Skip to content
Lizely

Day of Year Calculator

See what day number of the year any date is, plus ISO week.

Privacy: your files never leave your device. All processing happens locally in your browser.

How to use

  1. 1.Pick any date in the date field β€” it defaults to today.
  2. 2.Read the day number of the year and the days remaining, updated instantly with no button to press.
  3. 3.Check the ISO 8601 week number, the total days in the year, and whether it is a leap year below the day count.

About Day of Year Calculator

The day of the year is simply the ordinal position of a date within its year: January 1 is day 1, and December 31 is day 365 in a common year or day 366 in a leap year. This day of year calculator takes any date and shows four things instantly: its day number, the number of days remaining in the year, the ISO 8601 week number, and the total days in that year. The result updates the moment you pick a date, with no button to press.

The day number is often called the ordinal date or, loosely, the Julian date in logistics, aviation, and manufacturing, where a stamp like 2026-187 (the ISO 8601 ordinal date for July 6, 2026) encodes the year and the day of the year together. Note this is the ordinal day of the year, not the astronomical Julian Day Number used by astronomers. The tool shows both the plain day number and the padded ISO ordinal date so you can copy whichever your workflow expects.

The ISO 8601 week number is where most calculators get it wrong, so this one follows the standard exactly. In ISO 8601 the week starts on Monday, and week 1 is the week that contains the year's first Thursday (equivalently, the week containing January 4). Because of that rule, the first days of January can belong to the last week (52 or 53) of the previous year, and the last days of December can belong to week 1 of the next year. For example, January 1, 2021 falls in week 53 of 2020, while December 30 and December 31, 2024 both fall in week 1 of 2025. The tool also shows the full ISO week date, such as 2026-W28-1, including the ISO week-numbering year so cross-boundary dates read correctly.

Leap years are handled with the full Gregorian rule: a year is a leap year if it is divisible by 4, except for century years, which must be divisible by 400. That is why 2000 was a leap year but 1900 and 2100 are not. The calculator uses pure arithmetic for this, so the total days in the year and the December 31 day number are always right, and the days remaining is just the total days minus the day number.

Common uses include: reading or writing Julian-style date codes on labels and batch stamps, filling in the day of year field in spreadsheets and scientific data, planning against ISO week numbers used in European business, payroll, and manufacturing schedules, counting how many days are left in the year for goals and deadlines, and quickly checking whether a given year is a leap year. Everything runs in your browser, so the date you enter never leaves your device and nothing is uploaded or stored. There is no signup and no limit on how many dates you check. To count the days between two dates instead, use a date difference calculator; to find the weekday of a date, use a day of the week calculator; and to work out an age or anniversary from a birth date, use an age calculator.

Frequently asked questions

What day of the year is it today?
The date field defaults to today, so the tool immediately shows today's day number out of 365 (or 366 in a leap year), for example day 187 of 365. The count starts at 1 on January 1 and updates the instant you change the date.
How is the day number of the year calculated?
It is the ordinal position of the date within the year: January 1 is day 1 and each following day adds one. December 31 is day 365 in a common year and day 366 in a leap year. The tool counts the days from January 1 to your date and adds one.
How is the ISO 8601 week number worked out?
ISO 8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday (the week that includes January 4). Because of this, early January can fall in week 52 or 53 of the previous year and late December can fall in week 1 of the next year. For example, January 1, 2021 is in week 53 of 2020.
Is the day of the year the same as the Julian date?
In business, logistics, and manufacturing the day-of-year number is often called a Julian date, and codes like 2026-187 combine the year with the day of the year. It is not the astronomical Julian Day Number used in astronomy, which counts days from 4713 BC. This tool gives the ordinal day of the year and the ISO ordinal date.
How does the calculator handle leap years?
It uses the full Gregorian rule: a year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. So 2000 was a leap year but 1900 and 2100 are not. In a leap year the total is 366 days and February has 29 days, which the day count reflects automatically.

Related tools

Calculators guides

View all