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Miraç Özdemir
Date & Time

Day of Year and ISO Week Number Calculator

Find a date's day of the year, ISO 8601 week number and week-year, its quarter, the days in its month and the days left in the year.

Version 1

Things to keep in mind:

  • Some software starts weeks on Sunday or always counts 1 January as week 1; their week numbers can differ from ISO numbers.
  • The week-year is shown as a number; the interface may add a thousands separator.

Calculator

Date

The date whose week number and day of year you want. For today, pick today's date.

After the first calculation, the result updates automatically when you change a value.

Result

Enter the values and press Calculate to see the result here.

About this tool

For any date this tool returns the day of the year, the ISO 8601 week number with its week-numbering year, the Monday and Sunday of that week, the quarter, the number of days in the month and year, and the days left until year end — useful for project plans, production and shipping weeks, payroll periods and reporting.

Formulas

  • Day of year = (date − 1 January) + 1
  • Thursday rule: thursday = date + (4 − weekday), weekday 1 = Monday … 7 = Sunday
  • ISO week-year = year of that Thursday
  • ISO week = ⌊(thursday − 1 January of the week-year) / 7⌋ + 1
  • Quarter = ⌊(month − 1) / 3⌋ + 1
  • Days left = days in year − day of year

In ISO 8601 weeks start on Monday and week 1 is the week containing the year's first Thursday. Days around New Year can therefore belong to the previous or next year's week: Sunday 3 January 2021 is week 53 of 2020, and 30 December 2024 is week 1 of 2025.

Example

3 October 2026 (Saturday): day 276, week 40 (28 September – 4 October 2026), quarter 4; October has 31 days and 89 days remain in the year.

How to use

  1. Pick the date (today's date for today).
  2. Click Calculate.
  3. Read the week number together with its week-year (e.g. 2020-W53).

Notes

Timezone-free Gregorian calendar (1583 onwards). Week numbers follow ISO 8601; US-style (Sunday-based) week numbers may differ. Days left in the year exclude the chosen day.

Frequently asked questions

3 questions

  1. In ISO 8601 weeks start on Monday and week 1 is the week with the year's first Thursday. A year has 52 or 53 weeks.

Next step

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