ISO Week Number Calculator

The ISO week number calculator converts any calendar date into its ISO 8601 week number, ISO week-year, weekday name, and full week-date notation such as 2026-W33-1. It applies the standard’s Thursday rule, so the answer is the same everywhere ISO 8601 is used — from ERP week codes and sprint planning to manufacturing calendars and export documents.

What this calculator does

Pick any date, and the calculator reports:

• ISO week number — a number from 1 to 53 identifying the week within the ISO week-year.
• ISO week-year — the year that the week belongs to, which can differ from the calendar year for dates around January 1.
• Weekday — the day’s English name, with Monday as the first day of the ISO week.
• ISO week date — the standard’s compact notation YYYY-Www-D, where D runs from 1 (Monday) to 7 (Sunday).

How the ISO week works

ISO 8601 defines a calendar week as a Monday-to-Sunday block. Week 1 of a year is the week that contains the year’s first Thursday; equivalently, week 1 is the week containing January 4, and January 1 always lands in either week 1 of its own year or the last week of the previous year.

That Thursday rule is what makes ISO weeks consistent: every week has exactly seven days, every year has 52 or 53 weeks, and no week ever straddles two week-years with a partial week.

Because week 1 must contain the first Thursday, dates before the first Thursday belong to the previous week-year. For example, January 1, 2027 is a Friday, so it is in week 53 of ISO week-year 2026, even though the calendar year is 2027.

Formula

For a date d, convert its weekday to ISO numbering with Monday as day 0:

$\text{isoDay} = (\text{getDay}(d) + 6) \bmod 7$

The Thursday of the same ISO week is:

$\text{thursday} = d + (3 - \text{isoDay})$

The ISO week-year is the calendar year of that Thursday. With jan1 as January 1 of the week-year:

$\text{week} = 1 + \left\lfloor \frac{\text{thursday} - \text{jan1}}{7} \right\rfloor$

Worked example

Take August 10, 2026, a Monday. Its ISO day is 0, so the week’s Thursday is:

$2026\text{-}08\text{-}10 + 3 = 2026\text{-}08\text{-}13$

August 13, 2026 is the 225th day of the year:

$31 + 28 + 31 + 30 + 31 + 30 + 31 + 13 = 225$

January 1, 2026 is itself a Thursday, so it anchors week 1. The number of full weeks elapsed by August 13 is:

$\left\lfloor \frac{225 - 1}{7} \right\rfloor = 32$

and the week number is:

$1 + 32 = 33$

Monday is ISO day 1, so the full week date is 2026-W33-1: Monday of week 33 of 2026.

Interpreting the result

The week number is a numbering scheme, not a measure of elapsed time. Week 33 of 2026 starts on Monday, August 10 — the same date that begins any Monday-based week numbering that aligns with ISO 8601. If a partner system prints 2026-W33, the week date notation removes the ambiguity that “week 33” alone can have around year boundaries.

The ISO week-year matters most in December and early January. A payroll or sprint labeled “week 1 of 2027” by a different convention may actually be the last week of 2026 under ISO rules, which can shift reporting periods by a full week.

For a different take on day numbering, the Julian date calculator converts dates into the continuous astronomical day count, and the Unix time converter works with epoch timestamps instead of weeks. If your planning question is about hours and minutes rather than weeks, the time duration calculator measures clock intervals.

Tips

• When a week code must match a partner’s system, confirm they use ISO 8601 week numbering before relying on the result; some industries use their own week conventions.
• Near year end, read the week-year column, not the calendar year. A date of December 31 can already be in week 1 of the next week-year.
• Use the ISO week date output when writing dates in logs or documents: 2026-W33-1 cannot be mistaken for a different week the way a bare weekday can.

Common pitfalls

• Assuming week 1 always contains January 1. Under the Thursday rule, January 1, 2, or 3 can fall in the previous year’s week 52 or 53.
• Mixing calendar years and week-years when aggregating by week; the same calendar date can appear in two different reporting years.
• Expecting the week to start on Sunday; ISO weeks start on Monday.

Sources

• ISO, ISO 8601-1:2019 — Date and time — Representations for information interchange — the standard that defines week numbering and week-date notation.
• Wikipedia, ISO week date — explanation of the Thursday rule and week-year handling around New Year.