What is the ISO week date system?

The ISO week date system (ISO 8601) is a world-wide compatible calendar system where each week has exactly seven days, weeks start on Monday, and week 1 is the week containing the first Thursday of the year. A year has either 52 or 53 weeks. Because the ISO week year can start on a Monday in late December, the ISO year may differ from the calendar year for a few days at the turn of the year.

How many weeks are in a year?

Most years have 52 ISO weeks. A year has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. 2026 has 52 ISO weeks.

Why use ISO weeks instead of US weeks?

ISO weeks start on Monday and are consistent worldwide, which makes them the safer default for an international audience. US week numbering often starts weeks on Sunday and can number weeks differently. This site uses ISO 8601 week numbers so the numbering is predictable for a global readership.

What is the week number used for?

Week numbers are used for scheduling, reporting, pay periods, project timelines, and logs where a date is easier to refer to as "week 36" than by a full calendar date. They pair naturally with epoch-style tools because a Unix timestamp can be converted to its ISO week number, week start, and week end.

ISO week number

What's the Current Week Number? — Today's ISO Week

Today is ISO week 39 of 2026, running from Sep 21, 2026 (Mon) to Sep 27, 2026 (Sun) (Monday–Sunday). That is the world-wide ISO 8601 week-date convention. Use this page to answer "what week of the year is it" and to see the start and end dates of the current week.

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What week of the year is it today?

This week is ISO week $39 of $2026. The week runs from Sep 21, 2026 (Mon) to Sep 27, 2026 (Sun) under the ISO 8601 convention, where every week starts on Monday and ends on Sunday.

current week number

of 2025 · ISO week

this week's range

Sep 21, 2026 (Mon)

—

Sep 27, 2026 (Sun)

weeks in 2026

ISO weeks in 2026 ($52-week year)

How ISO week numbers are calculated

Under ISO 8601, week 1 of a year is the week that contains the first Thursday of that year. Because a week is always Monday to Sunday, this also means week 1 is the week containing January 4 — a date that is always in week 1. From that anchor, every following week adds one, so week numbers march forward in seven-day steps through the year. A year ends with either week 52 or week 53, depending on how the days fall.

Why the ISO week year can differ from the calendar year

Because week 1 is defined by the first Thursday, a few days at the start of January can belong to the last week of the previous year, and a few days at the end of December can belong to week 1 of the next year. When that happens, the ISO week-numbering year is one less or one more than the calendar year for those days. This page uses the ISO week year consistently so the numbering never jumps unexpectedly at the turn of the year.

52-week and 53-week years

A year has 53 ISO weeks when the year starts on a Thursday, or when it is a leap year that starts on a Wednesday. Otherwise it has 52 weeks. That means the total number of weeks in a year is not always 52 — it is 52 most of the time, and 53 about once every five or six years.

2024 — 52 weeks (leap year starting on Monday).
2025 — 52 weeks (starts on Wednesday).
2026 — 53 weeks (starts on Thursday).
2027 — 52 weeks (starts on Friday).
2028 — 52 weeks (leap year starting on Tuesday).

Week numbers and Unix time (epoch)

A Unix timestamp is a single number for an instant in time. To get the ISO week number from a timestamp, convert the timestamp to a UTC date and then compute the week per the ISO rule above. Because a timestamp is an instant, the week number depends on the date it falls on — and across midnight the week number can change even though the timestamp is only seconds apart. Converting a timestamp with this site's epoch converter and then reading the week from the resulting date is the quick way to go from an epoch value to its week number.

Epoch 1767225600 (Jan 1, 2026 00:00 UTC) → week 1 of 2026.
Epoch 1786060800 (Jan 7, 2026 00:00 UTC) → week 2 of 2026.
Epoch 1767225600 + 365×86400 (Dec 31, 2026 00:00 UTC) → week 53 of 2026.

Where week numbers show up

Week numbers are everywhere once you start looking: weekly reports, pay periods, sprint planning, maintenance windows, and exports from systems that prefer "week 36" to a full calendar date. They are especially useful when you need a compact label for a whole week and you want that label to mean the same thing to every reader, regardless of country.

Full week-number tables by year

Each year has its own page listing every week with its ISO week number, the Monday start date, and the Sunday end date. Pick the year you need:

Examples

January 1, 2026 is in ISO week 1 of 2026 (Monday).
Today (Sep 18, 2026 (Fri)) is in ISO week 39 of 2026.
December 31, 2026 is in ISO week 53 of 2026 (Thursday), because 2026 has 53 weeks.

First published Aug 4, 2026 · Last reviewed Sep 18, 2026 · Maintained and developed by the Real Epoch Converter team · [email protected] · Contact · Methodology

Sources & references

More timestamp tools

Need the reverse direction, another precision, or a whole list at once? The epoch time converter on the home page converts dates to Unix timestamps and back — seconds, milliseconds, or microseconds — with batch conversion, and the Unix timestamp to date converter handles one value at a time.

Prefer reading? The guides explain Unix time, timestamp formats, and common bugs in depth, and the methodology page documents exactly how this site handles negative timestamps, fractional seconds, leap seconds, and daylight-saving transitions.