How many months is 183 days?

The arithmetic

• In months: just over six months. Half of a normal year is 182.5 days (365 ÷ 2), so 183 is the first whole day past halfway; in a leap year 183 days is exactly half of 366. Six consecutive calendar months are 181 to 184 days long, so counted from a given date 183 days can fall either side of the six-month mark: from 1 January it ends on 2 July, from 1 July on 30 December.
• In weeks: 26 weeks and 1 day (26 × 7 = 182).
• As a share of the year: 50.1% — a bare majority.
• On a calendar: 1 January to 2 July inclusive is 183 days in a non-leap year; 1 January to 1 July in a leap year.

Why 183 and not 180 or six months

Tax law needs a bright line for “more than half the year.” “Six months” is ambiguous — months are 28 to 31 days long — and 180 days is slightly less than half a year. 183 is the smallest whole number of days that is more than half of a normal 365-day year (in a leap year it is exactly half), so it became the standard threshold in domestic residency rules and in tax treaties (the OECD model uses 183 days to decide which country may tax a short-term worker’s salary). That is the whole story behind the 183-day rule.

Two nearby numbers you will also see are not tax tests: the Schengen area’s short-stay limit of 90 days in any 180, and the UK’s ILR rule allowing 180 days of absence in any 12 months. Those are immigration limits with their own counting rules.

Counting 183 days correctly

The months-to-days conversion is the easy part. What trips people up is which days count and over which period:

• Part days. The US counts a day if you are physically present for any part of it, and Canada counts any part of a day you are sojourning (staying temporarily) there, so arrival and departure days both count (each with limited exceptions, such as commuters); the OECD model treaty’s 183-day test for employment income counts part days too. The UK counts a day only if you are there at midnight, again with limited exceptions.
• The window. A calendar year, the country’s own tax year (UK: 6 April–5 April; Australia: 1 July–30 June), or any rolling 12-month period (Chile, Colombia). A rolling window means 183 days across two calendar years can still trigger residency.
• Weighted counts. The US Substantial Presence Test reaches 183 by adding this year’s days to one-third of last year’s and one-sixth of the year before — 122 days in each of three years gives 122 + 40⅔ + 20⅓ = 183, so you can “hit 183” with only 122 days in the current year.

The free 183-day calculator counts from your actual trip dates across a calendar year, tax year, or rolling window, so you never have to convert months to days by hand.

Sources

• IRS — Substantial Presence Test
• OECD Model Tax Convention (condensed version 2017) — Article 15(2) and Commentary on Article 15 (183-day threshold for employment income)
• Canada Revenue Agency — Income Tax Folio S5-F1-C1, ¶1.33 (any part of a day counts as a day)
• HMRC — RDR3: Statutory Residence Test (days counted at midnight)