Week Number Calculator
ISO 8601 and US week numbers, day of year and days remaining.
Pick a date and instantly see its ISO 8601 week, US week number, day of year, days in the month and days remaining in the year. Handy for project tracking and reporting.
Common use cases
- Reference ISO week numbers in project trackers and Gantt charts
- File weekly reports tagged with the correct week
- Plan annual budgets or campaigns by day of year
- Confirm whether a year contains 52 or 53 ISO weeks
Frequently asked questions
What is the ISO 8601 week number?▾
ISO 8601 numbers weeks starting on Monday and assigns week 1 to the week containing the year's first Thursday. Most of Europe and many software libraries use this convention.
How does the US week number differ?▾
The US convention treats week 1 as the week containing January 1, with weeks starting on Sunday. So the same calendar date can have different numbers in each system.
Why does my year sometimes have 53 weeks?▾
ISO years with 53 weeks happen when January 1 is a Thursday or, in a leap year, a Wednesday.
Related tools
⏱
Unix Timestamp Converter
Convert between Unix timestamps and human-readable dates.
🌐
Timezone Converter
Convert a time between any two time zones.
📅
Date Difference Calculator
Calculate days, weeks, months and years between two dates.
⏳
Countdown Timer
Live countdown to any future date and time, ticking second by second.