Week Number
ISO week number for any date
The Week Number Calculator is a clean calendar utility designed to identify the week number for any selected date. By implementing the standard ISO-8601 week date system alongside traditional US calendar systems, it calculates the week number, day of the year, and remaining weeks, supporting your business scheduling.
Week numbers are standard in production scheduling, supply chain logistics, finance, and corporate planning. Many businesses plan projects by week (e.g., "completed in week 42") to avoid the confusion of specific calendar dates. This calculator simplifies the lookup: select a date, and it identifies the week number instantly.
All calculations run locally in your web browser. No date inputs or scheduling details are sent to external servers, protecting your privacy. It is a secure, fast, and free tool designed to support your project management and logistics.
What this calculator determines
This calculator computes the week number and chronological details for your selected date. The primary result is the ISO-8601 week number, which is the global standard for business and manufacturing calendars.
The results also display the traditional US week number (which starts the week on Sunday instead of Monday), the day of the year (out of 365 or 366), and the total weeks remaining in the year. This information helps you track project progress and align timelines with corporate targets.
The ISO-8601 week definition
The ISO-8601 standard defines week 1 of the year as the week containing the first Thursday of January. This means:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains January 4th.
- If January 1st falls on a Friday, Saturday, or Sunday, the days before the first Monday are counted as part of the last week of the previous year (week 52 or 53).
This consistent structure ensures that every week has exactly seven days, and week numbers align across international business borders without regional variations.
Worked example of week lookup
Let us look at a practical example: finding the week number for June 26, 2026. The calendar calculations are performed as follows:
- ISO-8601 Week Number: Week 26.
- Traditional US Week: Week 26 (weeks start on Sunday).
- Day of the Year: Day 177 out of 365 days.
- Weeks Remaining: 26 weeks left in the year 2026.
- Weekday: The selected date falls on a Friday.
This quick lookup ensures you align your schedules with production plans and coordinate deliverables with team members.
When to use this week calculator
Use this tool when planning project timelines, coordinating supply chain deliveries, or analyzing weekly sales data in business reports. It is also an excellent resource for developers and administrators scheduling automated tasks, server backups, or database maintenance cycles that run on weekly intervals.
By displaying both the ISO and US week numbers, the calculator helps you coordinate schedules across different regional standards. Adjusting the date updates all calculations instantly, making it a fast reference.
Calendar system differences to note
It is important to remember that calendar systems define week numbers differently. The ISO-8601 standard is used in Europe and global business, starting the week on Monday. The United States, Canada, and Japan traditionally start the week on Sunday, which can cause a date to fall in different week numbers depending on the system used.
Additionally, because of how week 1 is defined, the first few days of January or the last few days of December can fall in week 52 or 53 of the adjacent year. Always verify which standard your organization uses when scheduling deliverables near the start of the year.
Frequently asked questions
Why is January 1 sometimes in last year's week?
Because ISO weeks always start on Monday and Week 1 is defined to contain January 4. If January 1 falls on a Friday, Saturday, or Sunday, it is in the last week of the previous ISO year.
How many ISO weeks in a year?
Either 52 or 53. Roughly every 5–6 years has 53 ISO weeks.
Is week 1 always January?
Yes, week 1 always contains some days in January (specifically, January 4).
Why do some years have 53 weeks instead of 52?
A standard year has 365 days, which is equal to 52 weeks and 1 day (or 2 days in leap years). These extra days accumulate, resulting in a 53-week year approximately once every five to six years in the ISO system.
What is the start of the week in the ISO standard?
Under ISO-8601, the week starts on Monday at 00:00. This is the global standard for business, whereas the US standard calendar traditionally starts the week on Sunday.