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Week Number Calculator for ISO Weeks

Find the ISO week, week-year, day of year, quarter, and simple calendar week numbers for any date.

Quick answer

A week number calculator maps a date to its ISO calendar week and week-year.

It helps with sprint planning, reporting, production calendars, and schedule labels.

Best inputs

Use ISO for planning

ISO week numbers are common in business, manufacturing, analytics, and sprint calendars.

Check year boundaries

Early January and late December dates can belong to a different ISO week-year.

Date input

Calculate ISO week number, week-year, day of year, and quarter.

Try a common date

Date

Thursday, May 21, 2026

Week results

ISO weeks start on Monday, and week 1 is the week with the first Thursday.

ISO week

2026-W21

Sunday-start week

Monday-start week

Day of year

141

Quarter

224 days remain in the calendar year.

Example

Choose 2026-05-21 to see its ISO week label, day-of-year number, quarter, and days remaining in the year.

Assumption

ISO week calculations use Monday as the first day of the week and the first Thursday rule.

Limitation

It does not account for fiscal calendars, retail 4-4-5 calendars, local holidays, or custom company week rules.

Suggested workflow

Calendar planning workflow

Find the week label, count working days, then build a deadline countdown.

Step 1

Calculate week number

Find ISO week, day of year, and quarter.

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Step 2

Count business days

Count working days between project dates.

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Step 3

Create countdown

Track time remaining until a milestone.

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Frequently asked questions

What is an ISO week number?

ISO weeks start on Monday, and week 1 is the week containing the first Thursday of the ISO week-year.

Can the ISO week-year differ from the calendar year?

Yes. Dates near January 1 or December 31 can belong to the previous or next ISO week-year.

Does this tool need a backend?

No. Date calculations run locally in the browser.

Does it show day of year?

Yes. It shows day of year, days remaining, and calendar quarter.