A CP521 is a reminder that your monthly installment agreement payment is due. An installment agreement is a long-term IRS payment plan. The CP521 is a routine statement, not a warning. If you send the payment shown by the due date, your plan stays on track. Spanish-language versions are labeled CP521 (SP), and a similar notice is numbered CP621.
Why you got a CP521
You have an active installment agreement with the IRS, and a payment is coming due. IRS reference material describes the CP521 as an "Installment Agreement Reminder Notice." You may get one even if you've never missed a payment.
What the CP521 says
- The payment amount due and the due date.
- Your remaining balance, which includes tax, penalties and interest that continue to accrue while you pay.
- Recent payments the IRS has credited.
- A payment stub and envelope for mailing your payment.
- An IRS phone number at the top for questions.
Compare the payments listed with your own records. If a payment you made isn't there, the IRS says to allow one to three weeks for it to be credited, or three weeks for non-electronic payments.
What to do
- Pay the amount shown by the due date. Mail it in the envelope with the bottom part of the notice, or pay online through your IRS Online Account or IRS Direct Pay. Checks should be payable to the "United States Treasury," with your name, address, SSN, daytime phone, tax year and form number on them.
- Keep every new return on time and fully paid. The IRS lists this as a condition of staying in good standing. Your future refunds will be applied to the debt until it's paid off.
- Keep making scheduled payments even if a refund is applied. The IRS says a refund applied to your balance doesn't replace your monthly payment.
- If the payment no longer fits your budget, change it before you miss one. In your Online Account you can change the monthly amount, change the due date, switch to direct debit, or update bank details. The IRS lists a $6 fee to revise a plan online and $89 by phone, mail or in person (reimbursable for low-income taxpayers; $0 for changes to existing direct debit plans).
- If you moved, update your address online or with Form 8822 so you keep getting notices.
Consider direct debit. A Direct Debit Installment Agreement pulls payments from your checking account automatically, so you're less likely to miss one. Converting an existing plan to direct debit can be done online. Our payment plans guide compares plan types.
An optional installment agreement kit is available on this site if you're reworking your plan, but most changes can be made online in minutes.
What happens if you ignore it
The IRS says that if you don't send a payment, the interest and penalties you owe will increase, and you could default on your installment agreement. Default leads to a CP523, the notice of intent to terminate your installment agreement and levy your assets. Reinstating a defaulted plan may involve a fee.
While your plan is in effect, two cost rules work in your favor:
- Lower penalty. If you filed your return on time as an individual, the IRS cuts the failure-to-pay penalty from 0.5% to 0.25% per month during an approved payment plan.
- No new levies. Publication 594 says the IRS can't issue a new levy while a payment plan is in effect.
Interest still applies. The IRS underpayment rate for individuals was 7% for most of 2026 (6% from April through June) and is reset each quarter.
Say $6,000 of unpaid tax remains on your plan. At the reduced 0.25% rate, the failure-to-pay penalty is $15 a month. If the plan defaults and the reduced rate no longer applies, the regular 0.5% rate makes it $30 a month on the same balance, before interest.
Is this notice real?
You should recognize the balance from your own payment plan. To double-check, sign in to your IRS Online Account, which shows your plan type, due dates and payment amount. Pay only through IRS.gov/payments or by check to the "United States Treasury." The IRS won't ask you to pay a plan installment with a gift card or prepaid debit card. See IRS scams.
Frequently asked questions
Do I need to call the IRS when I get a CP521?
No. Just pay the amount due by the date shown. Call only if something on the notice doesn't match your records.
Can I pay more than the amount on the CP521?
Yes. Paying extra reduces your balance faster and cuts the interest and penalties that keep accruing.
My refund was applied to my balance. Can I skip this month?
No. The IRS says to make all scheduled payments even if it applied your refund to your balance.
Does the IRS still file a lien while I'm on a payment plan?
It can. Publication 594 says that even after approving a payment plan, the IRS still charges interest and applicable penalties and may file a Notice of Federal Tax Lien. If a lien has been filed, it says the IRS may withdraw it once you've entered into a payment plan, unless the agreement says otherwise. For certain direct debit plans it routinely grants withdrawal requests if you meet other conditions. See our federal tax lien guide.
What if I can't make this month's payment?
Contact the IRS before the due date. Go to your Online Account to lower the payment or move the due date, or call the number on the notice. Acting early is much easier than fixing a default. If you can't pay anything, ask about currently not collectible status.
Sources
- Internal Revenue Service (IRS), "Understanding your CP521 notice". irs.gov
- IRS, "Payment plans; installment agreements". irs.gov
- IRS, "Failure to pay penalty". irs.gov
- IRS, "Quarterly interest rates". irs.gov
- IRS, Publication 594, The IRS Collection Process (Rev. 1-2026). irs.gov
- IRS, Document 6209, Section 9: Notices and Notice Codes (2018). irs.gov
IRSV.com is an independent guide and is not affiliated with the IRS. This page is general information, not tax or legal advice. Rules change; confirm details on IRS.gov or with a tax professional. Found an error? Tell us.