The US Substantial Presence Test, explained

The test, in one formula

You're a US tax resident under the Substantial Presence Test if you were physically present in the US for at least 31 days this year AND 183 days over three years, counting:

Add those up. If the weighted total is 183 or more (and you were here 31+ days this year), you meet the test. Any part of a day in the US — including arrival and departure days — counts as a full day.

The days that don't count

Some days are excluded: a regular commuter from Mexico or Canada, days in transit under 24 hours between two foreign points, days as a crew member of a foreign vessel, days you couldn't leave for a medical condition that arose in the US — and days as an exempt individual.

Exempt individuals: students and teachers

If you're on certain visas, your days don't count at all while you're exempt: students (F, J, M, Q) generally for their first 5 calendar years; teachers and trainees (J, Q) generally if they weren't exempt for 2 of the last 6 years; and foreign-government-related people (A, G). The catch: you must file Form 8843 every year to claim it — and once the exemption period runs out, your days start counting.

Meeting the test — and the two escape hatches

Meet the test and you're a US resident alien: your worldwide income is US-taxable, reported like a citizen's. But two things can still change the outcome:

Where a calculator stops

A day count tells you whether you cross the threshold — the essential first question. What it can't decide for you: your exact residency start and end dates, whether you truly qualify as an exempt individual, and how a treaty applies. Those turn on facts and documents. Use the count to know if you're anywhere near the line, then take it to a cross-border tax advisor if you are.