Week Number Calculator

How to use the week number calculator

1. Choose a date.
2. Read the ISO week and day of year.
3. Check the note if the ISO year differs from the calendar year.

About this tool

ISO week 1 is the week containing the first Thursday of January — equivalently, the week containing 4 January. That definition has a consequence people find surprising and calculators frequently get wrong: the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next.

Both cases are real. 1 January 2021 was a Friday, so it fell in week 53 of 2020. 30 December 2024 was a Monday, so it fell in week 1 of 2025. Any calculator that simply divides the day of the year by seven gets both wrong, which is why the ISO week-numbering year is reported separately here and flagged when it differs from the calendar year.

Most years have 52 weeks; some have 53. A year has 53 when it starts on a Thursday, or is a leap year starting on a Wednesday. That matters for anyone planning against week numbers, since a 53-week year quietly gains a period.

The day of the year is shown alongside, running 1 to 365 or 366.

Every calculation is done in UTC. Working in local time silently gains or loses an hour across a daylight saving boundary, which turns a difference of exactly seven days into six days and twenty-three hours — a bug that appears twice a year and is therefore rarely caught.

Common uses

• Finding the current week number for planning or reporting.
• Checking which ISO year a date at the year boundary belongs to.
• Working out how many days remain in the year.

Frequently asked questions

Why does 1 January sometimes fall in week 52 or 53?

Because ISO week 1 is the week containing the first Thursday. If January starts late in the week, those first days belong to the previous year's final week.

How many weeks are in a year?

52 usually, 53 when the year starts on a Thursday or is a leap year starting on a Wednesday. The figure for the year in question is shown.

Is this the same week number my country uses?

If it uses the ISO standard, yes. Some countries and some software count weeks from 1 January or start the week on Sunday, which gives different numbers.