Week Number Calculator

A week number is a simple way to label weeks in a year, like “Week 06” or “Week 42”. Many teams use week numbers for planning, reporting, shipping, payroll, and sprint schedules because it makes timelines easier to compare than using long date ranges.

This calculator turns any date into an ISO week number (the global standard for many businesses) and shows the matching week-year — which can be different from the calendar year near New Year’s. For example, a date in early January might still belong to the last ISO week-year.

If you also work with a Sunday-based calendar, you can switch to the US week number system (Sun–Sat), or enable “Also show the other system” to compare both results side-by-side in stacked cards (never beside inputs).

If you want more tools like this, browse All Calculators or explore our Math Calculators hub for date, number, and planning utilities.

Calculate your week number
Choose a date, select a week system, then calculate.
You can type manually as YYYY-MM-DD or use the picker.
Week starts on: Monday
ISO rule (simple) ISO weeks start on Monday, and Week 1 is the week with the year’s first Thursday (often called the “Jan 4 rule”).

How it works

This Week Number Calculator supports two common systems: ISO-8601 (default) and a practical US week number convention. Both produce an integer week number, but the start day and the “week 1” definition differ. That’s why the same date can show different week labels depending on the system you choose.

ISO week number (Mon–Sun)

  • Weeks start on Monday.
  • Week 1 is the week that contains the year’s first Thursday (also described as “the week containing Jan 4”).
  • Because of that rule, the ISO week-year (WY) can differ from the calendar year for dates near New Year’s.
  1. Let D be your selected date.
  2. Move to the Thursday of D’s week to determine the week-year (WY).
  3. Count weeks from the ISO Week 1 anchor to get the ISO week number (W).
  4. Compute the ISO week range as Monday to Sunday for that ISO week.

US week number (Sun–Sat)

  • Weeks start on Sunday.
  • We define Week 1 as the week that contains January 1 (a common simple convention).
  • The US week-year here is the calendar year of the selected date.
  1. Let D be your selected date and Y be D’s calendar year.
  2. Find the Sunday that begins D’s week (Sun–Sat range).
  3. Find the Sunday that begins the week containing Jan 1 of year Y.
  4. Week number is the number of whole weeks between those Sundays, plus 1.

Variables

DSelected date
YCalendar year
WWeek number
WYWeek-year
DOYDay of year (1–365/366)
StartStart date of the week
EndEnd date of the week

Terminology

  • Week number: The index of the week within a year under a defined system.
  • Week-year: The year assigned to that week (ISO can differ near year boundaries).
  • Week range: The start and end dates that define the week.
  • Day-of-year: The count of the day within the year (Jan 1 = 1).

Common Mistakes

  • Mixing ISO weeks (Mon–Sun) with US weeks (Sun–Sat) and assuming they match.
  • For ISO, forgetting that week-year can differ from the calendar year near New Year’s.
  • Parsing YYYY-MM-DD as UTC and getting a one-day shift due to timezone drift.
  • Assuming every year has exactly 52 weeks (some years have week 53 depending on the system).
  • Comparing results across apps that use different “Week 1” conventions for US-style numbering.

Quick Tips

  • For business reporting across regions, choose ISO to reduce confusion.
  • When tracking payroll or shifts, confirm the week start day used by your organization.
  • For agile sprints, label sprint weeks using the same system your project tools use.
  • When exporting data to spreadsheets, store both date and week-year/week number.
  • Near New Year’s, double-check the week-year before assigning annual totals.

Use cases

Payroll scheduling

Many payroll systems group hours by week. A clear week number helps reconcile pay periods, overtime cutoffs, and approvals without misreading date ranges.

Agile sprint planning

Sprint calendars often reference weeks (e.g., “deliver in Week 14”). ISO weeks are especially common when distributed teams need one shared definition.

Logistics and shipping labels

Warehouses and carriers use week labels for dispatch, arrival windows, and batch routing. Knowing the correct week range prevents mis-shipments.

Academic timetables

Universities and training programs often publish schedules by week number. This tool helps students map dates to the right teaching week quickly.

Manufacturing batch tracking

Production lines tag lots by week to identify when items were produced. Including week-year avoids confusion when week 1 overlaps the prior calendar year.

Worked examples

Example 1: 2026-02-04

Choose 2026-02-04 and keep the default ISO system. The calculator will return an ISO week number and the ISO week-year. It will also show the week range (Mon–Sun) that contains the date, plus the day-of-year and day-of-week.

Example 2: 2026-01-01 (near New Year’s)

Dates near New Year’s are where ISO week-year differences often appear. Enter 2026-01-01 and note the ISO week-year shown. This happens because ISO Week 1 is determined by the first Thursday rule, not strictly by January 1.

Example 3: Compare ISO vs US for the same date

Pick a date like 2025-12-31, then enable Also show the other system. You may see different week numbers or ranges because ISO weeks start Monday while US weeks start Sunday and use a different Week 1 rule.

FAQ

What is an ISO week number?
ISO week numbers follow ISO-8601 rules used widely in business and planning. In ISO, weeks run from Monday to Sunday. ISO Week 1 is defined as the week containing the year’s first Thursday (also described as the week that contains January 4). This makes week numbering consistent across years and prevents short “partial weeks” at the start of January from being treated as Week 1. The result is an ISO week number (1–52/53) and an ISO week-year.
Why can the week-year be different from the calendar year?
The week-year depends on the week numbering system, not just the date’s calendar year. Under ISO rules, a week is assigned to the year that contains that week’s Thursday. Because of this, some dates in early January can still belong to the last ISO week-year, and some dates at the end of December can belong to the next ISO week-year. This is normal and is exactly why ISO outputs include both a week number and a week-year.
Can a year have Week 53?
Yes. Depending on the system, some years have 53 numbered weeks. ISO years have 53 weeks when the calendar layout causes an extra week to fit under the “Week 1 contains the first Thursday” rule. In US-style numbering, the presence of a week 53 depends on how many week boundaries (starting Sundays) occur from the week containing January 1 through the end of the year. This calculator flags when the selected system’s year is a 53-week year.
Why do ISO and US week numbers differ for the same date?
The main differences are (1) which day starts the week, and (2) how Week 1 is defined. ISO weeks start on Monday and Week 1 is the week with the year’s first Thursday. US week numbering commonly starts on Sunday, and many tools define Week 1 as the week containing January 1. Because these rules shift the boundaries, a date can fall into a different week label under each system. That’s why comparing both systems is useful for reporting.
What happens if my date is very close to New Year’s?
New Year’s boundary dates are the most common place to see surprising week-year results. For ISO, the first few days of January can still be part of the last ISO week-year if that week contains a Thursday in the previous year. Likewise, late December dates can sometimes fall into ISO Week 1 of the next week-year. This calculator shows the week range and week-year clearly so you can label reports correctly without guessing.
Does timezone affect a week number?
If a tool parses your date as a timestamp (with time and timezone), you can get an unexpected “one-day shift,” especially around midnight UTC. That shift can change the computed week number. This calculator avoids that problem by parsing the date as year, month, and day, then building a local date object without converting to UTC. Because the input is date-only, the output stays stable across devices as long as the same calendar date is used.
How can I use week numbers in Excel or Google Sheets?
Most spreadsheets provide a week function, but you must choose the correct week system. Some spreadsheet functions can return ISO-style weeks, while others return US-style weeks depending on the “return type” setting. The key is to store both the week number and the week-year (especially for ISO), and to keep the date itself as the source of truth. If your reporting spans multiple years, week-year prevents Week 01 from being incorrectly grouped with the wrong year.
Which week system should I use for business reporting?
If you work with international teams, logistics, or standardized reporting, ISO is often the safest choice because it is widely recognized and consistent. ISO week-year labeling reduces ambiguity near year boundaries. If your organization’s calendars and staffing schedules are Sunday-based and your internal tools label weeks that way, the US system may match your expectations better. When unsure, enable “Also show the other system” to compare outputs and align with your policy.

Trust & Accuracy

  • Runs locally: Calculations run in your browser. No data is sent anywhere.
  • Precision policy: Week number is an integer. Day-of-year is an integer. Dates are displayed in ISO format (YYYY-MM-DD).
  • Privacy-first: Your selected date stays on your device.
Last Updated: February 4, 2026

Sources & References

  • ISO-8601 week date concept (week number + week-year)
  • Common US week numbering conventions (week starts Sunday; Week 1 contains Jan 1)

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