This guide covers Week Numbers Explained | TimeFlow — what it means, how it works, and how to use it. The explanations below are followed by worked examples with real dates and real times, a set of frequently asked questions, and links to the tools that do the calculation for you: the time zone converter, the world clock, and the UTC converter. Where a number is given — an offset, a date, a count — it is computed from the IANA Time Zone Database for the date stated, not copied from a table that may be out of date.

Try the tool: Time Zone Converter — convert any time between any two cities, with the daylight saving rules for the date you choose.

A week number is the number of a week within its year, counted from the first week of the year. The standard most of the world uses is ISO 8601: weeks start on Monday, and week 1 is the week containing the first Thursday of the year (equivalently, the week containing 4 January).

The ISO rule has a quirk: the first days of January can belong to the previous year's last week, and the last days of December can belong to the next year's week 1. 1 January 2026 is a Thursday, so it is in week 1 of 2026. But 1 January 2027 is a Friday, so it is in week 53 of 2026. And 1 January 2028 is a Saturday, so it is in week 52 of 2027.

Most years have 52 ISO weeks, but some have 53. A year has 53 weeks if it starts on a Thursday, or if it is a leap year starting on a Wednesday. 2026 has 53 weeks; 2020 had 53; 2027 will have 52.

Week numbers are used in business, manufacturing and government across Europe, where 'week 34' is a normal way to refer to a date. They are less common in the United States, where the week is usually counted from Sunday and the numbering is not standardised. The week number finder on this site uses the ISO system, and it shows the week's start and end dates so you can check the result.

In practice

The ISO week system is used across Europe and in international business, and it has three rules: the week starts on Monday, the first week of the year is the one that contains the first Thursday, and a week belongs to the year that contains its Thursday. Those three rules mean that every date belongs to exactly one week, and every week belongs to exactly one year — which is more than can be said for the calendar months.

The odd consequence is that some dates belong to the 'wrong' year. 1 January can be in week 52 or 53 of the previous year, and 31 December can be in week 1 of the next year. In 2026, 1 January is a Thursday, so it is in week 1 — but in 2027, 1 January is a Friday, so it is in week 53 of 2026. The year of the week and the year of the date are different things, and the ISO week number tells you the former.

The United States does not use ISO weeks, which is a source of confusion. American week numbering usually starts the week on Sunday and counts the first week as the one containing 1 January, so the same date can have a different week number on either side of the Atlantic. If you are scheduling across the two systems, agree on which one you mean — or just give the date.

The numbers: the year 2026

2026 is not a leap year: it has 365 days, 52 full weeks and one extra day, and 53 ISO weeks (because it starts on a Thursday). There are 261 weekdays in the year and 104 weekend days. The next leap year is 2028, and the one after that is 2032; the century year 2100 will not be a leap year, because it is divisible by 100 but not by 400. 1 January 2026 was a Thursday, and 31 December 2026 will be a Thursday.

The extra day is the reason the calendar shifts: because 365 days is 52 weeks and one day, every date falls one weekday later the following year — two days later after a leap year. That is why your birthday is on a different weekday almost every year, and why the calendar repeats every 28 years in most cases. The week number finder gives the ISO week for any date, the date calculator works out what date it is a number of days from now, and the days between dates calculator counts the days between any two dates.

2027 is also not a leap year: it has 365 days, 52 ISO weeks, and 261 weekdays. It starts on a Friday and ends on a Friday. The next leap year after 2026 is 2028, which will have 366 days and 262 weekdays. The business days calculator counts the business days in any year, with the public holidays subtracted.

Key facts

  • A normal year has 365 days and a leap year has 366, with the extra day on 29 February.
  • A year has 52 full weeks and one extra day — or two in a leap year.
  • The Gregorian leap year rule: divisible by 4, except centuries, except centuries divisible by 400.
  • A business day is a weekday that is not a public holiday — and the holidays depend on the country.
  • In the ISO week system, the week starts on Monday and the first week is the one containing the first Thursday.

Frequently asked questions

What is a week number?

The number of the week in the year, under the ISO system: the week starts on Monday, and the first week is the one containing the first Thursday of the year.

What week is 1 January in?

It depends on the year. In 2026, 1 January is a Thursday, so it is in week 1. In 2027, 1 January is a Friday, so it is in week 53 of 2026.

Does the US use ISO week numbers?

No. American week numbering usually starts the week on Sunday and counts the first week as the one containing 1 January, so the same date can have a different week number on either side of the Atlantic.

How many weeks are in 2026?

53, under the ISO system — 2026 starts on a Thursday, which is the rule for a 53-week year.

How do I find the week number of a date?

Use the week number finder: enter any date and it gives the ISO week.

How many weekdays are there in 2026?

261 weekdays and 104 weekend days, out of 365 days in total. 2026 is not a leap year. The business days calculator counts the business days, with the public holidays subtracted.

What is the time difference between London and Tokyo?

Nine hours in January 2026: London is UTC+00:00 and Tokyo is UTC+09:00. In July it is eight hours, because London moves to UTC+01:00 while Tokyo stays on UTC+09:00. The time zone converter gives the difference for any two cities and any date.

Related pages

Sources

  • IANA Time Zone Database, via Node.js Intl (Intl.DateTimeFormat with timeZone) — offsets, daylight saving dates and local times in this article were worked out from the IANA database, not typed from memory.
  • TimeFlow's own time-zone data (data/cities.ts, data/dst.ts), computed from the IANA database.
  • Time zone rules and offsets: the IANA Time Zone Database, as applied by this site's tools. Offsets and daylight saving dates on this page are computed from the database for the dates stated.
  • TimeFlow's time zone converter, world clock and UTC converter — the live tools behind the numbers on this page.
  • Time-zone and daylight saving rules are set by governments and can change. For flights, deadlines and meetings that cannot move, confirm the time with an official source.