What is the difference between the ISO and US week number?

ISO 8601 weeks start on Monday and week 1 is the week containing the first Thursday of the year. The US convention starts weeks on Sunday and counts the week containing January 1 as week 1, so the two numbers can differ near the new year.

How is the ISO week number worked out?

The calculator finds the Thursday nearest to your date, then counts how many full weeks separate it from the Thursday of week 1 (the week containing 4 January). Anchoring on Thursday is what makes the ISO rule unambiguous.

Why does the ISO year sometimes differ from the calendar year?

Early January dates can belong to the last ISO week of the previous year, and late December dates can belong to week 1 of the next year. The tool shows the correct ISO year alongside the week number.

How many weeks are in a year?

An ISO year has either 52 or 53 weeks. Most years have 52; a year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year.

What is the day of year?

It is the date's ordinal position counting from 1 January as day 1. For example, 1 February is day 32, and 31 December is day 365 (or 366 in a leap year).

Which week numbering does my country use?

Most of Europe and much of the world uses the ISO 8601 (Monday-start) numbering, while the United States commonly uses the Sunday-start convention. The tool shows both so you can match whichever your calendar or payroll system expects.

Is my date uploaded anywhere?

No — the week number is computed entirely in your browser and nothing is ever uploaded.

Week Number Calculator

Week number calculator

This tool tells you which week of the year any date falls in, using both the international and US conventions. Week numbers are widely used in payroll, project planning, manufacturing schedules, and shared calendars, where teams refer to “week 23” rather than a specific date.

How it works

For the ISO 8601 number (weeks start Monday; week 1 is the week containing the year’s first Thursday), the calculator shifts your date to its nearest Thursday, then counts the whole weeks between that Thursday and the Thursday of week 1 (the week containing 4 January), adding one. Because the Thursday always lands in the “majority” year, this also yields the correct ISO year, which can differ from the calendar year at the start or end of December and January.

It also shows the US week number (weeks start Sunday; the week containing 1 January is week 1), the day of year (1 January = 1), and the weekday.

Field	Convention
ISO 8601 week	Monday start, week 1 holds first Thursday
US week	Sunday start, week 1 holds 1 January
Day of year	Ordinal day, 1–365/366

Example

Take 1 January 2027, which is a Friday. Its nearest Thursday falls in late December 2026, so the ISO result is Week 53 of 2026, while the US convention counts it as Week 1 of 2027. The day of year is 1. This new-year overlap is exactly why showing both numbers matters.

Everything runs entirely in your browser — nothing is uploaded.