Best Free Online Developer Tools to Use in 2026
A practical, opinionated list of the best free online developer tools worth using in 2026, organized by category so you can find what you need fast.
Aug 08, 2026
Week Number Calculator
Calculate ISO week numbers for any date. View monthly calendars with week numbers and a complete yearly week overview.
Week Number Calculator
Select a date to see week number information
- Select a date — Pick any date using the date picker, or click "Today" for the current date.
- View week info — See the ISO week number, week date range, day of year, and ISO 8601 format.
- Browse the calendar — Use the month/year selectors to view a monthly calendar with week numbers for each day.
- Year overview — See all weeks of the year with their start and end dates.
What is ISO week numbering?
ISO 8601 defines week numbers where weeks start on Monday. Week 1 is the week containing the first Thursday of the year (or January 4th). Each week belongs to the year in which its Thursday falls, which means the first few days of January may belong to week 52 or 53 of the previous year.
How many weeks are in a year?
Most years have 52 weeks, but years that start on a Thursday (in common years) or Wednesday (in leap years) have 53 weeks. This calculator shows the exact number for the selected year.
What is the ISO week format?
The ISO 8601 format is YYYY-Www (e.g., 2025-W14). The full date format including day is YYYY-Www-D (e.g., 2025-W14-3 for Wednesday of week 14, 2025).
What day does the week start on?
The ISO week numbering system starts weeks on Monday (day 1) and ends on Sunday (day 7). This is the international standard used in most European and Asian countries.
Why does January 1 sometimes show week 52 or 53?
If January 1 falls on a Monday, Tuesday, Wednesday, or Thursday, it belongs to week 1 of the new year. But if it falls on a Friday, Saturday, or Sunday, it belongs to the last week (52 or 53) of the previous year.
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