residex.ioEvery trip accounted forThe US Substantial Presence Test
How four months a year, every year, can make you a US tax resident
Why this one catches people out
Most residency rules are a single number in a single year, so you can feel yourself approaching them. The US test is not. It reaches back three years and weights them, which means you can meet it without any individual year looking remarkable.
Spend four months a year in the US — 120 days, two-thirds of the way to 183, nothing that would worry anyone — and repeat it. In the third year the test reads 120 + 40 + 20 = 180. You are three days from US tax residency, and nothing in any single year told you so.
That is the whole reason residex.io computes it for you. It is arithmetic across three years of travel history, which is exactly the kind of thing a person does not do in their head.
How the days are counted
Any day you are physically present in the United States counts, even partially. The day you land counts. The day you leave counts. A day is not a night — a two-week trip is fifteen counted days, not fourteen.
Residex counts the same way, which is why your US day count may read one higher per trip than a nights-based tally would.
The exceptions that matter
Meeting the test is not the end of the story. A closer connection to another country can be claimed on Form 8840 if you were present under 183 days in the current year and can show your ties lie elsewhere. A tax treaty between the US and your home country may override the outcome entirely.
Some people are exempt individuals whose days do not count at all: certain students on F, J, M or Q visas, teachers and trainees, foreign government-related individuals. Days you could not leave because of a medical condition that arose in the US may also be excluded, as may days in transit of under 24 hours.
US citizens and green card holders are outside this test altogether — they are taxed on worldwide income regardless. If you tell Residex you hold either, it stops showing you this rule.
What residex.io does about it
Residex keeps a running weighted total from the trips you log, shows what it is made of — this year, last year, the year before — and tells you how many days of headroom remain. If a trip you have already planned would take you across the line, it can notify you on the date that happens, before you travel.
Who needs to watch this
- Anyone spending several months a year in the US without living there
- Snowbirds and seasonal workers
- Founders and consultants with recurring US client trips
- Anyone on a visa that is not a green card
Counting days is where this stops
Residex tells you exactly where you stand against this threshold and warns you before you cross it. Crossing it is not a conclusion, though: it does not tell you what you owe, whether a treaty offers relief, which forms to file, or how this interacts with residency you already hold elsewhere.
That is where Amanda picks up — turning a day count into a read on your real cross-border obligations.
Track it automatically
Free forever. No account required. Your data never leaves your device.
How residex.io works
residex.io tracks where you have physically been, and tells you where that puts you against each country's own residency threshold.
What it does: • Counts your days per country, and per area for the Schengen 90/180 rule • Applies each country's actual rule — 183 days for most, 180 for Thailand, the UK's 6 April tax year, Portugal's rolling twelve months, and the US weighted three-year test • Counts days the way each rule counts them, including the arrival and departure days that most tests count and a nights-based tally misses • Warns as you approach a threshold, and can notify you on the date a trip you have already planned would take you across it • Exports to CSV, or hands off to Amanda for the compliance picture
Built for digital nomads, frequent travellers, expats, and anyone whose year is split across borders.
On your device, and nowhere else. There is no account, no login, and no copy of your travel history on our servers — residex works offline because the data never needed to be anywhere else.
It leaves your device only when you decide to send it: • A backup file you save yourself • A transfer to another device, encrypted with a code only you hold — the server relaying it cannot read what it is carrying • A hand-off to Amanda, which you start
We do receive a small amount of anonymous usage information — how many trips are in a log, how old an install is, the language the app is used in, and a random identifier that is not linked to you. Never a trip, a date, a destination, or your name. It is listed in full in our Privacy Policy.
The trade-off worth knowing: because your device holds the only copy, clearing your browser data or deleting the app erases it, and we cannot restore it. Save a backup before you switch devices.
No — residex is a day counter, not a compliance engine. It flags when you cross a threshold like the 183-day rule, but turning that into specific filings, tax exposure, or reporting obligations depends on your jurisdictions and personal ties.
For that, our partner Amanda maps your full cross-border legal exposure across countries, surfaces the filings and deadlines you trigger, and turns the data residex collects into a concrete compliance picture you can hand to your accountant.
You can export your residex trips and import them straight into Amanda — the CSV formats are aligned.