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Week Number Calculator and Calendar

Find week numbers for any date with ISO 8601, US, Monday-start, and fiscal week systems, plus a clickable calendar, week-to-date lookup, date-range analysis, CSV export, ICS export, and share links.

Last Updated: July 2026

Week number lookup

Find any date's week number, calendar week, and year position

Compare ISO, US, Monday-start, and fiscal week systems, then use the calendar, reverse lookup, range analyzer, and export tools for planning.

ISO is the default for international planning. Fiscal mode uses your custom year start.

Current answer

ISO 2026-W29

Date

Jul 16, 2026

Weekday

Thursday

Quarter

Q3

Week of Month

Week 3

Selected Week

ISO 2026-W29

Week Start

2026-07-13

Week End

2026-07-19

Day of Year

197 / 365

Actions and Exports

Copy the result, share the setup, export the week plan, or download an all-day calendar event for the selected week.

Used in Fiscal mode. ISO and Monday systems always start Monday; US starts Sunday.

Fiscal year labels use the year in which the fiscal year ends.

If the day does not exist in a month, the calculator uses that month's last day.

Week System Comparison

The same date can belong to different week labels depending on the convention. This is why ISO week-year values sometimes differ from the calendar year near New Year.

SystemWeek labelWeek rangeRule
ISO 8601ISO 2026-W292026-07-13 to 2026-07-19Monday start, week 1 contains Jan 4 / first Thursday.
US styleUS 2026-W292026-07-12 to 2026-07-18Sunday start, week 1 is the week containing Jan 1.
Monday Jan 1Monday 2026-W292026-07-13 to 2026-07-19Monday start, week 1 is the week containing Jan 1.
FiscalFiscal 2027-W032026-07-13 to 2026-07-19Custom fiscal year starting July 1.

Week Number Calendar

Click any date to update the lookup. The left rail shows the week number for each calendar row under the selected week system.

Week
Mon
Tue
Wed
Thu
Fri
Sat
Sun

Date From Week Number

Convert a week-year, week number, and weekday back into an exact calendar date.

Converted date

2026-07-13

Jul 13, 2026 in ISO 2026-W29. This week-year has 53 weeks.

Date Range Week Analyzer

Count days, business days, weekend days, complete weeks, and every touched week label.

Days

31

Business Days

22

Weekend Days

9

Weeks Touched

5

Complete Weeks

3

Start Week

ISO 2026-W29

End Week

ISO 2026-W33

Range

2026-07-16 to 2026-08-15

ISO 2026-W29ISO 2026-W30ISO 2026-W31ISO 2026-W32ISO 2026-W33

Full Year Week Planner

Review every week in the selected week-year. Export the full table for schedules, payroll periods, editorial calendars, releases, and school planning.

WeekDate rangeMonthsQuarterTarget-year days
ISO 2026-W012025-12-29 to 2026-01-04Dec 2025 - Jan 20262025 Q4 - 2026 Q14 days
ISO 2026-W022026-01-05 to 2026-01-11Jan 20262026 Q17 days
ISO 2026-W032026-01-12 to 2026-01-18Jan 20262026 Q17 days
ISO 2026-W042026-01-19 to 2026-01-25Jan 20262026 Q17 days
ISO 2026-W052026-01-26 to 2026-02-01Jan 2026 - Feb 20262026 Q17 days
ISO 2026-W062026-02-02 to 2026-02-08Feb 20262026 Q17 days
ISO 2026-W072026-02-09 to 2026-02-15Feb 20262026 Q17 days
ISO 2026-W082026-02-16 to 2026-02-22Feb 20262026 Q17 days
ISO 2026-W092026-02-23 to 2026-03-01Feb 2026 - Mar 20262026 Q17 days
ISO 2026-W102026-03-02 to 2026-03-08Mar 20262026 Q17 days
ISO 2026-W112026-03-09 to 2026-03-15Mar 20262026 Q17 days
ISO 2026-W122026-03-16 to 2026-03-22Mar 20262026 Q17 days

Week Number Planning Note

This tool is for calendar planning, scheduling, and reporting support. Week-number conventions can differ by organization, country, payroll system, academic calendar, fiscal year, and retail calendar. Confirm the official convention before using week labels for payroll, tax, compliance, or contractual deadlines.

Checked by Jitendra Kumar

Week Number Calculator and Calendar is checked for formula labels, source links, and result limits.

Jitendra Kumar, Founder & Editorial Standards Lead. Updated July 2026. Scope: time calculators.

Sources & methodology · Review standards

How to Use This Calculator

  1. Step 1: Enter a date

    Choose the date you want to convert into week numbers.

  2. Step 2: Pick a week system

    Use ISO 8601 for international week numbers, US for Sunday-start week numbers, Monday Jan 1 for Monday calendars, or Fiscal for custom year starts.

  3. Step 3: Review week ranges

    Check the week label, week start date, week end date, day of year, quarter, and comparison table.

  4. Step 4: Use the calendar or reverse lookup

    Click dates in the calendar or convert a week-year and weekday back into an exact date.

  5. Step 5: Analyze or export

    Analyze date ranges, copy a summary, share the URL, export a CSV year plan, or download an ICS event for the selected week.

How This Calculator Works

The calculator parses the selected date as a calendar date and then applies the chosen week-number rule. ISO 8601 uses Monday as the first day of the week and assigns week 1 to the week containing January 4. The US option starts on Sunday and assigns week 1 to the week containing January 1.

Fiscal mode lets you set the fiscal year start month, start day, and week start day. The tool labels fiscal years by the year in which the fiscal year ends, which is the common convention for fiscal-year planning.

The calendar grid, reverse lookup, range analyzer, CSV export, and ICS export all use the same selected week system. That keeps the week label, date range, and exports consistent.

How to Use Week Numbers Correctly

Quick Answer

Use the Week Number Calculator when you need to answer "what week is this date?" or "what date is week 27, Monday?" Choose ISO 8601 for international week numbers, US for Sunday-start calendars, Monday Jan 1 for a simple Monday-start calendar, or Fiscal when your organization's planning year starts on a custom date.

Week systemStarts onWeek 1 ruleBest use
ISO 8601 weekMondayWeek 1 is the week containing January 4 / the first Thursday.International reporting, product launches, engineering plans, European calendars, and cross-border schedules.
US weekSundayWeek 1 is the week containing January 1.US-style calendars, Sunday-start planning, simple weekly reports, and personal schedules.
Monday Jan 1 weekMondayWeek 1 is the week containing January 1.Teams that prefer Monday calendars but do not use ISO week-year rollover.
Fiscal weekCustomWeek 1 is anchored to the fiscal year start date you choose.Retail periods, school years, fiscal calendars, editorial schedules, and internal planning cycles.

What This Week Number Tool Includes

FeatureWhat it doesWhy it matters
Date lookupEnter any calendar date and see selected, ISO, US, Monday-start, and fiscal week numbers side by side.Fast answer for "what week is this date?" without manual date math.
Week number calendarBrowse a month grid with a week-number rail and clickable dates.Planning payroll, sprints, publishing calendars, school weeks, and delivery windows.
Date from week numberConvert week-year, week number, and weekday back into an exact date.Turning ISO week references such as 2026-W27 Monday into a calendar date.
Range analyzerCount days, business days, weekend days, complete weeks, and all week labels touched by a range.Project windows, leave planning, reporting periods, and schedule audits.
ExportsCopy summaries, share URLs, download CSV year plans, and download an ICS week event.Moving results into spreadsheets, calendars, status reports, and team planning notes.

Week Number Mistakes to Avoid

MistakeWhy it happensBetter check
Using calendar year instead of week-yearISO 2027-W01 can start in late December 2026.Copy the full week label, not only the week number.
Mixing Sunday and Monday week startsThe same date can be W01 in one system and W52/W53 in another.Choose the standard used by your payroll, reporting, or team calendar.
Ignoring fiscal start datesRetail, school, and company calendars often do not begin on January 1.Use Fiscal mode and set the actual fiscal start month and day.
Assuming every year has 53 weeksMany week-years have 52 weeks; some have 53.Check the full-year planner before assigning W53 work.

ISO Week Example

In ISO week numbering, a week belongs to the year containing that week's Thursday. That is why a late-December date can be part of next year's ISO week 1, while an early-January date can still be part of the previous ISO week-year.

When to Use a Week Number Calendar

Week numbers are useful when teams plan in repeating weekly blocks: payroll periods, agile sprints, school terms, manufacturing schedules, release trains, editorial calendars, service rotations, and logistics windows. For exact date spans, pair this tool with the Date Duration Calculator. For meetings across countries, use the Time Zone Converter.

Keep the research moving with Date Duration Calculator, Time & Hours Calculator, Time Zone Converter, and Online Countdown.

Frequently Asked Questions

Use the date field or the Today button to see today in ISO, US, Monday-start, and fiscal week systems. The highlighted result uses the week system you selected.

An ISO week number follows ISO 8601: weeks start on Monday, and week 1 is the week containing January 4, also described as the week containing the first Thursday of the year.

Dates near January 1 can belong to the last ISO week of the previous ISO year, and dates near December 31 can belong to week 1 of the next ISO year. That happens because ISO weeks are anchored to Mondays and the first Thursday rule.

The US-style setting in this tool starts weeks on Sunday and treats the week containing January 1 as week 1. ISO starts on Monday and uses the first Thursday rule, so labels can differ near New Year.

Yes. Choose Fiscal mode, pick the fiscal year start month and day, and choose the fiscal week start day. The tool labels fiscal years by the year in which the fiscal year ends.

Yes. Use the Date From Week Number section. Enter a week-year, week number, and weekday to get the exact calendar date under the selected week system.

Yes. The Week Number Calendar shows a month grid with a week-number rail on the left. Click any date to update the lookup and comparison panels.

Yes. The full-year week planner exports a CSV with week labels, start dates, end dates, month spans, quarter spans, and target-year day counts.

Yes. Download the ICS file to create an all-day calendar event for the selected week range. Calendar apps can then display the selected week in your normal calendar.

No. Most week systems have 52 weeks in many years and 53 weeks in some years. The tool calculates the week count for the selected week-year and clamps impossible week numbers in the reverse lookup.

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Sources & References

  1. 1.ISO - ISO 8601 date and time format(Accessed July 2026)
  2. 2.Unicode CLDR - Week Data(Accessed July 2026)
  3. 3.NIST Time and Frequency Division(Accessed July 2026)
  4. 4.MDN Web Docs - Date(Accessed July 2026)
  5. 5.MDN Web Docs - Intl.DateTimeFormat(Accessed July 2026)