Quick answer
ISO week numbers start on Monday, and week 1 is the week containing the year's first Thursday. 1 January 2024 is a Monday, so it falls in ISO week 1 of 2024.
Enter a date. The calculator returns the ISO week number and the ISO week-year.
Related tools: Day of the Week Calculator: Find the weekday of any date. Day of the Year Calculator: Which day of the year a date falls on. Leap Year Calculator: Check whether a year is a leap year.
How to use the Week Number Calculator
- Pick date.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Date | A calendar date | — |
Formula and method
Formula
ISO week: weeks start on Monday; week 1 contains the year's first Thursday
Exact integer day arithmetic in UTC; no daylight-saving or time-zone effects.
The ISO week-year can differ from the calendar year around 1 January.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Date = 2024-01-01.
| Result | Value |
|---|---|
| ISO week number | 1 |
| ISO week-year | 2024 |
Example 2
Inputs: Date = 2021-01-03.
| Result | Value |
|---|---|
| ISO week number | 53 |
| ISO week-year | 2020 |
Example 3
Inputs: Date = 2024-07-04.
| Result | Value |
|---|---|
| ISO week number | 27 |
| ISO week-year | 2024 |
The first line is the headline result (iSO week number). Below it the week Number Calculator lists iSO week-year, which describe the same case from other angles. Exact integer day arithmetic in UTC; no daylight-saving or time-zone effects.
Edge cases at the year's ends
Late-December dates can belong to week 1 of the next year, and early-January dates to week 52 or 53 of the previous year. 30 December 2024, for example, is in ISO week 1 of 2025. That is why the week-year is shown.
Why it matters
ISO weeks are used in business planning, logistics and Europe. The US often numbers weeks from Sunday and counts the week containing 1 January as week 1, so numbers can differ by one.
Week Number from different start dates
The same calculator run from 1 January of four consecutive years.
| Date | ISO week number | ISO week-year |
|---|---|---|
| 2026-01-01 | 1 | 2026 |
| 2027-01-01 | 53 | 2026 |
| 2028-01-01 | 52 | 2027 |
| 2029-01-01 | 1 | 2029 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Date: left empty shows "Enter date." beside that field and no result.
Choosing between the Week Number Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Day of the Week Calculator](/date-calendar/day-of-the-week-calculator/) | date | Day of the week |
| [Day of the Year Calculator](/date-calendar/day-of-the-year-calculator/) | date | Day of the week |
| [Leap Year Calculator](/date-calendar/leap-year-calculator/) | year | Result |
| [Number Base Converter](/technology/base-calculator/) | number, from base and to base | Result |
| [Number Scale Converter](/converters/number-scale-converter/) | value, from and to | Crore → Million |
Background: dates and times
Date and time results depend on the conventions a tool uses: whether the end date counts, whether weekends and holidays are skipped, how months of different lengths are handled and whether time zones are applied. This page states its convention, and the worked examples show it.
Dates use the Gregorian calendar and calendar days unless the page says otherwise; check deadlines against the rule in your contract or regulation.
Assumptions and limitations
- Late-December dates may belong to week 1 of the next year; early-January dates to week 52/53 of the previous year
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for date to confirm this.
Frequently asked questions
How many weeks in a year?
A year has 52 or 53 ISO weeks; years with 53 weeks start on a Thursday, or on a Wednesday in a leap year.
What day does an ISO week start?
ISO weeks start on Monday and week 1 is the week containing the year's first Thursday.
What week is 1 July?
Usually week 26 or 27.
What does the Week Number Calculator calculate?
ISO 8601 week number of a date. It returns ISO week number and ISO week-year.
What formula does the Week Number Calculator use?
ISO week: weeks start on Monday; week 1 contains the year's first Thursday. The ISO week-year can differ from the calendar year around 1 January.
What do I need to enter in the week Number Calculator?
You enter Date.
How is the result rounded or limited?
Exact integer day arithmetic in UTC; no daylight-saving or time-zone effects. Late-December dates may belong to week 1 of the next year; early-January dates to week 52/53 of the previous year.
What is an example of a week Number result?
With date 2024-01-01, the iSO week number is 1.
Where does the method come from?
The method follows: ISO 8601-1:2019, section 5.2.5.
Related calculators
- Day of the Week Calculator — Find the weekday of any date.
- Day of the Year Calculator — Which day of the year a date falls on.
- Leap Year Calculator — Check whether a year is a leap year.
- Number Base Converter — Convert a number between bases 2 and 36.
- Number Scale Converter — Convert thousands, lakhs, crores, millions and billions.
- Number Sequence Calculator — Identify an arithmetic or geometric sequence and find the next terms.
Sources
- ISO 8601-1:2019, section 5.2.5
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.