Week Number Calculator

What the ISO 8601 week number standard says

ISO 8601 is the international standard for writing dates and times, and its week date system defines every calendar week as seven days running from Monday to Sunday. Week 1 is the only week allowed to touch the new year, and the standard pins it with two equivalent definitions: it is the week that contains the first Thursday of January, and it is the week that contains January 4. Those two tests always select the same week, because January 4 always falls inside the week holding the first Thursday.

Once Week 1 is anchored, the rest of the year follows automatically. Every year owns exactly 52 or 53 ISO weeks, and each week belongs entirely to one ISO week-numbering year. That is a real difference from the plain calendar year: the Monday of Week 1 often sits in the previous December, and the last few days of December often fall in Week 1 of the following ISO year. On 31 December 2024, a Tuesday, the ISO week date is already 2025-W01-2.

The practical consequence is that a week number calculator has to solve two separate problems. It has to place a given date inside the correct ISO week-numbering year, and it has to count the weeks from the anchored Week 1. Counting alone is not enough, because the anchor shifts between years.

The broadcast calendar resembles ISO numbering but anchors Week 1 differently, which is why broadcast Week 1 of some years starts a week earlier or later than ISO Week 1. Always confirm which system your audience expects before publishing a week number anywhere.

For the primary source, see the ISO 8601 date and time format page published by the International Organization for Standardization, and the NIST Time and Frequency Division notes on calendar and timekeeping conventions used in the United States.

How the week number calculation works

The algorithm behind the calculator is short. For any date, it finds the Thursday of that date's own week, then looks up January 4 of the Thursday's year, then walks back from January 4 to the Monday that starts Week 1. The week number is the number of whole weeks between those two Mondays, plus one.

In formula form: week number = ((Monday of the date's week) - (Monday of Week 1)) / 7 + 1. The Thursday lookup is what makes the year assignment correct. A Saturday near the end of December belongs to the ISO year that its Thursday belongs to, which can be the next calendar year.

Worked example for 27 October 2026, a Tuesday. The Monday of that week is 26 October 2026. January 4 of 2026 is a Sunday, so walking back to Monday lands on 29 December 2025, the Monday of Week 1. Counting: (26 Oct 2026 - 29 Dec 2025) = 301 days, and 301 / 7 = 43, so 43 + 1 = 44. The date sits in ISO week 44 of 2026.

Reversing the process answers the second common question: which dates belong to week N of year Y? Take the Monday of Week 1, add 7 x (N - 1) days to reach the Monday of week N, and the Sunday six days later closes the week. For week 44 of 2026, adding 7 x 43 = 301 days to 29 December 2025 gives 26 October 2026, and six days later is 1 November 2026.

Which years have 53 weeks

A year contains 53 ISO weeks in exactly two cases: January 1 falls on a Thursday, or January 1 falls on a Wednesday inside a leap year. Every other year has 52 weeks. That pattern repeats on a 400-year Gregorian cycle, so the long-run share of 53-week years is 71 out of every 400 years.

The reason is the length of a year in weeks. A common year is 365 = 52 x 7 + 1 days, so it drifts one weekday per year, and a leap year is 366 = 52 x 7 + 2 days, so it drifts two. A 53rd week appears only when the drift leaves an extra Thursday inside the year before the next Week 1 begins.

Recent and upcoming 53-week years include 2020, 2026, 2032, 2038, and 2044. 2020 opened on a Wednesday and was a leap year, 2026 opened on a Thursday, and 2032 opens on a Thursday as well. In a 53-week year, dates from the final Monday through the following Sunday all carry the same ISO year, even when the Sunday lands in the next calendar year. Week 53 of 2026 runs from Monday 28 December 2026 to Sunday 3 January 2027.

Payroll teams feel this difference directly. A company that pays weekly will issue 53 paychecks in a 53-week year instead of 52, and salaried weekly budgets need one extra week of money set aside. Scheduling software that assumes 52 weeks every year will misreport the last week of every 53-week year.

Working days and weekend days inside a week

Every ISO week holds 7 days, and a standard Monday to Friday work week puts 5 working days and 2 weekend days inside any week. Those counts never change from week to week, because the ISO week always starts on Monday and ends on Sunday. What changes across years is only how many weeks the year holds, so 2026 holds 53 x 5 = 265 working days and 53 x 2 = 106 weekend days before holidays, and a 52-week year holds 260 and 104.

Real workweeks shrink once holidays land inside them. In the United States, eleven federal holidays fall inside each year, and when one of those lands on a weekday inside a given week, that week delivers 4 working days instead of 5. Thanksgiving week is the classic example: the holiday always falls on a Thursday, so that week contains Monday, Tuesday, Wednesday, and Friday as working days, and the Thursday holiday plus Saturday and Sunday as non-working days.

Note that 364 = 52 x 7, but a 52-week ISO year still spans 365 or 366 calendar days, because Week 1 starts in the previous December and the final week ends in the next January. That is why the calculator reports both the ISO week count and the calendar day span for any year you enter.

Using the week number calculator step by step

The widget supports three modes, and all of them recompute on every keystroke. In date to week mode, pick any date and the calculator returns the ISO week number, the ISO week-numbering year, the weekday name, the day count inside the year, and the Monday and Sunday that bound the week. In week to dates mode, enter a year and a week number and the calculator returns that week's Monday through Sunday range, its working day count, and its weekend day count. In year summary mode, enter just a year and the calculator reports whether the year holds 52 or 53 weeks, the calendar date span of the first and last week, and the totals for working days and weekend days.

Try these reference values to confirm the widget is behaving the way you expect:

Empty fields fall back to safe defaults, so a blank date input resolves to today's date and a blank year resolves to the current year. Nothing is saved or transmitted anywhere.

Where week numbers show up in real work

Manufacturing, retail, and logistics teams live on week numbers because production schedules, promotional calendars, and delivery waves are all planned in whole weeks. A factory that stamps part lots with the production week needs every supplier reading the same standard, or a lot marked week 40 will mean two different spans on two different continents.

European payroll and invoicing commonly quote week numbers in the ISO sense, so a contractor invoicing week 44 of 2026 is billing for Monday 26 October through Sunday 1 November. Project managers use the same ranges for sprint boundaries, because a sprint that starts on a Monday and ends on a Sunday maps exactly onto one ISO week.

Other common uses include expiry tracking on perishable stock, where a week number on a label compresses a date range into two digits, and historical research, where archival records use week dates. Anyone reconciling a week-numbered report against a daily calendar should run both through the calculator to catch the year-boundary shifts that catch people out at the end of December.

Common week number pitfalls and how to avoid them

The most frequent mistake is assuming week 1 starts on January 1. In ISO numbering, January 1 sits in Week 1 only when it falls on a Monday, Tuesday, Wednesday, or Thursday. When January 1 lands on a Friday, Saturday, or Sunday, those days belong to the final week of the previous ISO year. On 1 January 2027, a Friday, the ISO date is still week 53 of 2026.

The second mistake is mixing up week-numbering systems. US calendars often number weeks starting on Sunday and anchor week 1 on the week containing January 1, which produces different numbers for most dates than ISO does. The third mistake is treating a week number as a date span without stating the year, because the same week number exists in every year but covers different dates.

Frequently asked questions about week numbers

The questions below come up again and again when teams standardize on ISO week dates. Each answer reflects the ISO 8601 definition, which is what this calculator uses. If your organization follows a different convention, translate once into ISO and keep the rest of your tooling on a single standard.