What is Week Number Calculator?
A week number calculator identifies the ISO 8601 week-year, week number, and weekday for a calendar date. ISO weeks run Monday through Sunday, and weekday numbers run from Monday 1 through Sunday 7.
ISO week 1 is the week containing the year’s first Thursday. Equivalently, it is the week containing January 4. This means dates near New Year can belong to the previous or next ISO week-year.
The selected date is calculated locally in your browser and is not uploaded.
Why Use This Tool?
Use ISO week numbers for production plans, sprint labels, logistics schedules, reporting periods, and international documents that organize work by numbered weeks.
Displaying the week-year separately prevents a common error: assuming every late-December date belongs to that calendar year’s final ISO week.
- Find ISO week number for any date
- See the correct ISO week-year
- Identify weekday on a Monday-first scale
- Standardize week references across teams
How Does This Tool Work?
The calculator creates a UTC date from the selected local year, month, and day to avoid local DST affecting week arithmetic. Sunday is converted from JavaScript’s 0 to ISO weekday 7.
It then shifts the date to Thursday of the same ISO week. The year containing that Thursday is the ISO week-year. The number of seven-day groups from the start of that year determines the week number.
This implements the first-Thursday rule. ISO years contain 52 or sometimes 53 weeks; a week is never numbered 0.
- Monday is weekday 1
- Sunday is weekday 7
- Week 1 contains the first Thursday
- Week-year can differ from calendar year
Understanding Your Results
A result such as 2027-W01 means the date belongs to week 1 of ISO week-year 2027. The date itself may still be in late December 2026.
ISO week numbering is not the only week convention. Some calendars begin weeks on Sunday or define week 1 differently, so confirm which standard a partner or reporting system uses.
- Use week-year and week number together
- Leading and trailing calendar days can cross ISO years
- ISO week dates are designed for Monday-first weeks
Why Tracking This Matters
A bare “week 1” can refer to different dates under different conventions. Naming ISO 8601 and retaining the week-year makes schedules, reports, and file labels less ambiguous.
Benefits of Using Week Number Calculator
- ISO 8601 week number
- Correct ISO week-year
- Monday-through-Sunday weekday
- First-Thursday rule
- Reliable New Year boundary handling
- Private browser-local calculation
How Is the Result Calculated?
The selected date is moved to its ISO week’s Thursday before the week-year and ordinal are derived.
thursday = date + (4 − ISOweekday); week = ceil(((thursday − Jan 1 of weekYear) ÷ 86,400,000 + 1) ÷ 7)
- ISO weekday
- Monday = 1 through Sunday = 7.
- Thursday
- The anchor day that determines the ISO week-year.
- Week-year
- The calendar year containing the ISO week’s Thursday.
- Week number
- The 1-based ordinal of the Monday–Sunday week in its ISO year.
- Week numbers range from 1 to 52 or 53
- UTC arithmetic avoids local DST distortion
- The input date remains the user’s chosen calendar date
Tips for Better Results
- Write labels as year-Wweek, such as 2026-W30.
- Include the week-year near New Year boundaries.
- Confirm partners use ISO rather than Sunday-first numbering.
- Use weekday numbers only with the stated Monday-first convention.
- Check week 53 support in reporting systems.
- Store the original date alongside derived week labels.
Standards and References
Conclusion
The week number calculator applies ISO 8601’s Monday-first, first-Thursday convention and returns both week number and week-year.
Keep those two values together, especially around New Year, and confirm that collaborators use the same week convention.