The 183-day rule is used by most countries to determine if someone should be considered a resident for tax purposes. In the U.S., the Internal Revenue Service (IRS) uses 183 days as a threshold in the substantial presence test, which determines whether people who are neither U.S. citizens nor permanent residents should still be considered residents for taxation.
The 183rd day of the year marks a majority of the days in a year, and for this reason countries around the world use the 183-day threshold to broadly determine whether to tax someone as a resident. These include Canada, Australia, and the United Kingdom, for example. Generally, this means that if you spent 183 days or more in the country during a given year, you are considered a tax resident for that year.
Each nation subject to the 183-day rule has its own criteria for considering someone a tax resident. For example, some use the calendar year for its accounting period, whereas some use a fiscal year. Some include the day the person arrives in their country in their count, while some do not.
Some countries have even lower thresholds for residency. For example, Switzerland considers you a tax resident if you have spent more than 90 days there.

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