Date calculators · Calendar lookups

ISO Week Number Calculator

Find a date's ISO week—or turn a week number back into a date. Keep the week-year clear around New Year.

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How to use this calculator

Choose date to week and enter a calendar date. For a reverse lookup, enter an ISO week-year, week number and weekday. The result gives the ISO label, corresponding calendar date and Monday–Sunday range.

ISO weeks start on Monday. Week 1 contains January 4 and the first Thursday. A week-year has 52 or 53 weeks, and some early-January or late-December dates belong to a neighboring week-year. Invalid week 53 inputs are rejected. A week endpoint beyond years 1–9999 is labeled outside the supported range.

A worked example

January 1, 2027 is 2026-W53-5: Friday in week 53 of ISO year 2026. Its week runs from December 28, 2026 through January 3, 2027.

Does this use US Sunday-based or fiscal week numbers?

No. The result is explicitly ISO, with Monday first. Fiscal calendars and other week-number conventions can assign different numbers; do not substitute this output for an employer's fiscal week without checking its rules.

Which calendar and years are supported?

These tools use Gregorian calendar dates for years 1–9999, without historical calendar-switch dates. They count dates, not elapsed hours. Today buttons use the current date in your browser's time zone.

What happens to my dates?

Calculations run on your device. Saving a calculation is optional and keeps it in this browser. A setup link contains the dates and settings you entered; anyone with the link can read them. Use one tab per saved calculator. Clear saved data from Privacy & settings.