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ISO Week Calculator

Compute ISO-8601 week numbers and week-years.

About ISO Week Calculator

ISO 8601 week numbers are the standard way to refer to a specific week in a year across Europe, finance, and international project management. A week number such as 'W19' conveys a precise seven-day span without ambiguity — but only if everyone agrees on the numbering rules. ISO 8601 defines week 1 as the week containing the year's first Thursday, weeks always start on Monday, and the numbering runs from W01 to W52 or W53 depending on the year. These rules create a subtle but important edge case at year boundaries: dates in late December can belong to week 1 of the following year, and dates in early January can belong to the last week of the previous year. Most calendar software and date libraries implement these rules correctly, but verifying them manually is tedious. This ISO week calculator handles both directions of the conversion.

Why use ISO Week Calculator

ISO 8601 Compliant Calculation

The tool applies the official ISO 8601 rules: weeks start on Monday, and week 1 is the week containing the first Thursday of the year. This matches what European calendars, financial systems, and project-management tools use.

Date to Week Number

Enter any calendar date and instantly see its ISO week number, the week-year it belongs to, and the full Monday–Sunday span of that week. Useful when a meeting note says 'W22' and you need to know the actual dates.

Week Code to Date Range

Enter a YYYY-Www code and get back the precise Monday and Sunday dates that bracket that week. Essential for populating planning boards, writing sprint summaries, or aligning a week number with a delivery calendar.

Year-boundary Edge Case Handling

Late December dates sometimes belong to week 1 of the following year, and early January dates can fall in the last week of the prior year. The tool surfaces the week-year explicitly so you never confuse calendar year with ISO week-year.

53-week Year Detection

Some years have 53 ISO weeks rather than 52, creating an extra sprint or fiscal period. The tool flags this so sprint planners and fiscal teams can account for the extra week in their roadmaps and budgets.

Browser-side Privacy

All ISO week arithmetic runs locally in your browser. No date is sent to a server, no account is required, and results appear instantly without any network latency.

How to use ISO Week Calculator

  1. Choose a mode: Date to Week (enter a date, get the ISO week number) or Week to Dates (enter a week code, get the date range)
  2. For Date to Week: enter the date using the date picker or type it in YYYY-MM-DD format
  3. For Week to Dates: type the week in YYYY-Www format (e.g., 2026-W19)
  4. Read the ISO week number and week-year alongside the Monday-to-Sunday date span
  5. Check the 53-week indicator to know if the selected year has an extra week
  6. Copy the week code or date range for use in sprint boards, fiscal reports, or calendar invites

When to use ISO Week Calculator

  • When a project ticket or sprint board references a week number and you need the actual calendar dates
  • When a financial report uses ISO week notation and you need to map it to a specific Monday–Sunday period
  • When scheduling recurring meetings aligned to ISO weeks and you want to confirm the exact dates
  • When planning sprint cycles that span a year boundary and you need to know if the year has 52 or 53 weeks
  • When a logistics or manufacturing system schedules production by week number and you need to translate to dates
  • When writing international meeting invites that use YYYY-Www notation to avoid date-format ambiguity

Examples

Date to ISO week

Input: Date: 2026-05-07

Output: ISO Week 2026-W19 · Mon 2026-05-04 to Sun 2026-05-10

Late-December edge case

Input: Date: 2024-12-30

Output: ISO Week 2025-W01 (week-year is 2025, not 2024)

Week code to date range

Input: Week: 2026-W01

Output: Mon 2025-12-29 to Sun 2026-01-04

Tips

  • ISO weeks always start on Monday — if a tool shows week starting on Sunday, it is not ISO 8601
  • Watch for week-year drift in late December and early January: December 30 can be in W01 of the following year
  • 2026, 2032, and 2037 are 53-week years — sprint planners should budget for an extra planning period
  • The week-date format YYYY-Www-D is unambiguous in international emails and avoids the MM/DD vs DD/MM confusion
  • If a system shows 'week 0' or 'week 53' unexpectedly in January, it is likely using a non-ISO numbering convention

Frequently Asked Questions

What is the ISO 8601 week numbering rule?▾
ISO 8601 defines week 1 as the week containing the first Thursday of the year. Weeks always start on Monday and end on Sunday. This means that the first week of the year may start in late December of the previous calendar year.
Why does December 31 sometimes belong to week 1 of next year?▾
If December 31 falls on a Monday, Tuesday, or Wednesday, the Thursday of its week is in the following January, making that week ISO week 1 of the new year. The date itself is in December but its ISO week-year is the next calendar year.
How can a year have 53 weeks?▾
Most years have 52 ISO weeks. A year has 53 weeks when January 1 falls on a Thursday (giving that week a Thursday in the new year and making it W01), or when it falls on a Wednesday in a leap year. Years 2026, 2032, and 2037 are examples.
What is the difference between ISO weeks and US calendar weeks?▾
US calendars traditionally start weeks on Sunday rather than Monday, and the first week of the year is defined differently — often as the week containing January 1. This means US week numbers can differ from ISO week numbers by one, especially in early January.
Why do ISO weeks start on Monday?▾
ISO 8601 chose Monday as the start of the week to reflect the European and international business convention. The Thursday-based week-1 rule was chosen to ensure that week 1 always contains at least four days of the new year.
What is the ISO week-date format?▾
The full ISO week-date format is YYYY-Www-D where YYYY is the week-year, ww is the two-digit week number, and D is the day of the week (1=Monday through 7=Sunday). For example, 2026-W19-4 means Thursday of week 19 in 2026.
Are week numbers consistent across all software?▾
No. Different software uses different week-numbering conventions — ISO (Monday start, Thursday-rule), US (Sunday start, Jan 1 rule), and others. Always confirm which standard a system uses before comparing week numbers across tools.
How do I know if my fiscal year uses ISO weeks?▾
Check whether your fiscal calendar aligns sprint or reporting periods to Monday–Sunday boundaries and whether week 1 starts on the Monday of the week containing the first Thursday of January. Many European multinationals and SAP-based ERP systems use ISO weeks by default.

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Glossary

ISO 8601
The international standard for representing dates and times. It specifies week numbering rules, date formats (YYYY-MM-DD), time formats, and durations to eliminate ambiguity in cross-border data exchange.
ISO Week Number
A two-digit number (01–53) identifying a specific week within an ISO week-year. Week 1 contains the year's first Thursday. The number is part of the ISO week-date format YYYY-Www-D.
Week-year
The year component of an ISO week date, which can differ from the calendar year for dates near the start and end of January. For example, 2024-12-30 belongs to ISO week-year 2025.
Week-date Format (YYYY-Www-D)
The full ISO 8601 week-date notation identifying a specific day: YYYY is the week-year, Www is the week number with a W prefix, and D is the weekday number (1=Monday, 7=Sunday).
First Thursday Rule
The ISO 8601 rule that defines week 1 as the week containing the first Thursday of the calendar year. This ensures week 1 always contains at least four days of the new year and that no week is split across two week-years.
53-week Year
An ISO year that contains 53 weeks rather than the usual 52. This occurs when January 1 is a Thursday (or a Wednesday in a leap year). The extra week must be accounted for in annual sprint and fiscal planning.