🗓️ Week Number Calculator

🔧 Operation

🧭 Scheme

📅 Inputs

All four schemes are calculated together. Calendar dates use the proleptic Gregorian calendar.

Returns the total number of weeks (52, 53, or 54 depending on scheme and year).

Enter a year and week number for the selected scheme to get the corresponding calendar dates.

📤 Result

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ISO 8601

{{ isoSummary }}

US

{{ usSummary }}

Simple

{{ simpleSummary }}

MMWR

{{ epiSummary }}

📊 Week Range

Scheme
{{ weekRangeResult.scheme.toUpperCase() }}
Year-week
{{ weekRangeResult.year }}-W{{ weekRangeResult.week }}
Start
{{ weekRangeResult.start }}
End
{{ weekRangeResult.end }}
Day count
{{ weekRangeResult.days.length }}

📊 Weeks in Year

Scheme
{{ weeksInYearResult.scheme.toUpperCase() }}
Year
{{ weeksInYearResult.year }}
Total weeks
{{ weeksInYearResult.weeks }}
⚠️

ℹ️ About Week Number Calculator

A week number identifies a seven-day calendar window, but "week of year" is not one universal rule. This week number calculator compares the ISO 8601 week date, US Sunday-first week numbering, CDC MMWR epidemiological weeks, and a simple Jan 1 based calendar week. It can turn a date into week numbers, convert a week number back to its date range, count how many weeks a year has, and show the exact days in a selected week.

The important detail is the week-numbering year: a date near New Year's Day can belong to week 1 of the next week year or week 52/53 of the previous one. That is why an ISO week number, an MMWR week, and a US week can disagree for the same date even though each answer is internally consistent.

Week numbering schemes compared

Scheme Week starts Week 1 rule Common searches and workflows
ISO 8601 Monday The week containing the first Thursday, equivalently the week containing January 4. ISO week number, week date, ERP exports, analytics, European business calendars.
US Sunday The week containing January 1 is week 1; it can start in late December. US week of year, Sunday calendar weeks, payroll, broadcast-style schedules.
Simple Jan 1 (no weekday anchor) Week 1 is January 1-7; week N starts on day (N - 1) * 7 + 1. Quick day-of-year grouping, simple week range lookup, nonstandard internal reports.
MMWR / Epi Sunday The first Sunday-Saturday week with at least four days in the new year. CDC epi week calculator, MMWR week lookup, public health surveillance reporting.

ISO week dates and 53-week years

In ISO 8601-1:2019, an ISO week starts on Monday and is written as a week date such as 2026-W22-2 for Tuesday of ISO week 22. ISO week 1 is the week that contains January 4, so it always has at least four days in the new calendar year. Some years have 53 ISO weeks: years that begin on Thursday, plus Gregorian leap years that begin on Wednesday. For example, 2020 and 2026 are 53-week ISO years.

RFC 3339 profiles ISO 8601 for internet timestamps. This tool uses date-only values such as 2026-05-26 for input and output; it does not parse full timestamp offsets in the week calculator itself.

US and simple calendar weeks

The US convention here is Sunday-first: week 1 starts on the Sunday on or before January 1. A date in late December can therefore be reported as week 1 of the next US week-numbering year. This is useful when you need a week-of-year value that follows Sunday-start wall calendars rather than Monday-start ISO business calendars.

The simple scheme is deliberately not a cultural calendar rule. It splits the year into Jan 1-7, Jan 8-14, and so on, with a shortened final week when needed. Use it for rough intra-year buckets, not for ISO week dates, retail fiscal calendars, or epidemiological reporting.

CDC MMWR / epidemiological week

The CDC's MMWR week overview defines epidemiological weeks as Sunday-to-Saturday windows. MMWR week 1 is the first week with at least four days in the new year, so it starts on the Sunday on or before January 4. Because of that four-day rule, an MMWR week can differ from both ISO weeks and the simpler US "week containing January 1" convention.

Common week-number tasks

  • Find the week number for a date: compare ISO, US, simple, and MMWR values for the same calendar day.
  • Convert week number to date range: turn 2026-W01 into its start and end dates for the chosen scheme.
  • Check 53-week years: validate reporting periods before building dashboards, sprints, payroll exports, or scheduled jobs.
  • Translate between systems: reconcile ISO week columns from databases with Sunday-first US calendars or CDC epi week reports.
  • Debug boundary dates: inspect late-December and early-January dates where the week-numbering year differs from the calendar year.

Worked examples

Date ISO US Simple MMWR
2026-05-26 (Tue) 2026-W22 2026-W22 2026-W21 2026-W21
2026-01-01 (Thu) 2026-W01 2026-W01 2026-W01 2025-W53
2025-12-29 (Mon) 2026-W01 2026-W01 2025-W52 2025-W53
2020-12-31 (Thu) 2020-W53 2021-W01 2020-W53 2020-W53

For reverse lookup, ISO 2026-W01 spans 2025-12-29 through 2026-01-04, while MMWR 2026-W01 spans 2026-01-04 through 2026-01-10.

Calendar limits and interpretation

  • Supported years are -9999 through 9999; invalid dates such as February 30 are rejected.
  • All arithmetic is date-only and uses UTC internally so daylight saving time and host timezone settings do not move week boundaries.
  • The calculator uses the proleptic Gregorian calendar. It does not model Julian dates, regional calendar adoption dates, lunar calendars, or local business holidays.
  • US week numbering is not the same as a 4-4-5 retail calendar, fiscal year week, or custom first-day-of-week setting in spreadsheet software.
  • MMWR weeks are designed for CDC-style epidemiological reporting. Other countries or agencies may publish different epi-week calendars.

Source references

  • ISO 8601-1:2019 defines calendar dates, week dates, and the week-numbering year used by ISO week calculations.
  • ISO's 8601 overview summarizes the standard date and time notation family.
  • RFC 3339 profiles ISO 8601 for internet date-time exchange and is a useful companion when week dates sit beside API timestamps.
  • CDC MMWR Week Overview documents the Sunday-start, four-day-rule epidemiological week convention.
  • U.S. Naval Observatory leap-year guidance explains the Gregorian leap-year rule behind 52- and 53-week calculations.

API Usage Example:

POST /api/tools/time/week-number
Content-Type: application/json

{
    "operation": "fromDate",
    "year": 2026,
    "month": 5,
    "day": 26
}
Ready