Form 6166: How to Get a US Residency Certification Letter
In this guide
Quick answer: Form 6166 is a letter from the IRS certifying that you are a resident of the United States for income tax purposes. Many treaty partners ask for it before they give you treaty benefits. You get it by submitting Form 8802, Application for United States Residency Certification, and paying a user fee. The IRS says the fee rises to $105 for individuals from 1 October 2026.
If you live in the US and earn income, pensions or dividends from another country, that country’s tax office may want proof of where you are taxed before it applies a lower treaty rate. Form 6166 is that proof. This guide sets out what it is, how to request it and what it costs, using the IRS page on certification of US residency for tax treaty purposes. For the wider picture, read our guide to tax treaty benefits.
What is Form 6166?
The IRS says many US treaty partners require US citizens and US residents to provide a US Residency Certificate to claim income tax treaty benefits, or certain other tax benefits, in those foreign countries. The IRS provides this certification on Form 6166, a letter of US residency certification.
It is a computer-generated letter printed on Department of the Treasury letterhead. It certifies that the individuals or entities listed are residents of the US for the purposes of US income tax law.
How do you get Form 6166?
You must submit a completed Form 8802, Application for United States Residency Certification. A user fee is charged to process every Form 8802. The IRS points to the Form 8802 instructions for details, and to its pages on foreign certification requests and on additional certification requests.
How much does it cost?
| Applicant | Fee before 1 Oct 2026 | Fee from 1 Oct 2026 |
|---|---|---|
| Individual | $85 | $105 |
| Business (non-individual) | $185 | $230 |
The IRS page announces these increases as starting on 1 October 2026. Check the current Form 8802 instructions before you pay, because fees can change.
Who is the letter for?
The letter is for dealing with the foreign country, not the IRS. Typical cases are claiming a reduced withholding rate on foreign income or claiming another tax benefit that country grants to treaty residents. The IRS page does not list specific countries or the documents each country requires, so ask the foreign tax authority or payer what it needs.
What should you check before applying?
- That you are actually a US tax resident for the year concerned. See the green card test and the substantial presence test.
- Which tax year the foreign country wants certified.
- Whether the foreign payer needs the original letter or accepts a copy. The IRS page does not say.
Common mistakes
- Applying before confirming you meet the residency tests.
- Using an old fee amount.
- Assuming the letter itself reduces foreign tax. It only proves residency; the foreign country decides the benefit.
Frequently asked questions
Is Form 6166 the same as Form 8802?
No. Form 8802 is the application. Form 6166 is the letter the IRS issues.
Can a US citizen get one?
Yes. The IRS says treaty partners require citizens and residents to provide it.
How long does it take?
The IRS page does not state a processing time, so allow extra time and check the Form 8802 instructions.
Key takeaways
- Form 6166 is the IRS letter that proves US residency to a foreign country.
- You request it with Form 8802 and a user fee.
- The individual fee is $105 from 1 October 2026 per the IRS page.
Related guides
- US tax treaty benefits
- Foreign tax credit
- Expat tax for US citizens abroad
- The green card test
- Your first 90 days in a new country
This page is general information, not legal or tax advice. Last updated: October 2026.
Sources
Internal Revenue Service: Certification of U.S. residency for tax treaty purposes.