Offices, factories and project plans often talk in week numbers: "delivery in week 42". This calculator gives the ISO week number of any date, which is the international standard used in most of the world, along with the day of the year and the quarter.
How to use it
Pick a date (today by default).
How ISO week numbers work
- Weeks start on Monday.
- Week 1 is the week that contains the year's first Thursday (so it always holds 4 January).
- A year has 52 or 53 weeks. Dates in early January can belong to the last week of the previous year, and dates in late December to week 1 of the next year.
Example
Friday, 25 September 2026 is in week 39 of 2026. It is day 268 of 365, in the third quarter (Q3), with 97 days left in the year. That week runs from Monday 21 to Sunday 27 September.
2026 has 53 ISO weeks, so Friday, 1 January 2027 still belongs to week 53 of 2026.
FAQ
What week is it today?
Open the calculator; it shows today's week by default.
Why does my calendar show a different week number?
Some calendars, often in the US, start weeks on Sunday and call the week containing 1 January "week 1". This calculator uses the ISO standard.
Which quarter is a date in?
January to March is Q1, April to June Q2, July to September Q3 and October to December Q4. India's financial year is different: it runs from April to March.