Week number calculator
Two converters: find the week number of any date, or the exact dates of any week number. Results are shown in ISO 8601 and US numbering, with copy-ready week codes.
How the week number is calculated
Under ISO 8601, take the date's Thursday-anchored week: find the Thursday of the week the
date falls in (weeks run Monday–Sunday). The year of that Thursday is the ISO week-year, and the week
number is how many weeks that Thursday is from the year's first Thursday, plus one. In practice you never
need to do this by hand — this page, ISOWEEKNUM in spreadsheets, and
isocalendar() in Python all implement the same rule.
The US convention instead numbers the week containing January 1 as week 1 and starts weeks on Sunday, which is why it can disagree with ISO by one (and briefly show "week 53/54" in late December). More in week start conventions.
Frequently asked questions
Which week numbering does the calculator use?
The main result uses ISO 8601 (Monday start, week 1 contains January 4) — the international standard. It also shows the US convention (Sunday start, week 1 contains January 1), which is what Excel's default WEEKNUM returns.
Why does my date get a week from a different year?
Around New Year, ISO weeks can belong to the other year's numbering: December 29–31 can fall in week 1 of the next ISO year, and January 1–3 can fall in week 52/53 of the previous one. The calculator shows the ISO week-year so the code is always unambiguous.
Can I calculate week numbers in Excel or code instead?
Yes — use ISOWEEKNUM() in Excel/Sheets, date.isocalendar() in Python, or see the snippets in the Excel guide and ISO week guide.