How to Calculate the Day of the Year, From 001 to 365 or 366, With a Month Table

The day of the year is a date’s position in its calendar year: Jan. 1 is day 001, and Dec. 31 is day 365 in a common year or day 366 in a leap year. To calculate it, add the days in every completed month to the day of the month. Oct. 30, 2026, for example, is day 303, because January through September hold 273 days and 273 + 30 = 303.

This guide gives the month table for common and leap years, a spreadsheet formula, and the steps to turn a day number back into a date. For printable charts of every day, see our Julian date calendar 2027 and 2026; this story is the method behind them.

What does a day-of-the-year number mean?

It counts days within one year, starting at 001 on Jan. 1. The same idea has other names: ordinal date, day number and, in some industries, “Julian date.”

The numbers are written with three digits. A summary of the International Organization for Standardization (ISO) date standard, ISO 8601, hosted by NASA, shows the ordinal format with the example 1995-035 and describes day numbers as 001 to 365, or 366 in leap years. The U.S. Department of Agriculture (USDA) Agricultural Marketing Service uses the same style on its shell egg chart, where egg cartons show the pack date as a day number from 001 for Jan. 1.

Whether a year has 365 or 366 days follows the U.S. Naval Observatory (USNO) rule: years evenly divisible by 4 are leap years, except century years that are not evenly divisible by 400. Our leap year guide lists the leap years to 2100.

How do you work out the day of the year?

Use one formula:

Day of the year = days in completed months + day of the month

1. Take the month of your date and add up the days of all the months before it.
2. Add the day of the month.
3. In a leap year, February counts 29 days, so any date from March 1 on gains one.

Worked example for today, Oct. 30, 2026. January to September have 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 = 273 days, and 2026 is a common year. Add 30 and the answer is day 303. We checked every month of the table below against JavaScript’s Date object and against an independent Julian-day-number formula, and both agree.

Date	Common year (2026)	Leap year (2028)
Jan. 1	001	001
Feb. 28	059	059
Feb. 29	does not exist	060
Mar. 1	060 (31 + 28 + 1)	061 (31 + 29 + 1)
Apr. 10	100	101
Oct. 30	303	304
Dec. 31	365	366

Day 100 is April 10 in a common year and April 9 in a leap year; our 100th day of the year story has the details.

Day number of the 1st of each month

Find the month, add your day of the month to the “add” number, and you have the answer. The “1st is day” columns give the first day of each month.

Month	Common year: add	Common: 1st is day	Leap year: add	Leap: 1st is day
January	0	001	0	001
February	31	032	31	032
March	59	060	60	061
April	90	091	91	092
May	120	121	121	122
June	151	152	152	153
July	181	182	182	183
August	212	213	213	214
September	243	244	244	245
October	273	274	274	275
November	304	305	305	306
December	334	335	335	336

The two columns are identical through February. From March 1, the leap-year number is one higher, because Feb. 29 is already behind you. The same table is on the free printable below, set for 2026 and 2028.

What formula gives the day of the year in Excel or Google Sheets?

With the date in cell A1, use the function that builds a date from a year, month and day (DATE) and the function that reads the year from a date (YEAR) like this:

=A1-DATE(YEAR(A1),1,0)

Microsoft Support says a day value below 1 in the DATE function subtracts that many days, plus one, from the first day of the month. So DATE(YEAR(A1),1,0) is Dec. 31 of the previous year, and subtracting it from A1 leaves the day number. Microsoft does not publish this formula as such; it is our formula built on the documented DATE behavior. Google Docs Editors Help says DATE silently recalculates values outside the valid month or day range, so the same approach applies in Google Sheets. Set the result cell to a number format, or the sheet may show it as a date.

A second route uses the days-between function (DAYS), which Microsoft describes as returning the number of days between two dates. Its example of Dec. 31, 2021 minus Jan. 1, 2021 gives 364, a difference that leaves out one end. Add 1 to get a day number: =DAYS(A1,DATE(YEAR(A1),1,1))+1.

How do you turn a day number back into a date?

Reverse the steps, and check the limit first.

1. Confirm the year is common or leap. A day number above 365 is valid only in a leap year.
2. Find the last month whose “add” number in the table is smaller than your day number.
3. Subtract that “add” number from the day number. The result is the day of the month.

Example: 2026-300. The “add” number for October is 273, and 300 - 273 = 27, so the date is Oct. 27, 2026. In 2028, October’s number is 274, so day 300 is Oct. 26, 2028. Day 366 is Dec. 31, 2028, but 2026-366 does not exist: the year has 365 days.

Is day of the year the same as a Julian date?

No, and mixing them up causes wrong dates. The day of the year is the 001 to 366 count above. Some industries call the day number a “Julian date” in everyday use, and our Julian date calendar charts use that label for the same reason.

The astronomers’ Julian date is a different measure. The USNO says Julian dates are a continuous count of days and fractions since noon Universal Time on Jan. 1, 4713 BC. It never restarts on Jan. 1, so it runs to numbers in the millions. If a form asks for a Julian date, check whether it wants the three-digit day number or the astronomical count.

Printable day of the year chart

The one-page chart lists the number to add for every month in 2026 (a common year) and 2028 (a leap year), plus the Oct. 30, 2026 example. It comes in a standard and an ink-saver version. For whole-year pages, use our printable yearly calendar or the February calendars when checking a leap day.