If your notice number is different from this one, use the page for that number. If you are unsure a notice is real, the IRS contact information on the notice is the starting point; this page does not verify any notice.
IRS reference tools listed for this notice
The IRS page lists: Publication 1 (Your Rights as a Taxpayer).
More IRS notice pages
Other notices explained in this set: IRS CP523 notice, IRS CP53 notice, IRS CP59 notice, IRS CP63 notice, IRS CP74 notice, IRS CP60 notice, IRS CP88 notice, IRS CP52 notice, IRS CP76 notice, IRS CP77 notice.
Sources
- IRS: Understanding your CP521 notice: every statement in the table above. Read November 2, 2026.
The IRS may update its notice pages. Wording here is summarized from the page as read on the date above. Not tax, legal or financial advice.
Frequently asked questions
What is a CP521 notice?
According to the IRS, it reminds you that an installment agreement payment is due. The same page covers CP521 (SP) and CP621.
What do I need to do after a CP521?
Pay the amount due shown on the notice by the due date, mailing your payment in the envelope the IRS sent with the bottom part of the notice.
What happens if I do not pay after a CP521?
The IRS page says the interest and penalties you owe will increase and you could default on your installment agreement.
Who do I contact about a CP521?
Call the number at the top of the notice. A customer service representative will assist you.
Does the CP521 give a deadline?
The IRS page says the payment is due by a specific date, which is printed on your notice. The page itself does not state a number of days.
Not tax advice
This page summarizes an IRS web page and is not tax, legal or financial advice, and it does not describe your account. A CPA, an enrolled agent or the IRS can confirm what your notice means for you. Everything runs in your browser; nothing you enter is sent anywhere.