How does the US substantial presence test count days?

Year	Each day counts as	122 days a year gives
Current year	1	122.00
First prior year	1/3	40.67
Second prior year	1/6	20.33
Weighted total		183.00

Plus a floor: at least 31 days in the current year, or the test is not met at all.

The substantial presence test has two conditions that must hold at once, and failing either means it is not met.

First, at least 31 days in the current calendar year. Fewer than that and the test stops there, regardless of history.

Second, 183 or more weighted days across the current year and the two before it:

• days in the current year × 1
• days in the first prior year × 1/3
• days in the second prior year × 1/6

The trap is that a steady pattern crosses the line. 122 days a year, every year, gives 122 + 40.67 + 20.33 = 183 — exactly the threshold, from a pattern that never once looks like half a year. Anyone splitting their time roughly a third in the US is closer to US tax residency than the "183 days" shorthand suggests.

Any part of a day of physical presence counts as a full day, so arrival and departure days both count. Several categories of day are excluded, and they are narrower than people hope — among them days you are in transit between two places outside the US for under 24 hours, days you regularly commute to work from Canada or Mexico, days as a crew member of a foreign vessel, days you cannot leave because of a medical condition that arose while there, and days as an "exempt individual" such as certain students, teachers and trainees on specific visas.

Exempt-individual status is a status question, not a day-count question, and it has its own limits and filing requirements. It is the part of this test most often assumed rather than checked.

Someone who meets the substantial presence test may still be treated as a non-resident under the closer connection exception, which broadly requires being present under 183 days in the current year, having a tax home in a foreign country, and having a closer connection to that country than to the US. It is claimed on a form, not assumed, and it is unavailable to anyone who has taken steps toward permanent residence.

A tax treaty may also allocate residence elsewhere. Both routes are fact-specific and both require filing, so this is a point for advice on your own position rather than a rule of thumb.

• IRS — Substantial Presence Test
• IRS — Introduction to residency under US tax law
• IRS — Conditions for a closer connection to a foreign country