Week Number Calculator
The ISO 8601 week number and week-year of any date, the Monday-to-Sunday dates of any ISO week, and the US-style week number too.
Result…
How it's calculated
ISO 8601: weeks run Monday to Sunday; week 1 is the week holding the year's first Thursday (equally, 4 January) Week-year = year of the week's Thursday · week = ⌊(day of year of that Thursday − 1) ÷ 7⌋ + 1 US style: week = ⌊(day of year − 1 + weekday of 1 January, Sunday = 0) ÷ 7⌋ + 1
ISO 8601 numbers weeks from Monday to Sunday, and week 1 is the week with the year's first Thursday in it, which is always the week holding 4 January. So a year has 52 or 53 ISO weeks, and 29 to 31 December can sit in week 1 of the next year while 1 to 3 January can sit in week 52 or 53 of the year before: that year is the ISO week-year. The US-style number (Excel WEEKNUM's default, system 1) starts weeks on Sunday and counts the week holding 1 January as week 1, so its last week can be week 53 or even 54 and is often only a few days long. Every date uses the Gregorian calendar. For the weekday of a date, use the day of the week calculator.
Your numbers, step by step
About the Week Number calculator
This week number calculator gives the ISO 8601 week of any date, the week-year it belongs to and the Monday-to-Sunday dates of that week. Turn it around to find the dates of a week number: enter a week-year and a week, and it lists all seven days from Monday to Sunday. Offices, factories, schools and planners across much of the world schedule by week number, and the first and last days of a year are where mistakes happen, because ISO week 1 is the week with the year's first Thursday. So 1 January can fall in week 52 or 53 of the year before. The calculator also shows the US-style number, with weeks from Sunday and week 1 holding 1 January, which is what many spreadsheets give by default. Every step is shown, from finding the week's Thursday to counting the weeks.
How to use it
- Choose “Week of a date” to number a date, or “Dates of a week” to list the days of a week number.
- For a date, enter it in “Date”; it starts at today.
- For a week, enter the “Week-year” and the “Week number” (1 to 52, or 53 in a long year).
- Read the ISO week, the week-year and the Monday-to-Sunday dates, with the US-style number below them.
Worked example
1 January 2027 is a Friday, ISO weekday 5. The Thursday of its week is 31 December 2026, so the week belongs to 2026: it is week 53, written 2026-W53-5, and runs from Monday 28 December 2026 to Sunday 3 January 2027. ISO year 2026 has 53 weeks. In the US style, 1 January is always in week 1, so the same date is week 1 of 2027.
Updated
Questions people ask
How is the ISO week number worked out?
Find the Thursday of the date's Monday-to-Sunday week. That Thursday's year is the week-year, and the week number is ⌊(day of the year of that Thursday − 1) ÷ 7⌋ + 1. Week 1 is always the week that holds 4 January.
Why does a year sometimes have 53 weeks?
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. 2026 and 2020 are both 53-week years; 2027 has 52.
Why do Excel and my diary show different week numbers?
Excel's WEEKNUM starts weeks on Sunday and counts the week holding 1 January as week 1 unless you choose return type 21, the ISO system, as Microsoft's WEEKNUM page explains. Many European diaries use ISO weeks, so the two often differ by one.
What dates are in week 1 of 2026?
ISO week 1 of 2026 runs from Monday 29 December 2025 to Sunday 4 January 2026, because its Thursday is 1 January 2026. Choose “Dates of a week” to list any other week.
Which week numbering should I use?
Use the one your workplace, school or supplier uses, and say which when you share a week number. For the weekday of a single date, try the day of the week calculator, and to add or count days use the date calculator.