# Day of the Year Calculator

URL: https://speedycalculator.com/calculators/everyday-life/day-of-the-year/
Last updated: 2026-10-25

> The day of the year calculator shows the day number (1–366) of any date, how many days are left in the year and the percentage of the year gone – and converts a day number back into a calendar date.

## Inputs

- Date
- Year (for day number → date)
- Day number

## Results

- Day of the year
- Days in the year
- Days left in the year
- Year completed
- ISO week number
- Date of day number

## Formula

```
Day of year = date − 31 December of the previous year
Days left = days in year − day of year
Date from day number = 1 January + (day number − 1)
```

## Explanation

Day-of-year numbers (sometimes wrongly called Julian dates) are used in manufacturing date codes, food packaging, aviation, the military and science. Day 256 is Programmers' Day; day 100 marks the 100th day of the year.

### How to use the Day of the Year Calculator

- **Date**
- **Year (for day number → date)**
- **Day number**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Pick a date, or enter a year and day number to convert back.

### Tips

- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so "one month" is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.

### Common mistakes to avoid

- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).

### Inputs explained

FieldWhat to enterDateA dateYear (for day number → date)A numberDay numberA number

### What each result means

- **Day of the year** = dayofyear([Date])
- **Days in the year** = 365 + isleap(year([Date]))
- **Days left in the year** = [Days in the year] - [Day of the year]
- **Year completed** = [Day of the year]÷[Days in the year]×100
- **ISO week number** = (floor((dayofyear(([Date]) - mod(weekday([Date]) + 6, 7) + 3) - 1)÷7) + 1)
- **Date of day number** = date([Year (for day number → date)], 1, 1) + [Day number] - 1

### Example values

How the main result changes when **Year** changes and everything else stays at the example values above:

Year (for day number → date)Day of the year1,0132981,519.52982,0262982,532.52983,0392984,052298

## Worked example

Inputs — Date: 2026-10-25; Year (for day number → date): 2,026; Day number: 256.

Results — **Day of the year = 298**; Days in the year = 365; Days left in the year = 67; Year completed = 81.6 %; ISO week number = 43; Date of day number = Sun, September 13, 2026.

## FAQ

**What day of the year is it?**
See the result – 1 January is day 1 and 31 December is day 365 (366 in a leap year).

**What is a Julian date code?**
In manufacturing, a code like 26298 often means year 2026, day 298.

**What is day 256?**
13 September (12 September in leap years) – Programmers' Day.

**What formula does the Day of the Year Calculator use?**
Day of year = date − 31 December of the previous year.