About the Day of the Year Calculator
The day of the year counts the days from 1 January, which is day 1, up to 31 December, which is day 365, or 366 in a leap year. Programs, farms, factories and weather stations use it as a simple date code. This calculator gives the day number for any date, together with the days left, the weekday, the quarter and the ISO week number. It also works in reverse, from a year and a day number to a date. Type a date, or a year and a day number and the day of the year appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the Day of the Year Calculator
- 1Enter a date, or a year and a day number.
- 2Pick the units or options from the menus if the day of the year tool offers them.
- 3Read the day of the year in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the day of the year
Scientists and engineers log data by day of year to avoid month lengths. Gardeners and farmers plan around days from the first frost. Software uses the ordinal date, written like 2026-277, in log files and in some date formats.
The ISO week starts on Monday, and week 1 is the one with the first Thursday of the year. This means that the first days of January can belong to week 52 or 53 of the previous year, and the last days of December to week 1 of the next.
How the day of the year is worked out
Day number = days between 1 January and the date, plus 1. A leap year (divisible by 4, except most century years) has 366 days. 4 October 2026 is day 277 of 365. There are 88 days left in the year, it falls on a Sunday in the fourth quarter and in ISO week 40, and 75.9 percent of the year has gone. Day 60 is 1 March in 2026 but 29 February in 2024.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- Date to number and back
- Weekday, quarter and ISO week
- Days left in the year
Mistakes to avoid with day of the year
Make sure that you use the right year: the day number of 1 March shifts by one in leap years. For day numbers, 366 is only valid in a leap year. The tool works with the Gregorian calendar used today.
On phone and computer
On a phone, type into the boxes one after another and read the day of the year underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the day of the year move. The day of the year calculator works the same on both. Dates have many views. Use Days Between Dates for gaps, Week Number Calculator for ISO weeks, Date Calculator for adding or removing days, and Leap Year Calculator to check February.
Features
- Leap years shift day numbers
- ISO weeks start on Monday
- Day 366 needs a leap year
- Clear messages when the day of the year inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate day of the year?
Day number = days between 1 January and the date, plus 1. A leap year (divisible by 4, except most century years) has 366 days. 4 October 2026 is day 277 of 365. There are 88 days left in the year, it falls on a Sunday in the fourth quarter and in ISO week 40, and 75.9 percent of the year has gone. Day 60 is 1 March in 2026 but 29 February in 2024.
What is the formula for day of the year?
Day number = days between 1 January and the date, plus 1. A leap year (divisible by 4, except most century years) has 366 days. The day of the year calculator applies it exactly and rounds only the displayed answer to six significant figures.
How many days are in a year?
365, or 366 in a leap year.
What is an ordinal date?
A date written as a year and a day number, like 2026-277.
Why can early January be in week 52?
The ISO calendar starts week 1 with the first Thursday, so some January days belong to the last week of the previous year.
Is this day of the year calculator free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.