The SPT 3-year weighted formula, with worked examples
The IRS Substantial Presence Test uses a 3-year weighted day-count formula. Here it is in plain math, with multiple worked examples for visa types and travel patterns.
The Substantial Presence Test (SPT) is the IRS's day-count test for non-citizen tax residency. It's not a 183-day rule; it's a 3-year weighted formula that catches non-citizens who cycle through the US year after year. Understanding the math is the difference between knowing your status and finding out via a tax bill.
The formula
You meet the SPT for the current year if both are true:
- You were physically present in the US for at least 31 days in the current year.
- Your weighted day total is 183 or more:
(current year days) + (1/3 × prior year days) + (1/6 × two-years-prior days).
The 31-day current-year minimum is a floor, not a ceiling. If you spent 25 days in the current year, you can't meet the SPT regardless of prior-year days.
Example 1: Frequent business traveler
An executive based in London visits the US for client meetings:
- 2026: 90 days × 1 = 90
- 2025: 90 days × 1/3 = 30
- 2024: 90 days × 1/6 = 15
- Total: 135 days, under 183. Not a US tax resident.
Sustainable. The executive can do this annually without triggering SPT.
Example 2: 6-month assignment
A French manager comes to NYC for a 6-month project:
- 2026: 180 days × 1 = 180
- 2025: 0 × 1/3 = 0
- 2024: 0 × 1/6 = 0
- Total: 180 days, under 183. Not a tax resident.
Tight margin. If the assignment runs 4 days longer (184 days), the manager meets the SPT and triggers worldwide US taxation. The 3-day buffer matters.
Example 3: Rotating assignments
A consultant alternates between London and NYC, ~120 days each year:
- 2026: 120 × 1 = 120
- 2025: 120 × 1/3 = 40
- 2024: 120 × 1/6 = 20
- Total: 180 days, under 183. Not a tax resident.
Now suppose the consultant pushes 2026 to 130 days: 130 + 40 + 20 = 190. Triggers SPT. Adding 10 days in the current year matters more than 30 days in the prior year because of the weights.
Example 4: Stair-step pattern that triggers SPT
An expat returns to the US gradually:
- 2026: 100 × 1 = 100
- 2025: 150 × 1/3 = 50
- 2024: 200 × 1/6 = 33.3
- Total: 183.3 days, over 183. SPT met.
Even though the expat spent 'only' 100 US days in the current year, the weighted total triggers full SPT because of the prior-year tail. This is the formula's most surprising result.
Exempt individuals (days don't count)
Some categories of people exclude their US days entirely from the SPT calculation:
- Foreign government-related individuals (A or G visa).
- Teachers/trainees on J or Q visa (limited, typically 2 years).
- Students on F, J, M, or Q visas (limited to 5 calendar years).
- Athletes in the US for charitable sports events.
- Days you couldn't leave due to a medical condition that arose while in the US.
Transit days that don't count
Days you spend in the US in transit (less than 24 hours, traveling between two foreign points) don't count as US days for SPT purposes. A layover at JFK on the way from London to São Paulo isn't a US day. But a layover where you exit the airport, stay overnight, or board a US-domestic flight typically does count.
Track the SPT correctly
Tax Days runs the full 3-year weighted SPT calculation in real time, applies exempt-individual statuses correctly, and handles transit-day exclusions. Notifications fire 30, 14, 7, 3, and 1 days before you'd cross the 183 weighted-day threshold.
If you're close to 183 weighted days, file Form 8840 to claim the closer-connection exception (if you maintain a foreign tax home). The form's claims rest on day count, yours and the IRS's reconstruction of yours.
Frequently asked questions
What is the SPT 3-year weighted formula?
You generally meet the Substantial Presence Test if you spent at least 31 days in the US in the current year and your weighted total reaches 183 days: all current year days, plus one third of prior year days, plus one sixth of days from two years prior. It is not a simple 183 day rule, because prior years carry partial weight and can push frequent travelers over the line.
Can I meet the Substantial Presence Test with fewer than 183 days in one year?
Generally yes, because prior year days carry weight. For example, 100 US days in the current year combined with 150 days the prior year and 200 days two years before produces a weighted total of about 183.3, which meets the test. That prior-year tail is the formula's most surprising result.
Do layover and transit days count toward the SPT?
Days spent in the US in transit for less than 24 hours while traveling between two foreign points generally do not count. A layover at JFK on the way from London to São Paulo typically is not a US day, but exiting the airport, staying overnight, or boarding a US domestic flight typically does count.
What is the 31-day minimum in the Substantial Presence Test?
The 31 day current-year requirement is a floor, not a ceiling. If you spent fewer than 31 days in the US in the current year (for example 25), you generally cannot meet the SPT regardless of how many days you spent in the two prior years.
How many days per year can I spend in the US without triggering the SPT?
A steady pattern of about 90 days per year produces a weighted total of 135, comfortably under 183, and about 120 days per year produces 180, just under the line. Pushing the current year to 130 days in that pattern yields 190 and would typically trigger the test, since current year days carry more weight than prior year days.