Dates

Week Number Calculator

Find the ISO-8601 or US week number for any date.

Date

Week 29 (ISO)

ISO week: 29 of 2026
US week: 29

"What week number is this" has two genuinely different real answers depending on the convention: ISO-8601 (the international standard) and the simpler US-style week system don't always agree, and dates right at the turn of the year are exactly where they diverge most visibly.

Payroll systems, academic term schedules, and some countries' government forms all reference week numbers directly, and a mismatch between the ISO and US conventions in that context isn't just an academic curiosity — it can mean a form or schedule referencing "week 12" means a genuinely different date range depending on which system generated it.

How the Week Number Calculator works

Under ISO-8601, week 1 of any year is defined as the week containing that year's first Thursday (equivalently, the week containing January 4th), and weeks run Monday to Sunday — which means the very last days of December can actually belong to week 1 of the following year, and the first few days of January can belong to the last ISO week (52 or 53) of the previous year. The US-style convention instead starts week 1 on January 1st itself and runs Sunday to Saturday, giving a different number for the same calendar date.

Manufacturing, logistics, and some payroll systems frequently reference week numbers directly in scheduling and shipping codes, which is exactly where the ISO-vs-US convention mismatch causes real operational confusion — two systems disagreeing on a shipment's "week 3" can genuinely mean two different 7-day spans.

Worked example

January 1, 2027 falls on a Friday. Under ISO-8601, the week containing January 4, 2027 (a Monday) runs from Monday Jan 4 to Sunday Jan 10 and is week 1 of 2027 — which means January 1, 2, and 3, 2027 (Fri/Sat/Sun) actually belong to week 53 of 2026, not week 1 of the new year, because 2026 happens to be one of the years with a 53rd ISO week.

Edge cases this tool handles correctly

53-week years: A year gets a 53rd ISO week specifically when January 1 falls on a Thursday, or when it's a leap year and January 1 falls on a Wednesday — this happens roughly once every 5 to 6 years, not every year.
Year-boundary dates: Dates in the very first days of January or the very last days of December are the most likely to report a week number that seems to belong to "the wrong year" under ISO-8601 — that's expected, correct behavior, not an error.
Spreadsheet mismatches: Excel's WEEKNUM function defaults to a Sunday-start, US-style numbering system rather than ISO-8601 unless you explicitly select the ISO option, which is a common real source of numbers not matching between a spreadsheet and this tool.
Comparing week numbers across years: Because each year's week 1 start date shifts depending on which weekday January 1 falls on, the same calendar week number (like week 20) doesn't correspond to the exact same range of calendar dates from one year to the next.
Checking a specific year's total week count: A quick way to tell whether a given year has 52 or 53 ISO weeks without working through the full rule: check this tool for December 28th of that year (which always falls in that year's final ISO week) and read off the week number shown.
ISO week numbers and ISO year can differ from the calendar year: A date's ISO week number is reported alongside its ISO week-year, which for a handful of late-December or early-January dates each year is a different number from the plain calendar year that date's month/day would suggest — this is expected under the standard, not a display bug.

Frequently asked questions

Why does January 1st sometimes show as week 52 or 53 instead of week 1?

Because ISO-8601 anchors week 1 to the week containing January 4th, not to January 1st itself.

Why does the tool sometimes show an ISO year that's different from the calendar year of the date I entered?

Because a date's ISO week-year is tied to which week it falls in, not its calendar year — a few late-December or early-January dates each year belong to the adjacent year's week numbering, which is standard ISO-8601 behavior.

What is a "53-week year"?

A year that needs an extra 53rd week to fit all its days into whole Monday-to-Sunday weeks — it happens when January 1 lands on a Thursday, or on a Wednesday in a leap year.

Does Excel use ISO week numbers by default?

No — its WEEKNUM function defaults to a Sunday-start system unless you specifically choose the ISO-8601 option, which is why its numbers sometimes don't match this tool.

Which convention does this tool use by default?

ISO-8601, since it's the international standard, with the US-style Sunday-start convention available as an alternative.

Does week 20 always cover the same calendar dates every year?

No — because each year's ISO week 1 starts on a different date depending on the weekday January 1 falls on, week 20's actual date range shifts slightly from year to year.