Week Number Calculator

Find the ISO week number, day of year, and quarter for any date.

By Konstantin Iakovlev · Updated April 2026 · Source: TimeAndDate

ISO Week

Week 30

Day of Year

201

Quarter

Q3

Date Details

ISO Week Number30
Day of Year201 / 365
Days Remaining164
QuarterQ3
Leap YearNo

Use the Week Number Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.

Disclaimer: This calculator is for informational purposes only and does not constitute tax, financial, or legal advice. Results are estimates based on the information you provide and current rates. Always consult a qualified tax professional or financial advisor for advice specific to your situation.

How It Works

Enter any date and you get back three things at once: its ISO week number, its position in the year counted as a day-of-year, and the fiscal quarter it lands in. Teams lean on these figures for project timelines, financial reporting, and scheduling that has to line up across borders. January 1, 2026, for example, sits in ISO Week 1, which lets annual plans snap into place from day one.

Week numbering here follows ISO 8601. Under that standard, Week 1 is the first week containing at least four days of the new year, and every week starts on a Monday. The day of the year is just a running count from January 1st onward. Quarters split the calendar into four three-month blocks: Q1 opens January 1st, Q2 April 1st, Q3 July 1st, and Q4 October 1st.

Not every region numbers weeks the way ISO 8601 does, so check which standard your application actually expects before relying on the output. It also helps to drop the assumption that January 1st always belongs to Week 1. When January 1st falls on a Friday, Saturday, or Sunday, ISO 8601 assigns it to the final week of the previous year. Fiscal calendars that don't match the standard quarters are worth verifying against your own starting date.

Example: Tracking a Project Deadline in 2026

  1. 1 Specific Input: A project deadline is set for March 15, 2026.
  2. 2 Calculation: Using the calculator, March 15, 2026, is identified as ISO Week 11. It's also the 74th day of the year and falls within Q1.
  3. 3 Result: The project deadline is in ISO Week 11, Day 74, of Q1 2026.
  4. 4 Context: This information helps the project manager communicate progress using standardized metrics. If the project's budget is reviewed quarterly, knowing it falls in Q1 is essential for financial reporting, and the ISO week number aids in international team coordination.

Source: TimeAndDate · Last updated: April 2026

Frequently Asked Questions

What week number is it right now?
The calculator shows the current ISO week number, which counts from the first week of January. ISO weeks run Monday to Sunday, and the first week of the year contains the first Thursday of January.
How are ISO week numbers calculated?
ISO 8601 defines week 1 as the week containing the first Thursday of January. Weeks run Monday to Sunday. A year has either 52 or 53 weeks. Some days in early January may belong to the previous year's last week.
What quarter of the year is a given date in?
Q1 is January-March, Q2 is April-June, Q3 is July-September, Q4 is October-December. The calculator shows the quarter along with the week number and day of year.

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