Is the US 183-day rule a simple day count?

No. The US uses a weighted formula: all days in the current year, plus one-third of the days in the previous year, plus one-sixth from two years ago. If the weighted total reaches 183 (and you were present at least 31 days in the current year), you pass the Substantial Presence Test.

Can I be under 183 days in a year and still become a US resident?

Yes — the weighting is exactly why. Someone present 150 days in each of three consecutive years has never crossed 183 in a single year, yet their weighted third-year total is 150 + 50 + 25 = 225. The formula reaches back two years.

Which days never count in the US?

Days you were an exempt individual (F/J/M/Q students and teachers within statutory limits), regular commuter days from Canada or Mexico, transit days under 24 hours between two places outside the US, and days you could not leave due to a medical condition that arose while present.

Do the arrival and departure days count in the US?

Yes. US presence counting includes any day on which you are physically present at any time during the day — arrival and departure days both count, unlike UK-style end-of-day counting.

US tax residency rules: the Substantial Presence Test, explained properly

Q: What is the Closer Connection Exception?

If the weighted formula reaches 183 but you were present fewer than 183 days in the year itself, maintained a tax home in a foreign country, and had a closer connection to that country, you can file Form 8840 and remain a nonresident. It is unavailable if you applied for a green card.

Most "US 183-day calculator" pages show a single number. The statute does not. The Substantial Presence Test is a two-part test with a three-year memory, a list of days that never count, and two escape hatches. Here is how it actually fits together — with the statutory citations.

The two-part test

Under IRC §7701(b)(3), an alien is a US resident for tax purposes if they meet the Substantial Presence Test for the calendar year. The test has two parts, and both must be satisfied:

The 31-day floor. You were physically present in the US on at least 31 days during the current calendar year.
The weighted 3-year formula. The sum of —

all the days you were present in the current year, plus
⅓ of the days you were present in the first preceding year, plus
⅙ of the days you were present in the second preceding year,

…reaches at least 183 days. Both parts matter: 400 weighted days with only 25 days of current-year presence does not make you resident, because the floor fails.

Why the weighting matters more than the headline number

The formula is what separates a rough estimate from a real answer. Because a third of last year and a sixth of the year before still count, heavy presence keeps "charging" against you for two years. Three consecutive 120-day years produce a weighted total of 120 + 40 + 20 = 180 — one day under. That is IRS Publication 519's own worked example: a single extra day in the third year flips the result. Someone planning around "183 days a year" without the formula can cross the line without ever spending 183 days in any one year.

The practical consequence for planning: your remaining safe days this year depend on what you did in the previous two. A calculator that asks only for this year's days cannot give you an answer — which is why ours takes a travel ledger and computes the weighted total with the exact fractional arithmetic (⅓ and ⅙ are handled as exact integers internally: the test is equivalent to 6·current + 2·prior₁ + prior₂ ≥ 1098).

Days that never count at all

Certain days of physical presence are excluded from every part of the formula:

Exempt individual days. F, J, M and Q visa holders — students for up to 5 calendar years, teachers and trainees for 2 of the prior 6 years — are "exempt individuals". Their days of presence do not count. This is why many international students only become countable in later years.
Regular commuters from Canada or Mexico, when commuting is more than 75% of their workdays.
Transit days — present less than 24 hours while travelling between two places outside the US (the classic connecting-flight case).
Medical condition days — you intended to leave but a medical condition that arose while present prevented it.

Our calculator does not yet model these exclusions from a ledger alone; the US page lists them as caveats. If they apply to you, your true counted days are lower than shown.

Escape hatch 1: the Closer Connection Exception

Meeting the weighted formula does not end the analysis. If you were present fewer than 183 days in the year itself, maintained a tax home in a foreign country, and had a closer connection to that country than to the US, you can remain a nonresident by filing Form 8840 on time. You cannot use it if you applied for a green card or otherwise took steps toward US residence.

Escape hatch 2: first-year and last-year elections

The year you arrive in or depart from the US can be split. First-year residents may elect a residency starting date that ignores pre-arrival days; departing individuals may end residency on the date of departure if they connect with a foreign country. These elections are fact-specific and are not modelled by a day calculator — but they change the day count itself, so they belong in a conversation with a cross-border adviser.

Check your own numbers

The US tax residency calculator applies exactly the rules above — floor, weighted formula, exact arithmetic, remaining safe days — to your travel ledger, and runs the same trips against 12 other countries' tests in one pass. For the contrast with other systems, see the 183-day rule compared and the UK Statutory Residence Test explained.

Rule text verified 2026-09-30

IRC §7701(b) — (b)(2) substantial presence test; (b)(3) weighted 3-year formula — official text
IRS Publication 519 (2025), Chapter 1 — The Substantial Presence Test (worked examples) — official text
IRS Form 8840 — Closer Connection Exception Statement — official page

Informational only — not tax advice. Based on the published tests, the calculator tells you what appears to follow; elections, treaties and visa facts can change the outcome. Confirm with a qualified cross-border tax adviser.