Free Week Number Calculator
‘Week 41’ - but which dates does that cover? Enter any date to get its ISO 8601 week number instantly, along with the ISO year, the weekday name and the full Monday-to-Sunday range, so week-based plans and reports always line up.
How to Use the Week Number Calculator
- Pick a date (or leave it as today).
- Click Get Week Number for the ISO week.
- See the ISO year, weekday name and the week's Monday–Sunday range.
Features
The tool returns the ISO 8601 week number (1–53) for any date, plus the ISO year - which can differ from the calendar year in early January - the weekday name, and the complete Monday-to-Sunday date range of that week. Weeks follow the Monday-start ISO standard used across European business.
Benefits & Use Cases
Project managers plan in week numbers, payroll teams align weekly cycles, and manufacturers schedule production by ISO week. Academics mapping terms to weeks and anyone decoding a "week 41" reference in an email get the exact dates instantly.
Tips
Watch the ISO year in early January: January 1 can belong to week 52 or 53 of the previous ISO year. Some years have 53 ISO weeks - 2026 does, because it starts on a Thursday. For the full seven-day grid view, try the Calendar Week Calculator.
Frequently Asked Questions
What is an ISO week number?
A number from 1 to 53 identifying a week under ISO 8601. Week 1 is the week containing the first Thursday of the year, and weeks always start on Monday.
Why is the ISO year different from the calendar year?
Days in early January can belong to the last ISO week of the previous year - for example, January 1, 2027 falls in ISO week 53 of ISO year 2026.
Do ISO weeks start on Monday or Sunday?
Monday. The ISO 8601 standard defines Monday as the first day of the week, which is the norm in European business calendars.
How many ISO weeks does 2026 have?
53. Years starting on a Thursday - like 2026 - get a 53rd week; most years have 52.
What dates are in a given week number?
It varies by year. The tool shows the full Monday-to-Sunday range for the week containing your date.
Is it free?
Yes - free, private and no sign-up.
Related Tools
Calendar Week Calculator
See the full Monday-to-Sunday calendar week for any date, with its ISO week number and date range. Free, instant results - no sign-up required.
Open tool →Day of Year Calculator
Find the day-of-year number for any date, plus days remaining in the year, ISO week number and quarter. Instant, free and private - no sign-up.
Open tool →Days in Month
Look up how many days any month has, with a full Monday-first calendar table and weekday counts included. Leap years handled. Free, no sign-up.
Open tool →Date Format Converter
Convert any date into 12+ formats: ISO 8601, US, European, long, short, RFC 2822, SQL and Unix timestamp. Free with copy buttons, no sign-up.
Open tool →Business Days Calculator
Count business days between two dates, skipping weekends and your custom holidays. Free online calculator with a full breakdown - no sign-up needed.
Open tool →Date Difference
Get the exact years, months and days between any two dates, plus totals in days, weeks and hours. Calendar-accurate, private and free - no sign-up.
Open tool →