ISO reporting
Use this page for Monday-based ISO week dates shared across systems that explicitly require ISO 8601.
One standard, all output fields visible
Find ISO week-year, week, weekday, and bounds
Enter one valid Gregorian date as YYYY-MM-DD. The output identifies the complete ISO week date and the Monday and Sunday that bound the same reporting week.
Loading the ISO week number calculator…
Enable JavaScript to enter a YYYY-MM-DD date. The ISO week-one rule, year-edge examples, week-53 behavior, and scope remain readable below.
The first Thursday anchors the week-year
ISO week 1 contains January 4
ISO 8601 weeks begin on Monday and end on Sunday. Week 1 is the week that contains January 4. The same rule can be stated as the first week with a Thursday in the new calendar year, or the week containing a majority of its days in that year. These equivalent definitions prevent a partial one- or two-day fragment from becoming week 1.
An ISO week date has three components: week-year, week number, and weekday. A complete representation such as 2020-W53-5 says Friday, weekday 5, in week 53 of ISO week-year 2020. Writing only “week 53” loses the year and can make data ambiguous.
The input remains a Gregorian calendar date in YYYY-MM-DD form. Conversion changes its labeling within a weekly system; it does not change the underlying date, locale, time zone, or elapsed duration.
- Week-year
- The ISO year that owns the containing week.
- Week
- Two-digit number from 01 through 52 or 53.
- Weekday
- Monday 1 through Sunday 7.
- Bounds
- Monday and Sunday Gregorian dates for that week.
Test the dates where assumptions fail
Late December and early January can belong next door
Gregorian dateISO identityReason
2018-12-312019-W01-1Its Monday-to-Sunday week contains January 4, 2019.
2021-01-012020-W53-5The Friday belongs to the final ISO week of 2020.
2021-01-042021-W01-1This Monday begins the week containing January 4.
2026-12-31Derived, not assumedWeek 53 exists only when the year's weekday arrangement allows it.
Use boundary fixtures when implementing weekly reports. A common defect concatenates the Gregorian year with a separately computed week number, producing an impossible pair around New Year. Store or transmit the ISO week-year returned by the same calculation as the week number.
ISO is precise, not universal
Fiscal, locale, and payroll weeks need their own definitions
Locale display
A locale may start its visible week on Sunday or Saturday and may define its first week differently. Do not label that result ISO.
Fiscal schedule
Retail 4-4-5 and custom fiscal calendars require a fiscal-year anchor and leap-week policy not accepted here.
Date movement
Use the Date Calculator to add seven-day weeks; use Days Between Dates to count date boundaries.
ISO week answers
Week number calculator FAQ
What is ISO week 1?
ISO week 1 is the Monday-to-Sunday week containing January 4, equivalently the first week containing a Thursday. It can begin in the previous Gregorian calendar year.
Why can the ISO week-year differ from the calendar year?
The first and last few dates of a Gregorian year can belong to a week whose Thursday lies in the neighboring year. The ISO week-year follows that week assignment, not merely the date's year field.
Which day starts an ISO week?
Monday starts the ISO week and is weekday 1. Tuesday through Sunday are 2 through 7. This differs from displays or APIs that put Sunday first.
Can an ISO year have week 53?
Yes. An ISO week-year has 53 weeks when its calendar arrangement supplies a fifty-third Thursday-based week. The calculator derives the result instead of assuming every year ends at week 52.
Is this the same as a locale or fiscal week number?
No. This page implements the ISO 8601 Monday-based week-date system. Locale calendars, retail 4-4-5 calendars, payroll periods, and organization-specific fiscal weeks can use different starts and numbering.