US state residency is where a lot of otherwise careful people get caught. States tax residents on all of their income, and the rules that make you a resident are not the same as the federal ones - a state can claim you through your domicile or through a 183-day statutory line, whichever bites first. This guide explains how state residency actually works, why the burden of proof sits with you, and how to track state residency for taxes automatically on your iPhone so no per-state day count ever slips past the line.
Why state residency is its own problem
US states tax residents on their entire income, wherever in the world it was earned - not just income sourced inside the state. That makes residency the switch that turns a whole tax bill on or off, and states with aggressive audit programs know it. If you split your time between, say, a high-tax state and a no-income-tax state like Florida or Texas, the exact number of days you spent in each place is the difference between owing a lot and owing nothing.
The trap is that people assume "residency" means one thing. In practice there are two separate doors into state residency, and you only have to walk through one of them to be taxed as a resident.
The two ways a state makes you a resident
Almost every state defines residency through some combination of these two tests. Understanding which one applies to you decides what you actually need to track.
- Domicile. Your domicile is your true, fixed, permanent home - the place you always intend to return to. You have exactly one domicile at a time, and it is deliberately sticky: once a state is your domicile, it stays your domicile until you clearly establish a new one somewhere else. Where you keep your home, your family, your driver's licence, your voter registration and your "center of life" all feed into it.
- Statutory residency. Even if you are domiciled elsewhere, a state can still treat you as a resident if you (a) are present in the state for a threshold number of days - commonly 183 or more - and (b) maintain a permanent place of abode there. A permanent place of abode generally means a home you actually use as a residence, not merely a dwelling you happen to own: recent New York case law has narrowed this so that a rarely-used or vacant second home you never really live in may not count. Meet both conditions and you are a statutory resident, fully taxable, regardless of where your "real" home is.
So you can be caught by domicile without hitting any day count, or caught by day count even though your domicile is somewhere else entirely. Most audits turn on the second one, because days are countable - and that is exactly what an app can watch for you.
The 183-day line, and how a "day" is counted
Many states use a bright 183-day statutory-residency line. The exact number and wording vary - for example, New York triggers statutory residency at 184 days in the state (with a permanent place of abode), so 183 days or fewer keeps you a nonresident on that test. Not every state stops at 183, either: some set the bar higher, such as Idaho at 270 days, North Dakota at 210 and Oregon at 200, which is why a per-state threshold matters more than a single number. The mechanics of counting are where people quietly go over:
- Any part of a day counts. Under the rule most states apply, spending even a few minutes in the state makes the whole day a day of presence. Arrive at 11 pm and leave at 1 am the next morning and that is two days, not one.
- Arrival and departure days both count. A weekend that touches Friday night through Monday morning is four counted days under any-part-of-a-day.
- The margin is thin. Because both ends of every trip count, a heavy-travel year can add up far faster than a mental estimate suggests - which is why guessing is dangerous.
The burden of proof is on you
This is the part that surprises people most. In a state residency audit, you carry the burden of proving where you were on any given day. Auditors do not assume good faith about undocumented days - a day you cannot account for is typically counted against you, i.e. treated as a day in the high-tax state. States like New York and California are known for running detailed, aggressive audits that reconstruct your year from phone records, tolls, credit card charges and swipe data.
That means a residency position is only as strong as your evidence. A clean, dated, contemporaneous record of where you were - backed by documents - is what wins these audits. A rough recollection built after the fact does not.
Breaking residency in the old state
If you are moving to a lower-tax state, changing your address is not enough. To genuinely break residency you generally have to do two things at once:
- Establish a new domicile. Actually build your permanent home elsewhere - move your family and belongings, change your licence, register to vote, spend real time there, and shift your "center of life."
- Reduce your days and abode in the old state. Cut your day count well under the statutory line and, ideally, give up the permanent place of abode. Keeping a home you can walk into any time you like is exactly what a statutory-residency audit looks for.
Do only one and the sticky old domicile can follow you for years. This is precisely why counting days in both the state you are leaving and the state you are entering - at the same time - matters so much.
Track state residency for taxes in the app
Doing all of this by hand, across multiple states each with its own threshold, is exactly the task that goes wrong under audit pressure. Tax Residency Tracker keeps it running in the background:
- US State Tracking. The app shows your current state, your recent states, and your total days by state for the tax year - the exact numbers an auditor would want to reconstruct.
- Per-state residency thresholds with a live count. Each state carries its own statutory-residency threshold from a bundled dataset, and the app shows how many days you've used against it and how many days remain before you cross the line - so you see a state approaching its limit long before you get there. States with no bright-line day test are flagged as facts-and-circumstances rather than given a false number.
- Threshold alert notifications. The app warns you before you cross a line with a warning ladder as your remaining days shrink, so a per-state limit is never something you only discover after the fact. An optional discreet mode hides the details on your lock screen.
- States tracked in parallel with countries. Your state day counts run alongside your country counts, the Schengen 90/180 rolling window and the US Substantial Presence Test, so nothing is measured in isolation.
- Automatic GPS state detection. Real-time and background location monitoring detects when you cross state lines and creates a dated stay for you - even when the app is closed - so your record doesn't depend on remembering to log every trip. You can also add stays manually.
- Planned-stays preview. Add a future trip and see the projected per-state totals, SPT and Schengen numbers before you book, so you can plan around a line instead of tripping over it.
- Document attachments as proof. Attach camera photos, library photos, PDFs and scanned documents to any stay, building the contemporaneous, evidenced record that flips the burden of proof back in your favour.
- CSV export of stays and days. Export your stays and your daily records by country and US state as CSV, for a full tax year or a custom range, and hand your tax consultant an audit-ready file.
Everything is processed on your device and never uploaded, with optional iCloud sync through your own private account, so your movements stay private while still being fully documented if a state ever asks.
Frequently asked questions
Can I be a resident of a state where I don't live full-time?
Yes. Under statutory residency, if you spend 183 or more days in a state (New York uses 184) and keep a permanent place of abode there, the state can tax you as a resident even though your true home, your domicile, is somewhere else. The abode generally has to be a home you actually use as a residence rather than a vacant property you merely own, but the day count alone can still catch you.
Do arrival and departure days both count toward the 183-day line?
Under the any-part-of-a-day rule most states use, yes. Any day you set foot in the state counts as a full day, so both the day you arrive and the day you leave count - a same-night in-and-out can even count as two separate days.
What happens to undocumented days in an audit?
They generally go against you. The burden of proof is on the taxpayer, so a day you can't account for is usually treated as a day spent in the high-tax state. A dated record with attached documents is what keeps those days on your side.
How do I stop being a resident of a high-tax state?
You typically need to both establish a new domicile elsewhere and reduce your days and abode in the old state. Doing only one leaves the old, sticky domicile intact. Tracking days in both states at once shows you whether you've actually made the move on paper.
Next, see the California 183-day rule, the New York 183-day rule and statutory residency, or the Substantial Presence Test calculator, or browse all tax-residency guides.