What is tax residency?

Tax residency determines which country has the right to tax your income. It is decided by each country's own residence tests and is separate from citizenship and immigration status.

How residence is determined

Most countries apply some combination of a day-count test, a permanent-home test and a center-of-vital-interests test. The United States applies the substantial presence test, which counts days over a three-year weighted period. The United Kingdom applies a statutory residence test with automatic overseas, automatic UK and sufficient-ties limbs.

Residence versus citizenship versus visa status

These are three different things and conflating them is the most common reader error. A person can hold citizenship in one country, a residence visa in another, and be tax resident in a third.

When two countries both claim you

Tax residency is the status a country assigns to a person it treats as subject to tax on worldwide income. The precise test varies by jurisdiction, but the consequence is consistent: a tax resident is liable to that country's full domestic tax regime, not merely to tax on income sourced within its borders. That distinction matters here because it is the basis on which two countries can each make a simultaneous, legitimate claim on the same person.

How a country reaches that conclusion depends on its domestic rules. Some jurisdictions apply a day-count threshold. The United States, for example, uses the substantial presence test, which counts days across a three-year weighted period: all days present in the current year, one-third of days in the preceding year, and one-sixth of days in the year before that. A person who crosses 183 weighted days and was present for at least 31 days in the current year meets the test and is treated as a US resident for that year. Other countries apply different criteria, habitual abode, center of vital interests, domicile, and none of those tests are coordinated with one another. A person who spends enough time in two countries, or who maintains a home in one while working in another, can satisfy both countries' domestic residency rules at the same time, through no ambiguity or error. Dual residence is a structural outcome of uncoordinated national rules, not an edge case.

Where two countries each treat a person as resident, a tax treaty between them may contain tie-breaker rules that assign residence to one of them for treaty purposes. Those rules work through a hierarchy of factors, typically permanent home, center of vital interests, habitual abode, and nationality, applied in that order, and stop at the first factor that produces a clear result. Treaty tie-breakers do not change a person's domestic-law status in either country; they determine which country's treaty benefits apply and which country has primary taxing rights over particular categories of income.

To use a treaty tie-breaker, a person generally needs to demonstrate residency in one of the treaty countries to the other. In a US context, that demonstration takes the form of a residency certificate. The IRS issues US residency certificates on Form 6166 following an application on Form 8802. Form 6166 is the document a foreign tax authority or withholding agent accepts as evidence that the holder is a US resident for treaty purposes in the relevant tax year. Without it, treaty-rate withholding and treaty-based exemptions are difficult to substantiate in practice.

Consider a hypothetical: a person is present in the United States for 200 days in 2025 and maintains a permanent home in a treaty-partner country where they are also registered as a resident. Both countries assert worldwide taxing rights. The applicable treaty's tie-breaker article would be applied, starting with permanent home. If the person has a permanent home available in only one country, that country wins the tie-breaker. If both countries qualify, the analysis moves to center of vital interests. The outcome determines which country's full treaty network applies, and which country the person would approach for a residency certificate to present to the other.

Managing the documentation across two jurisdictions, each with its own filing calendar and portal, is where fragmentation creates compliance risk. Remitian's platform standardizes and automates tax payment execution across jurisdictions, replacing disconnected systems with a single point of control and an auditable record. For the dual-residence position specifically, where the paper trail between two tax authorities needs to be precise and timely, that kind of infrastructure reduces the margin for error.

For a closer look at how treaty tie-breakers operate article by article, see the dual residence spoke. For an overview of how treaty networks interact with residency certificates across jurisdictions, see the treaty hub.

Proving your residence

Many processes require documentary proof of residence. In the United States this is Form 6166, requested using Form 8802.

Frequently asked questions

How do I know which country I am a tax resident of?

Each country applies its own statutory test independently. Start with the country where you spent the most time, then check whether a second country's rules also capture you: the UK's statutory residence test (RDR3) and the US substantial presence test are typical starting points. If both tests are satisfied simultaneously, a double-taxation treaty's tie-breaker provisions govern which residence takes priority. Those rules are covered in our guide to double taxation.

Can I be a tax resident of two countries at once?

Yes. Two countries can each conclude, under their own domestic rules, that you are a tax resident for the same period. This is dual residence, and it is a common outcome for individuals who split time across borders or relocate mid-year. Where a tax treaty exists between the two countries, its tie-breaker article, typically structured around permanent home, center of vital interests and habitual abode, determines which residence governs for treaty purposes.

Is tax residency the same as citizenship?

No. Tax residency is determined by where you live and how many days you are present, not by the passport you hold. A US citizen living abroad may remain subject to US tax on worldwide income regardless of where they are resident; equally, a non-citizen can become a tax resident of a country simply by meeting its day-count or ties test. Citizenship and immigration status are separate questions from tax residence.