Week Number Tool

See the ISO week number for today or any date — US convention beside it — and turn a week number back into its Monday–Sunday dates, on this device.

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Date

Accepts 2026-08-07, 7 Aug 2026, or an ISO week date like 2026-W32-5.

Pick a date

Today is this device's local date — a viewer in another time zone may already be in a different week.

ISO year

Week (1–53)

Result

ISO week date—

Share is available once the active mode's fields have a value.

ISO week: —
US week: —
ISO week-numbering year: —
Weekday: —
Monday–Sunday range: —
Weeks in ISO year: —

US week: week 1 contains 1 January and weeks start on Sunday — the Excel WEEKNUM default.

Monday—
Tuesday—
Wednesday—
Thursday—
Friday—
Saturday—
Sunday—

Press ? for shortcuts

Do it offline:$ date +%G-W%V-%u

How to use it

Pick a mode: Date → week for a calendar date, Week → dates for a year plus week number.
Type a date (2026-08-07, 7 Aug 2026, even 2026-W32-5), use the picker, or press Today; Sample loads a year-boundary case.
Read the card: copyable ISO week string, week-numbering year, weekday, Monday–Sunday range, weeks in that ISO year, and the labeled US week.
In Week → dates, set a year and a week (1–53) to list that week's seven dates.

Enter any date — or press Today — and read its ISO 8601 week: the W01–W53 number, week-numbering year and weekday, with the US convention beside it and every disagreement flagged in a chip. Switch modes to turn a year and week number back into its Monday–Sunday dates. Both directions compute on this device.

How it works

ISO 8601 weeks run Monday to Sunday; week 01 is the week holding the year's first Thursday — equivalently, the week of 4 January. A date belongs to the year owning its Thursday, which is why 2007-12-31, a Monday, is already 2008-W01-1: that week's Thursday is 3 January 2008. The same rule pulls early-January days into the old year's final week.

Fifty-two weeks cover 364 days; the leftover day — two in a leap year — makes a 53rd week exactly when 1 January is a Thursday or a leap year opens on a Wednesday. 2015, 2020, 2026 and 2032 qualify; 2025 stops at W52. The card prints the count for every answer's ISO year.

The US convention: week 1 is whichever week contains 1 January, weeks begin on Sunday, and a date keeps its calendar year — Excel's WEEKNUM default, the definition this page pins. Without a Thursday anchor the counts split at year boundaries and can sit one apart far longer: 2021-06-15 is ISO W24 but US week 25. strftime's %U is a third scheme, filing days before the year's first Sunday under week 00.

Examples

2026-08-07

2026-W32-5

Mid-year the systems usually agree — both say week 32, weekday 5 meaning Friday. A chip still flags that ISO year 2026 runs to 53 weeks: 2026 opens on a Thursday.

2005-01-01

2004-W53-6

New Year's Day 2005 sits in the old ISO year: week 53 of 2004, weekday 6 (Saturday). The US count says week 1 of 2005 — the disagreement chip prints both figures.

2026-W53

2026-12-28 – 2027-01-03

W53 of 2026 runs Monday 2026-12-28 to Sunday 2027-01-03, across the year boundary; a bare week string with no weekday digit points at its Monday.

Every example on this page is executed against the same engine that powers the tool, on every build.

Gotchas worth knowing

Early January, previous ISO year

2021-01-01 answers 2020-W53-5 — the week's Thursday is 31 December 2020, so ISO keeps it all in 2020; the US column says week 1 of 2021.

The US count can reach 54

2000-12-31 is ISO 2000-W52-7 but US week 54: a leap year opening on a Saturday leaves one day in week 1, pushing the tail past 52 — WEEKNUM's documented result, never clamped here.

Pair %V with %G — never %Y

date +%Y-W%V breaks at New Year: for 2027-01-01 it prints 2027-W53, a week 2027 does not have — it stops at W52, and the 53 is ISO year 2026's. Pair %V with %G instead.

Week-date strings: tolerated and refused forms

Extended 2026-W32-5 is exact; compact 2026W325 and lowercase 2026-w32 parse with a chip naming the normalized form; bare 2026-W32 resolves to its Monday and says so. W00 or weekday 8 refuse with the specific rule.

Years 1 to 9999, proleptic Gregorian

Before 1583 the arithmetic is proleptic Gregorian — the same answers as Python's isocalendar() — no claims about historical calendars. At the edge, 9999-W52 honestly ends on 10000-01-02; leap day 2024-02-29 computes normally, 2024-W09-4.

Today means this device's date

Today reads this device's local calendar date; the engine gets that civil date only — no clock, no zone. A reader elsewhere may already be in a different week around midnight; type the date to pin it.

Common errors

2025 has 52 ISO weeks — W53 does not exist in that ISO year.: The refusal names the year's real count — most years end at W52; nothing rolls over silently.
"2026-W3" is not an ISO week date — the form is YYYY-Www or YYYY-Www-D (4-digit year, uppercase W, 2-digit week, optional weekday 1–7), e.g. 2026-W32-5.: The week part needs two digits: W03, not W3; the message spells the full shape.
"03/04/2026" could be 3 April or 4 March — type the month name (4 Mar 2026) or use YYYY-MM-DD.: Slash dates with two small numbers read differently across locales; the shared date parser refuses to guess, and its message shows unchanged.
Year 0000 is outside the supported range 0001–9999.: Week-numbering years run 0001 through 9999 — refusal, not clamping, outside that.

FAQ

Why does a late-December or early-January date show the other year's week?

An ISO week is indivisible and files under the year holding its Thursday: 2027-01-01 is the Friday of a week whose Thursday is 31 December 2026, so it lands in 2026-W53. A chip names both years whenever they differ.

Which years have 53 ISO weeks?

Years beginning on a Thursday, plus leap years beginning on a Wednesday — 2015, 2020, 2026, 2032. Most years end at W52, which is why 2025-W53 is refused rather than rolled into 2026.

Why do the ISO and US week numbers disagree?

Different anchors: first Thursday with Monday starts, versus contains-1-January with Sunday starts. The counts split at year boundaries and can sit one apart into June — 2021-06-15 is ISO W24, US week 25. Shell %U is neither: days before the year's first Sunday are week 00.

Keyboard shortcuts

⌘/Ctrl + Enter: Recompute (results already track every valid edit).
⌘/Ctrl + C: Copy the ISO week string when nothing is selected — never intercepted while a number or date-picker field has focus.
⌘/Ctrl + Shift + C: Copy even while text is selected.
⌘/Ctrl + S: Download the result rows as week-number.txt, overriding the browser's save-page dialog.
t: Cycle the theme (never fires from a focused field).
? or ⌘/Ctrl + /: Open the shortcut help for this page.
Esc: Close the topmost dialog.

Nothing you paste here leaves this tab. State only ever travels in links you explicitly create.

ISO 8601-1:2019 — Date and time — Part 1: Basic rules · strftime(3) — Linux manual page, man7.org (%G, %V, %U, %u) · Microsoft — WEEKNUM function (return type 1) · Python — datetime.date.isocalendar()

Last reviewed: 2026-08-07