Week Number Calculator
Look up ISO and US week numbers, week ranges, day-of-year, and year-boundary context for any date.
Calculate with Week Number Calculator
Selected ISO week layout
The ISO calendar week starts on Monday and ends on Sunday.
ISO vs US week rules
Use this when a report, planner, or timesheet names a different week convention.
Mar 16, 2026 through Mar 22, 2026.
Mar 15, 2026 through Mar 21, 2026.
Reusable week summary
Your result
Check before you use it
What this result means
Review these details before you use the number for a deadline, schedule, bill, trip, or household plan.
The planning label to use in reports, schedules, or recurring checklists.
Highlights dates that sit close to a week-year transition.
Pair with Day of Week Calculator when the weekday also needs to be stated.
Planning context
Use this when a date such as January 5, 2026 needs to be translated into a calendar week label before it goes into a planner, status report, invoice note, or family schedule.
Common jobs
Enter the exact date whose week number you need.
Compare the week label with the calendar-year context.
Check adjacent Monday or Sunday dates when the week boundary matters.
Inputs to confirm
Use a specific date such as January 5, 2026, not only a month name.
Confirm whether the recipient expects ISO-style Monday weeks or another local reporting rule.
Dates near January 1 and December 31 can belong to a neighboring week-year under some conventions.
Copy-ready handoff note
Copy this after entering the live calculator values and confirming the visible assumptions.
Week Number Calculator handoff note Task: Look up ISO and US week numbers, week ranges, day-of-year, and year-boundary context for any date. Use case: Check a date. Inputs checked: Target date, Week convention, Year boundary. Result use: Enter the live values on the calculator, review the result, then share it with the assumptions below. Assumptions: Week labels depend on the week-numbering convention used by the page and by the recipient. Dates near January 1 and December 31 should be checked individually. Next check: When sending the result, state the week convention if another person will rely on it.
Workflow method and assumptions
Next decision
Confirm the week before sharing a schedule
Enter the date
Start with the exact calendar date from the event, form, or reporting period.
Review the week result
Check whether the date falls in a normal week or near a year boundary.
Document the rule
When sending the result, state the week convention if another person will rely on it.
Assumptions worth checking
Using This Calculator
Scenario: label a January reporting week
A team preparing a report for Monday, January 5, 2026 can use this page to confirm the week label before naming the file or sprint note.
- Concrete input: January 5, 2026.
- Assumption to state: week numbering may depend on ISO or local business rules.
- Useful follow-up: check the weekday when the week starts near a holiday.
Boundary checks
Week numbers are easy to misread around New Year because the calendar year and week-year can diverge.
- Check December 29, 2025 and January 1, 2026 separately if a range crosses the year boundary.
- Do not assume every organization starts week 1 on January 1.
ISO week versus US week labels
Searches for week numbers often mix ISO week, US week, calendar week, and day-of-year language. Treat the result as a label tied to a convention, not just a raw number.
- Use ISO wording when the recipient expects Monday-start week labels.
- Use a US week note when Sunday-start reporting is used.
- Pair with Time Duration Calculator when the same report also needs hours or minutes.
Quick glossary
All days in the range, including weekends and holidays.
Working days after weekend exclusions, and sometimes after holiday exclusions.
Extra time intentionally added to absorb delay or review overhead.
Result checks before you use it
Calculator questions
What people usually check next
How does the week number calculator work?
Week Number Calculator uses the values you enter in the form and applies a browser-based calculation to produce an instant result. The goal is to help people planning dates, countdowns, school admin tasks, and small logistics questions test scenarios quickly and privately without rebuilding the same logic in a spreadsheet.
When should I use a week number calculator instead of a spreadsheet?
Use week number calculator when you need a fast answer for deadlines, date gaps, reporting windows, and repeat calendar lookups and you do not want to recreate the same setup every time. It is best for quick checks, comparisons, and planning conversations where speed matters more than a fully customized workbook.
Are the week number calculator results exact?
The result is exact for the formula and assumptions used by this tool, but calendar rules, local holidays, and real-world schedules can still change the final outcome. Use the output as a planning baseline and confirm any policy-sensitive detail before acting on it.
Workflow references and examples
Follow-up tools
Use the next calculator when it matches the workflow
Confirm the weekday for the same date.
Measure hours and minutes when the week label is part of a time report.
Plan a full week window after finding the week label.
Move from a week label to working-day deadline planning.