The US Substantial Presence Test, Step by Step
How the IRS three-year weighted formula works, which days are excluded, worked examples, and the closer-connection exception.
The United States does not use a plain 183-day rule for non-citizens. It uses the Substantial Presence Test (SPT), a three-year weighted formula that catches people who never spend more than half of any single year in the country. If you meet it, you are generally a "resident alien" for federal income tax and taxed on worldwide income, just like a citizen or green-card holder.
The formula
You meet the test for a calendar year if you were physically present in the US on at least:
- 31 days during the current year, and
- 183 days during the three-year period made up of: all days in the current year + one-third of the days in the previous year + one-sixth of the days in the year before that.
The IRS's own example: someone present 120 days in each of three years counts 120 + 40 + 20 = 180 days and does not meet the test. Add a few more days in the current year and they would.
What counts as a day
Any part of a day physically present in the 50 states or DC counts. Some days are excluded, including:
- Regular commuting days from Canada or Mexico for work.
- Days in transit between two foreign points of less than 24 hours.
- Days as crew on a foreign vessel.
- Days you were unable to leave because of a medical condition that arose while in the US.
- Days as an "exempt individual" — certain diplomats, teachers/trainees on J or Q visas, students on F, J, M or Q visas, and professional athletes at charitable events, subject to limits.
Exempt individuals generally must file Form 8843 to claim the exclusion.
Worked examples
Example A — the seasonal resident
Maria spends 150 days in Florida in 2024, 150 in 2025 and 150 in 2026. For 2026: 150 + 50 + 25 = 225. She meets the SPT for 2026 even though she never spent half a year in the US.
Example B — the short first year
Ahmed visited for 20 days in 2026 after 300 days in 2025. He fails the 31-day requirement for 2026, so he does not meet the test for 2026 regardless of the weighted total.
The closer-connection exception
Even if you meet the SPT, you may be treated as a non-resident if you were present fewer than 183 days in the current year, kept a tax home in a foreign country for the whole year, and had a closer connection to that country than to the US. You claim it on Form 8840 by the filing deadline. It is unavailable if you applied for a green card.
Treaty tie-breakers
If you are resident in both the US (under the SPT) and a treaty country under that country's law, the treaty's tie-breaker article may make you resident only in the other country for treaty purposes. This is claimed on Form 8833 and has knock-on effects for reporting, so it is a conversation for a professional.
Check your numbers
Run the SPT calculator or log every trip in the Day Tracker, which calculates your weighted US days automatically. State residency is a separate question — see our state guide.
Source: IRS, "Substantial presence test" and Publication 519. Thresholds as published at time of review.