Day of Year Calculator
Day of Year Calculator
Find the day number of any date in its calendar year. Enter a date to see its position from day 1 through day 365 or 366.
Calculate the day of the year
Select a date to find its day number.
How the day number works
The day of the year, also called the ordinal date, counts how many calendar days have elapsed from the beginning of the same year.
January 1 is always day 1. The final day is day 365 in a common year and day 366 in a leap year.
The calculator accounts for the selected date's month and day, including February 29 in leap years.
What is a day of year calculator?
A day of year calculator tells you the ordinal number of a date within its calendar year. For example, it identifies whether a selected date is the 32nd, 100th, or 250th day of that year. This is useful when working with schedules, reports, project timelines, scientific observations, agriculture, and date-based records.
How to use this calculator
- Choose the date using the date field.
- Select Calculate to find its day number.
- Read the result to see the date's position in the year.
- Select Reset to clear the date and start again.
Day of year formula
The day number is the total number of days from January 1 up to and including the selected date. In a non-leap year, the month lengths add up to 365 days. In a leap year, February has 29 days and the year has 366 days.
If the selected date is February 1 in a common year, January contributes 31 days. Therefore, February 1 is day 32 of the year (31 + 1).
Common and leap years
A common year contains 365 days. A leap year contains 366 days because February has an extra day. A year is generally a leap year when it is divisible by 4, except century years must also be divisible by 400. This means the day number after February 29 is one higher in a leap year than it would be on the same month and day in a common year.
When is the day of year useful?
- Planning and reporting: identify the exact day of a project, financial, or operational year.
- Data analysis: compare observations recorded on the same ordinal day across different years.
- Agriculture and weather: organize seasonal records and daily measurements.
- Personal planning: track milestones, goals, and annual schedules.
Frequently asked questions
1. What does day of the year mean?
It is the sequential number of a date within its calendar year. January 1 is day 1, and December 31 is day 365 or 366.
2. How do I calculate the day number of a date?
Count all calendar days from January 1 through the selected date, including the selected date itself. This calculator performs that count for you.
3. What is the 100th day of the year?
In a common year, the 100th day is April 10. In a leap year, it is April 9 because February has one additional day.
4. Does the calculator support leap years?
Yes. The day number accounts for February 29 in leap years, so dates after February have the correct ordinal number.
5. How many days are in a year?
A common calendar year has 365 days. A leap year has 366 days.
6. Is January 1 always day 1?
Yes. The day-of-year count restarts at 1 on January 1 every calendar year.
7. What day number is December 31?
December 31 is day 365 in a common year and day 366 in a leap year.
8. Is day of year the same as week number?
No. The day number counts individual days from the start of the year. A week number groups dates into calendar weeks under a week-numbering convention.
9. Why is the day number different in a leap year?
Leap years include February 29. As a result, dates from March 1 onward have a day number one greater than the same month and day in a common year.
10. Can I use the day number to compare dates in different years?
Yes, ordinal day numbers can help compare the same position in different years. Remember that leap years add an extra day after February.