ISO Week Number Calculator

What Is an ISO Week Number?

An ISO week number is the week of the year defined by the ISO 8601 standard. ISO weeks start on Monday, and Week 1 is the first week of the year that contains at least four days in January (equivalently: the week that contains the first Thursday of the year).

How the ISO Week Number Calculator Works

This calculator applies the ISO 8601 rules to your selected date and returns: the ISO week number, the ISO week-year (which can differ from the calendar year), and the Monday–Sunday date range for that ISO week.

ISO 8601 Week Rules (in plain English)

• Weeks start on Monday and end on Sunday.
• Week 1 is the first week with 4+ days in January.
• The ISO week-year is the year that the week belongs to—so early January can be week 52/53 of the previous ISO year, and late December can be week 1 of the next ISO year.

Examples (why week-year can differ)

ISO weeks are designed so weeks don’t get split across years too awkwardly. That’s why you sometimes see Week 1 beginning in late December, or Week 52/53 continuing into early January.

Can a year have 53 ISO weeks?

Yes. Some ISO week-years have 53 weeks. A common way to remember it: an ISO year has 53 weeks if it starts on Thursday (or if it’s a leap year that starts on Wednesday). In practice, the calculator handles this automatically.

Why Week Numbers Matter

• Project management: Track deliverables using Week 32, Week 45, etc.
• Business reporting: Compare performance by week across months and quarters.
• Scheduling: Coordinate tasks and meetings using a shared week numbering system.
• Logistics: Plan shipments and production cycles by ISO weeks.

Developer snippets (ISO week in code)

Many environments have ISO week helpers. Below are common approaches (useful for dashboards, BI, and reporting).

// Returns { year, week } for ISO 8601
function isoWeek(date) {
  const d = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()));
  const day = d.getUTCDay() || 7;        // Mon=1..Sun=7
  d.setUTCDate(d.getUTCDate() + 4 - day); // shift to Thursday
  const year = d.getUTCFullYear();
  const yearStart = new Date(Date.UTC(year, 0, 1));
  const week = Math.ceil((((d - yearStart) / 86400000) + 1) / 7);
  return { year, week };
}

isoWeek(new Date("2026-02-20T00:00:00"));

import datetime as dt
d = dt.date(2026, 2, 20)
iso_year, iso_week, iso_weekday = d.isocalendar()
print(iso_year, iso_week)  # e.g. 2026 8

-- ISO week number (01–53)
SELECT to_char(date '2026-02-20', 'IW') AS iso_week;

-- ISO week-year
SELECT to_char(date '2026-02-20', 'IYYY') AS iso_week_year;

=ISOWEEKNUM(DATE(2026,2,20))

Troubleshooting (common confusion)

Why does my “week number” differ from another website?

Many tools show a non-ISO week number (often weeks starting on Sunday, or “Week 1 starts on Jan 1”). This page uses ISO 8601 rules: weeks start on Monday and Week 1 is the first week with 4+ days in January.

Why is the ISO week-year not the same as the calendar year?

Because ISO week numbering is week-based. A date in early January can still belong to the last ISO week of the previous year, and late December can belong to Week 1 of the next ISO year. The “ISO week-year” output clarifies this.

Does time zone affect ISO week number?

For a pure calendar date (YYYY-MM-DD), time zone usually doesn’t matter. It can matter if your “date” comes from a timestamp near midnight UTC (it might be a different local date). If your source data is timestamps, consider converting to a local date first (see the Time Zone Converter).

Frequently Asked Questions

What day does an ISO week start on?

ISO weeks start on Monday.

Why does the ISO week year differ from the calendar year?

ISO week numbering is based on weeks. Dates at the start or end of a calendar year can belong to the previous or next ISO week-year depending on ISO 8601 rules.

Can a year have 53 ISO weeks?

Yes. Some years contain Week 53 depending on how the calendar aligns.

Is this ISO week number calculator free?

Yes. The tool is completely free and runs directly in your browser.