Live Week Number Calculator
It calculates ISO week year, ISO week number, US week number and day-of-year for any date.
Week 18
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Week Number Calculator
Free week number calculator for ISO 8601 week, US week number, weekday and day-of-year.
How to use Week Number Calculator
- Open the Week Number Calculator page and use the default example if you want a quick test.
- Enter any calendar date in the tool panel.
- Press the primary action button or use the listed keyboard shortcut when available.
- Read the result as iso week, iso week-year, us week number, weekday and day-of-year, then copy, export or print it if needed.
Formula and algorithm
ISO 8601 week 1 is the week with the year's first Thursday and weeks start on Monday.
This tool runs in your browser. If the tab is backgrounded or the device sleeps, visible updates or alerts may be delayed. Notifications and Wake Lock reduce that risk where supported, but they are not a guarantee.
Worked example
2026-04-30. The expected output is ISO Week 18, US Week 18, Day-of-Year 120.
{
"tool": "Week Number Calculator",
"input": "2026-04-30.",
"output": "ISO Week 18, US Week 18, Day-of-Year 120."
}Use cases
- Agile sprint planning
- Manufacturing calendars
- Academic schedules
- Payroll periods
- Editorial calendars
Pro tips and gotchas
- ISO week years can differ from calendar years near January 1.
- US weeks start on Sunday.
- ISO weeks start on Monday.
- Day-of-year is useful for logs.
- Use the same week standard across your team.
Frequently Asked Questions
What week number is it today iso
Enter today's date and the calculator returns the ISO week number — the week of the year according to ISO 8601 standards. ISO weeks start on Monday and the first week of the year is the one containing the first Thursday of January (or equivalently, the week containing 4 January). So today's date might fall in week 19 or 20 depending on the calendar layout. Useful for project tracking, sprint planning, and international scheduling where ISO weeks are the standard.
What is the iso week number for a date
Enter the date. The calculator returns the ISO week number, which can range from 1 to 53. ISO 8601 defines week 1 as the week containing the first Thursday of January, so dates in late December may belong to week 1 of the next year, and dates in early January may belong to week 52 or 53 of the previous year. Useful for international business, project management, and any context where weeks need a consistent global definition.
How to find week number from date
Type the date and click Calculate. The calculator returns the week number using the system you've selected — ISO 8601 (Monday-start) or US convention (Sunday-start). Both are common; ISO is the international standard, US convention is mostly used in the United States and parts of Asia. Useful for fiscal-year tracking, sprint planning, school terms, and any system that organises time by week rather than month.
How many weeks are in 2026 calendar year
2026 has 53 ISO weeks. Most years have 52 weeks, but a few have 53 — these are years where 1 January falls on a Thursday, or where 31 December falls on a Thursday in a leap year. The calculator's year-overview feature shows which years have 53 weeks. Useful for long-term project planning, payroll cycles based on weekly periods, and reporting periods that follow ISO week numbering. Plan your year accordingly.
What week of the year is a date
Enter the date and the calculator returns the week number using either ISO 8601 (Monday-start, Thursday rule) or US convention (Sunday-start, January 1 rule). The two systems can differ by a week for dates in early January or late December. Pick the system that matches your context — international business and project tools usually use ISO; US calendars and some HR systems use the US convention. The calculator shows both side by side when needed.
What is week 1 in iso calendar
ISO week 1 is the week of the year containing the first Thursday of January, which is equivalent to saying the week that contains 4 January. The week always starts on Monday. So if 1 January falls on a Friday, ISO week 1 starts on the following Monday (4 January or later), and the days from 1 to 3 January belong to the last week of the previous year. Useful for understanding how ISO 8601 handles year boundaries.
How to convert date to week number online
Enter the date in the input field and pick your week-numbering system — ISO 8601 or US. Click Calculate. The calculator returns the week number along with the day of the week and the year. Useful for project tracking, sprint reports, fiscal-year planning, and educational calendars. The two systems usually agree from January through December but can diverge by a week at year boundaries depending on which day of the week 1 January falls on.
Glossary
- UTC
- Coordinated Universal Time, the reference time standard used for offsets and date math.
- DST
- Daylight Saving Time, a seasonal clock change that can affect wall-clock schedules.
- IANA time zone
- A named time zone such as America/New_York used by browsers for DST-aware rendering.
- Tab-title timer
- A timer state shown in the browser tab title so it remains visible while multitasking.
- Wake Lock
- A browser feature that can ask a device to keep the screen awake during active timing.
- localStorage
- Browser storage used here for settings and saved tool state on the same device.
- Input format
- Any calendar date.
- Output format
- ISO week, ISO week-year, US week number, weekday and day-of-year.
References and sources
- ISO 8601 for week-number rules.
- 5 U.S.C. section 6103 for US federal holidays.
- IANA Time Zone Database for time zone identifiers.
- The Pomodoro Technique, Francesco Cirillo, 1987 for Pomodoro method.
- US Department of Labor FLSA for overtime context.
- NIST Time and Frequency Division for time-keeping standards.