The substantial presence test: how the 183-day count works
Short answer: you are a US resident for tax purposes if you were here at least 31 days this year and this year's days, plus a third of last year's, plus a sixth of the year before's, add up to 183 or more. Staying 120 days a year is not safe: it weighs 180 in the third year and 183 after three more days.
Everything below is arithmetic on invented dates. Not tax, legal or immigration advice. Exempt-individual status, tax treaties, the closer connection exception (Form 8840) and the first-year choice can change what you owe. Check IRS Publication 519.
A worked example
Three-month client stints: 2023-03-01 to 2023-06-28, 2024-03-01 to 2024-06-28 and 2025-03-01 to 2025-05-30, as of 2025-06-15, with 40 more days planned in 2025.
Days present: 91 in 2025, 120 in 2024, 120 in 2023.
Weighted: 91 + 120 ÷ 3 + 120 ÷ 6 = 151.0, which is under 183.
The most days you can spend in 2025 and stay under the test is 122, so 31 more days are safe.
The planned 40 days make 131 days in 2025 and the weighted total reaches 183, so that trip meets the test.
How to keep your count honest
Count every day you were in the US for any part of the day, including the days you land and leave.
Leave out only days the rules exempt: days as an exempt individual (certain F, J, M and Q visa holders), a medical condition that began here, crew members, Canada or Mexico commuters and transit stops under 24 hours.
Plan around the year ahead: this year's days become a third of next year's count.
If you meet the test but were here fewer than 183 days this year, ask a preparer about the closer connection exception on Form 8840.
Common questions
How are the three years weighted?
All of the days you were present this year, one third of the days last year, and one sixth of the days the year before. You must also have been present at least 31 days this year. The total must reach 183.
Does a day I land or leave count?
Yes. Any part of a day in the US is a day present, the opposite of the 330-day FEIE test, where only full days abroad count.
Is this tax advice?
No. It is arithmetic on invented dates. Check IRS Publication 519 or a qualified professional. No warranty.