Week Number Calculator

About this tool

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 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. Pick any date — it defaults to today — and click Calculate to see its ISO week number, the ISO week-year it belongs to (which can differ from the calendar year), and the combined code in YYYY-Www format, such as 2026-W19.

Why use it

• 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.

• Week-Year Shown Separately: 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 shows the ISO week-year explicitly so you never confuse it with the calendar year.

• Ready-made Week Code: The result includes the YYYY-Www code with a zero-padded week number, the format used by ISO 8601 and HTML week inputs.

• Week 53 Revealed: Dates in the extra week of a 53-week year, such as late December 2026, return W53, so you can spot the additional planning period.

• 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.

How to use

1. Pick a date with the Date picker — it starts on today
2. Click Calculate
3. Read the ISO Week number and the ISO Week-Year
4. Use the ISO format line (for example 2026-W19) in sprint boards, reports or invites

When it helps

• When a colleague asks which week number a date falls in for a sprint or release plan
• When a financial report uses ISO week notation and you need to label a date with the right week
• When checking whether a late-December or early-January date belongs to the previous or next ISO year
• When a logistics or manufacturing system schedules by week number and you need the week for a delivery date
• When writing international meeting invites that use YYYY-Www notation to avoid date-format ambiguity

Examples

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

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.