MasterTool

Week Number Calculator

Find the ISO-8601 week number and week-year of any date, plus the Monday and Sunday that bound its week.

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What this tool does

Find the ISO-8601 week number and week-year of any date, plus the Monday and Sunday that bound its week. Follows ISO-8601 exactly: early-January dates can belong to week 52 or 53 of the previous week-year, and late-December dates can be week 1 of the next. A year has 53 ISO weeks when it starts on a Thursday (or on a Wednesday in a leap year).

How to use the Week Number Calculator

  1. Enter or select date (yyyy-mm-dd).
  2. Read the calculated result; change any measurement to compare alternatives.

Formula

ISO-8601: weeks start on Monday; week 1 is the week containing the year's first Thursday (equivalently, January 4). A date's week-year is the Gregorian year of the Thursday of its week
date
Date (YYYY-MM-DD)

Follows ISO-8601 exactly: early-January dates can belong to week 52 or 53 of the previous week-year, and late-December dates can be week 1 of the next. A year has 53 ISO weeks when it starts on a Thursday (or on a Wednesday in a leap year).

Worked example

For week number calculator, the following measurements illustrate the exact method: Date (YYYY-MM-DD): 2026-01-01.

Inputs

  • Date (YYYY-MM-DD)2026-01-01

Result

  • ISO week number1
  • ISO week-year2,026
  • ISO week date2026-W01-4
  • WeekdayThursday
  • Week starts (Monday)2025-12-29
  • Week ends (Sunday)2026-01-04

Results explained

ISO week number
ISO week number from the formula above. Follows ISO-8601 exactly: early-January dates can belong to week 52 or 53 of the previous week-year, and late-December dates can be week 1 of the next. A year has 53 ISO weeks when it starts on a Thursday (or on a Wednesday in a leap year).
ISO week-year
ISO week-year from the formula above. Follows ISO-8601 exactly: early-January dates can belong to week 52 or 53 of the previous week-year, and late-December dates can be week 1 of the next. A year has 53 ISO weeks when it starts on a Thursday (or on a Wednesday in a leap year).
ISO week date
ISO week date from the formula above. Follows ISO-8601 exactly: early-January dates can belong to week 52 or 53 of the previous week-year, and late-December dates can be week 1 of the next. A year has 53 ISO weeks when it starts on a Thursday (or on a Wednesday in a leap year).
Weekday
Weekday from the formula above. Follows ISO-8601 exactly: early-January dates can belong to week 52 or 53 of the previous week-year, and late-December dates can be week 1 of the next. A year has 53 ISO weeks when it starts on a Thursday (or on a Wednesday in a leap year).
Week starts (Monday)
Week starts (Monday) from the formula above. Follows ISO-8601 exactly: early-January dates can belong to week 52 or 53 of the previous week-year, and late-December dates can be week 1 of the next. A year has 53 ISO weeks when it starts on a Thursday (or on a Wednesday in a leap year).
Week ends (Sunday)
Week ends (Sunday) from the formula above. Follows ISO-8601 exactly: early-January dates can belong to week 52 or 53 of the previous week-year, and late-December dates can be week 1 of the next. A year has 53 ISO weeks when it starts on a Thursday (or on a Wednesday in a leap year).

Frequently asked questions

Enter the date and read the ISO week number — for example 2026-01-01 is in week 1 of 2026, while 2027-01-01 belongs to week 53 of week-year 2026, not week 1 of 2027.

ISO weeks run Monday to Sunday and week 1 must contain January 4 (the first Thursday). If January 1 falls on a Friday, Saturday or Sunday, it still belongs to the last week of the previous week-year.

Years that start on a Thursday — like 2026 — and leap years that start on a Wednesday. Most years have 52; roughly 71 of every 400 years have 53.

Not always. US conventions often start weeks on Sunday and call the week containing January 1 week 1, which can differ by one from ISO. This tool always uses ISO-8601, the standard for business and logistics in most of the world.

It is written YYYY-Www-D: week-year, week number, then weekday 1 (Monday) to 7 (Sunday). The output shows all three parts for your date.