Day of Year Calculator
Find day number of year
About This Tool
The Day of Year Calculator tells you what day number any date falls on within the year. January 1st is day 1, December 31st is day 365 (or 366 in leap years).
This tool is useful for scheduling, data analysis, scientific calculations, and anyone who needs to work with ordinal dates or day numbers.
How It Works
How to Use This Calculator
Using our day of year calculator is easy:
- Enter a date - Select or type any date you want to convert.
- Click Calculate - Instantly see what day number that date is.
- Reverse lookup - You can also enter a day number and year to find the corresponding date.
The calculator shows the day number, days remaining in the year, and percentage of year completed.
Formula
How Day of Year is Calculated
DOY = Sum of days in all previous months + Current day of month
For example, March 15: January (31 days) + February (28 days) + March 1-15 (15 days) = 74 (or 75 in leap year).
Examples
Examples
Example 1: Common Dates
- January 1: Day 1
- February 14 (Valentine's): Day 45
- July 4 (US Independence): Day 185 (186 in leap year)
- December 25 (Christmas): Day 359 (360 in leap year)
Example 2: Today's Date
- January 27, 2026: Day 27
- Days remaining: 338
- Year progress: 7.4%
Example 3: Reverse Lookup
- Day 100 of 2026 = April 10, 2026
- Day 200 of 2026 = July 19, 2026
Frequently Asked Questions
What is the day of year number?
The day of year (DOY) is a number from 1 to 365 (or 366 in leap years). January 1 is day 1, February 1 is day 32, December 31 is day 365 or 366. The calculator converts any date to its day-of-year number and vice versa.
How do I convert a day number back to a date?
Enter the year and day number. Day 100 of 2025 = April 10. Day 200 = July 19. Day 365 = December 31. In leap years, day 60 = February 29 instead of March 1. The calculator handles leap year adjustments.
Where is the day-of-year format used?
Julian day numbers are used in astronomy, military date-time groups, agriculture (planting schedules), meteorology, and some file naming conventions. The format YYYYDDD (e.g., 2025100) is compact and unambiguous.
How do I calculate the day of year manually?
Sum the days in each completed month plus the current day. For March 15: January(31) + February(28) + 15 = 74. In a leap year: 31 + 29 + 15 = 75. The calculator eliminates this manual counting.
What percentage of the year has passed?
The calculator also shows the year progress: Day 182 of 365 = 49.9% complete. This is useful for tracking annual goals, budget spending rates, and project timelines against the calendar year.
Disclaimer
Disclaimer: This calculator provides results based on standard calendar calculations including leap year handling. Results are for general reference only. All processing happens in your browser.
💡 Tips
Tips
- Day numbers are commonly used in scientific and technical applications
- Remember that leap years have 366 days, affecting dates after February
- Useful for calculating days until year-end goals
- Great for data analysis and file naming conventions
- Some systems use 3-digit day numbers (001-365)