Week Number Calculator – Find the ISO Week Number for Any Date
About the Week Number Calculator
This calculator converts between dates and week numbers in both directions. Give it a date and it returns the week of the year that date falls in; give it a week number and a year and it returns the seven-day range that week covers. Advanced options let you change the numbering rules to match whichever standard or fiscal calendar you work to.
Week numbers are standard in European business, manufacturing, and logistics, where “delivery in week 34” is a normal way to communicate a schedule. Project plans, production runs, academic terms, and shipping windows all use them, because a week number identifies a period without the ambiguity of a date range and without depending on how your reader formats dates.
The default settings follow ISO 8601, the international standard: weeks begin on Monday, and week 1 is the first week containing at least four days of the new year, which is the same as saying it is the week containing the first Thursday. Under those rules a year has either 52 or 53 weeks. The calculation itself is straightforward once week 1 is fixed: \text{week} = \left\lfloor \frac{\text{days since week 1 began}}{7} \right\rfloor + 1.
The complication is that ISO is not the only convention in use. North American practice commonly starts weeks on Sunday and treats the week containing January 1 as week 1, which produces different numbers for the same dates. Broadcast and retail calendars have their own rules again. This is why two colleagues can look at the same date and disagree about which week it is, and why the tool exposes both the first day of the week and the week 1 rule rather than assuming one answer.
The Week Year Start Month and Start Date options exist for fiscal calendars. An organization whose financial year begins in April can set that as the start, so week 1 aligns with the start of their reporting year rather than with January.
One consequence of ISO numbering that surprises people: the last days of December can belong to week 1 of the following year, and the first days of January can belong to week 52 or 53 of the previous one. January 1, 2027 falls on a Friday, which puts it in the final ISO week of 2026 rather than week 1 of 2027. That is correct behavior, not an error, and it is why the ISO standard talks about a “week-numbering year” that does not always match the calendar year.
The BigToolSite developers who built the date suite tested this across year boundaries, 53-week years, leap years, and every combination of week 1 rule and first-day-of-week setting, confirming the ISO defaults return the same numbers as the standard.
When you need the number of working days inside a given week rather than its date range, the Workdays Calculator handles that count with a configurable working week.
To measure the span between two dates in weeks and days rather than by week number, the Date Difference Calculator gives that breakdown.
If you know a week number and need to project forward from its start date, the Date Calculator adds or subtracts time from any date you supply.
How to Use the Week Number Calculator
- Choose a mode: Date to Week Number or Week Number to Dates.
- In date mode, type the day, month, and year, use the calendar button to pick a date, or click Today to fill in the current date.
- In week mode, enter the Week Number and the Year.
- Leave the advanced options alone for standard ISO 8601 numbering, which is what most European business and international standards use.
- Set First day of week to Sunday if you follow the North American convention, which shifts every week boundary by one day.
- Change When does week 1 start to match your standard. “At least 4 days” is ISO. “Start date above is always week 1” matches the common US approach of treating the week containing January 1 as week 1.
- Use Week Year Start Month and Start Date for a fiscal year that does not begin on January 1.
- Click Calculate. Date mode returns the week number with the full date and weekday; week mode returns the start and end dates of that week.
Example Usage
Date to week number: August 20, 2026, ISO defaults
Result: Week 34, Thursday, August 20, 2026
Week number to dates: Week 34 of 2026, ISO defaults
Result: Monday, August 17 through Sunday, August 23, 2026
Switching First day of week to Sunday moves that range to August 16 through August 22. Nothing is wrong with either answer; they are different conventions. If you are coordinating with someone in another country, agreeing which standard you are both using is worth doing before you agree on the week.
For a fiscal year beginning April 1, set Week Year Start Month to April and Start Date to 1. Week 1 then covers the start of April, so week numbers line up with your reporting periods rather than the calendar year.
A practical check on any week-numbering setup: run December 31 and January 1 of the same changeover through it. Those two dates expose exactly how your chosen rules handle the year boundary, which is where nearly every week-numbering disagreement originates.
Frequently Asked Questions
The international standard for numbering weeks. Weeks run Monday to Sunday, and week 1 is the first week with at least four days in the new year, equivalent to the week containing the first Thursday. Years contain either 52 or 53 weeks under this rule.
Click the Today link to fill in the current date, then calculate. With the default settings you get the ISO week number, which is what most international business uses.
Because 365 days is 52 weeks plus one day, and 366 in a leap year. That surplus accumulates, so roughly every five or six years an extra week is needed to cover the year. 2026 is a 53-week year under ISO rules.
Under ISO rules a week belongs to whichever year holds the majority of its days. If January 1 falls late in the week, that whole week counts as the final week of the previous year. The ISO week-numbering year and the calendar year do not always match.
ISO starts weeks on Monday and requires four days in the new year for week 1. The common US convention starts weeks on Sunday and treats the week containing January 1 as week 1 regardless of length. The same date can therefore have two different week numbers.
Set Week Year Start Month and Start Date to the first day of your fiscal year. Week 1 then begins there rather than in January, so the numbering matches your reporting calendar.
Yes, any day of the week can be set as the first day. Monday is the ISO standard and Sunday is common in North America and parts of Asia.
52 in most years and 53 in some, depending on which day January 1 falls on and whether it is a leap year. Under ISO rules a 53-week year occurs when the year starts on a Thursday, or on a Wednesday in a leap year.
It means week 1 is the first week that has four or more of its seven days inside the new year. This is the ISO rule, and it is why a week split across the year boundary belongs to whichever year holds most of it.
Because it names a period unambiguously. “Week 34” is clearer across languages and date formats than a range written as 17/08 or 08/17, which is why manufacturing, logistics, and European business rely on it.
