Pick a date and the calculator returns its week number under both the ISO 8601 standard (Monday-start, used in Europe and almost all backend systems) and the US convention (Sunday-start, used in most US calendar software). The two can differ by 1 — and in some years one system has 53 weeks while the other has 52 — so the calculator shows both for cross-checking. ISO week numbers are the right choice for software, finance, and international scheduling; the US-style week number matches what your phone calendar shows. For the broader year context (day-of-year, year length), see day of year and year progress.
Common use cases
- Filing weekly reports under ISO numbering. Most enterprise reporting systems (SAP, Workday, NetSuite) reference weeks by their ISO 8601 number. Enter the date your report covers; the calculator returns the ISO week. Share that number with your finance and ops counterparts in any country; ISO weeks are unambiguous worldwide.
- Checking when ISO week 1 actually starts. ISO week 1 of any year is the week that contains the first Thursday — equivalently, the week containing January 4. So ISO week 1 can start as early as December 29 of the previous year (when Jan 1 is a Thursday) or as late as January 4. Enter a January date in 2026 to see the off-by-one cases.
- Timezone-agnostic content scheduling. When you publish "the post for week 24" the URL or filename should not depend on the reader's time zone. Use the ISO week number for the publish date in UTC; this calculator gives you the right integer to put in the URL. The result is identical for any reader on the planet.
- Reconciling ISO vs US calendar mismatch. A US-based calendar app may show "Week 27" for a date that an EU-based system records as "Week 26" — the same date, two different week numbers. Run the date through the calculator to see both numbers side by side. The mismatch usually comes from the Sunday-vs-Monday week-start choice.
How it works
The calculator parses your input as a UTC date, then asks date-fns for both week numbers. ISO week 1 is defined as the week containing the first Thursday of the year — equivalently, the week containing January 4 — and weeks start on Monday. The US convention starts weeks on Sunday and defines week 1 as the partial week containing January 1 (so it can span the new year). Both conventions are widely used; the calculator displays both so you can see the difference at a glance.
Worked examples
New Year 2026 (ISO)
Enter 2026-01-01.
Result: ISO week 1.
Jan 1 2026 is a Thursday, so it falls inside ISO week 1 of 2026 (the week containing the first Thursday). The week itself starts on Monday Dec 29 2025 and ends on Sunday Jan 4 2026.
First full Sunday-start week (US)
Enter 2026-01-04 with US/Sunday convention.
Result: US week 2.
Under the US convention week 1 is the partial week Sunday Dec 28 2025 → Saturday Jan 3 2026. Sunday Jan 4 starts week 2. ISO numbering gives the same date a different answer because of the Monday-start rule.
A 53-ISO-week year
Enter 2026-12-28 with ISO convention.
Result: ISO week 53.
2026 has 53 ISO weeks because Jan 1 2026 was a Thursday and Dec 31 2026 is a Thursday. Most years have 52 ISO weeks; ~71 years out of every 400 have 53. The Monday-Sunday week of Dec 28 2026 → Jan 3 2027 is week 53 of 2026.
Edge cases & gotchas
- ISO week 1 can belong to the previous calendar year. When January 1 falls on Friday, Saturday, or Sunday, the first few days of January belong to the last ISO week of the previous year. For example, January 1, 2027 is a Friday, so it sits in ISO week 53 of 2026. Software that prints "Week 53, 2026" for a date in January 2027 is doing the right thing, not making a mistake.
- Some years have 53 ISO weeks. ~71 years in every 400 have 53 ISO weeks rather than 52. The years are those that start on Thursday OR (in leap years) those that start on Wednesday. Recent and upcoming examples: 2015, 2020, 2026, 2032. Knowing this matters when planning a year-long schedule indexed by week number.
- US weeks 1 and 53/54 are partial. Under the US Sunday-start convention, week 1 always contains January 1 — even when January 1 is a Saturday and week 1 is just one day long. Year-end weeks are similarly truncated; some years have a one-day "week 54" containing only December 31. This makes US weeks more cosmetically familiar but harder to compute with.
- Locale settings can override defaults. date-fns supports a "weekStartsOn" parameter from 0 (Sunday) through 6 (Saturday). Many Middle Eastern calendars start on Saturday; Israel starts on Sunday with different week numbering than the US. The calculator only exposes ISO and US — for other locales, use date-fns getWeek with the appropriate parameter directly.
Frequently asked questions about Week of Year Calculator
Which week number should I use in software?
Use ISO 8601 unless you have a specific reason not to. It's the international standard; it's what enterprise software defaults to; it's deterministic across time zones; and it's the format URL paths and filenames usually expect (e.g. "2026-W24"). Reserve the US convention for end-user-facing displays where users expect the Sunday-start visual.
Why do ISO and US numbers sometimes differ by 1?
The starts of weeks differ. ISO weeks start on Monday; US weeks start on Sunday. So a Sunday in early January often sits in different weeks under the two systems. The mismatch wraps around: if ISO and US agree at the start of the year, they'll diverge mid-year and re-converge.
Why does my year have 53 weeks?
53 ISO weeks happen in years where January 1 is a Thursday (any year), or where January 1 is a Wednesday in a leap year. Both conditions cause the year to "wrap" an extra Thursday. Recent 53-week years: 2015, 2020, 2026. The leap-year case applies to 2032.
How does daylight saving time affect week numbers?
It doesn't. Week numbers depend on the calendar date, which is the same in any time zone. DST shifts only affect wall-clock times within a given day. The calculator interprets your input as UTC midnight and the result is location-independent.
What's the relationship between ISO week and ISO ordinal date?
ISO 8601 lets you write a date in three forms: calendar (2026-07-04), ordinal (2026-185), and week-date (2026-W27-6). The week-date form combines the ISO year, the ISO week number, and the day-of-week number (1=Monday, 7=Sunday). Note "ISO year" can differ from the calendar year for the first/last days of January/December.
Why don't my dates match what Outlook or Google Calendar shows?
Both apps default to local conventions: Outlook usually shows ISO weeks for non-US locales and US weeks for the US. Google Calendar by default shows US-style weeks; you can change to ISO in settings. The calculator shows both numbers so you can match either app.
Glossary
- ISO 8601 week
- A week that starts on Monday and is numbered such that week 1 contains the year's first Thursday (equivalently, the week containing January 4). The international standard for week numbering.
- US week (Sunday-start)
- A week that starts on Sunday and is numbered such that week 1 contains January 1 (which can make week 1 a partial week of one to seven days). Common in US consumer calendar apps.
- ISO year
- The year that owns an ISO week. Differs from the calendar year for January 1–3 in years where Jan 1 falls on Friday, Saturday, or Sunday — those days belong to the last week of the previous ISO year.
- 53-week year
- A year with 53 ISO weeks (instead of the more common 52). Occurs when January 1 is a Thursday, or when January 1 is a Wednesday in a leap year. About 71 years per 400 have this property.
- Week-date format
- ISO 8601 short form for a date written as YYYY-Www-d, where ww is the zero-padded ISO week number and d is the day of week (1=Monday). Example: 2026-W27-6 = Saturday of week 27, 2026.