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.
Today (Saturday, September 26, 2026) is week 39
— ISO code 2026-W39, running Sep 21 – Sep 27, 2026.
In US (Sunday-start) numbering it is week 40. Enter any other date below.
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.
Week numbers of popular dates
| Date | 2026 | 2027 |
|---|---|---|
| Easter Sunday | Apr 5 — week 14 | Mar 28 — week 12 |
| Independence Day (US) | Jul 4 — week 27 | Jul 4 — week 26 |
| Thanksgiving (US) | Nov 26 — week 48 | Nov 25 — week 47 |
| Christmas Day | Dec 25 — week 52 | Dec 25 — week 51 |
| New Year's Eve | Dec 31 — week 53 | Dec 31 — week 52 |
ISO 8601 week numbers. Each year's chart lists more holidays with both ISO and US week numbers.
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.