How it works
The 183 number shows up in lots of tax laws and tax treaties, but the details change by country: which 12 months get counted, whether it's "183 or more" or "more than 183", and what counts as a day.
Spain counts more than 183 days in the calendar year (1 January to 31 December). Short trips out of Spain still count as days in Spain unless you prove you're tax resident somewhere else.
Portugal counts more than 183 days in any 12-month period that starts or ends in the tax year, so the window can straddle two calendar years. A day counts if it includes a night spent in Portugal.
UK
A day counts if you're in the UK at midnight. There are two exceptions: passing through (you arrive as a passenger and leave the next day without doing anything unrelated) and being stuck by events beyond your control. A separate "deeming rule" can count some extra days for people with strong UK ties.
Besides the day count, many countries have other tests (a home there, family there, your main business there) that can make you resident with far fewer days. The 183 count is the clock the app can track.
An example
A US traveler arrives in Portugal on 1 August 2025 and leaves on 5 February 2026, sleeping there every night from 1 August to 4 February. That's 188 nights inside one 12-month window, which is more than 183, so Portugal's day test is met. If the exact same dates were in Spain, they'd split into 153 days in 2025 and 36 days in 2026, and neither calendar year goes over 183. Same trip, different answer, because the countries count differently.
Mistakes people make
- Thinking the count resets on 1 January everywhere. Portugal uses a rolling 12-month window and the UK's year starts 6 April.
- Treating "183 days" the same everywhere. Spain and Portugal need more than 183 (so 184), while the UK triggers at 183.
- Counting the wrong days. The UK looks at where you are at midnight, Portugal at where you sleep, and Spain counts any day you're shown to be there.
- Assuming staying under 183 means you're safe. Other tests, like having a home or your family or business there, can make you resident with fewer days.
- Mixing up immigration days and tax days. A 90 or 180-day visa clock is a separate count from any tax residency clock.
Quick answers
If I stay under 183 days, am I safe from tax residency?
Not always. Having a home, your family or your business in a country can make you resident with fewer days.
Does the 183-day count reset on January 1?
It depends on the country. Spain uses the calendar year, the UK's tax year starts 6 April, and Portugal uses any 12-month window.
Is 183 days the same rule everywhere?
No. Spain and Portugal need more than 183 days, the UK counts 183 or more, and each country decides what counts as a day.
Where this comes from
Government sources only. Read the rule yourself.
- Tax on foreign income: residence (GOV.UK)gov.uk ↗
- Persona física residente en España (Agencia Tributaria)sede.agenciatributaria.gob.es ↗
- Código do IRS, Artigo 16 (Portal das Finanças)info.portaldasfinancas.gov.pt ↗
- Finance Act 2013, Schedule 45, paragraph 22legislation.gov.uk ↗
- HMRC RFIG20710, meaning of a day spent in the UKgov.uk ↗
- HMRC RDR3, Statutory Residence Testgov.uk ↗
What changed
11 Oct 2026. Fixed the summary to match Spain and Portugal, and cited the UK midnight rule.
3 Oct 2026. Checked against HMRC, the Agencia Tributaria and the Portuguese IRS code. No change.
A plain-language summary of the official rules. It isn't legal advice. If your situation is unusual, confirm with the relevant authority or a qualified adviser.
