Day of Year & Week Number Calculator

How it works

This tool answers two calendar questions at once. The "day of year" is simply the count of days from 1 January up to your date, inclusive — so 1 January is day 1 and 31 December is day 365 (or 366 in a leap year). The "week number" follows the ISO 8601 standard used across most of the world, where week 1 is the one containing the first Thursday of the year.

ISO weeks can surprise people: a date in early January sometimes belongs to the last week of the previous year, because the ISO rule is based on Thursdays, not on the calendar month. The calculator applies that rule directly, so you get the same week number a standard ISO calendar would show.

Examples

Example: 1 July 2026

The calculator reports day 182 of 365 and ISO week 27 of 2026.

Example: 1 January 2026

Because Thursday falls on 1 January 2026, that date is in ISO week 1 — and it is day 1 of the year.

Example: 31 December 2026

The calculator reports day 365 of 365 and the final ISO week of the year (week 53, because 2026 has 53 ISO weeks).

Example: your birthday’s day number

Enter your birth date to see how far through the year it falls — for example 1 July is day 182, a handy way to picture the year as a 365-step line.

FAQ

What week of the year is it (ISO)?

The ISO 8601 week number treats the week containing the year’s first Thursday as week 1. Near a year boundary, early January can fall in the previous year’s final week. The calculator applies this Thursday-based rule, matching standard ISO calendars.

What is the day number of the year?

It is the count of days from 1 January up to your date, inclusive. In a common year 31 December is day 365; in a leap year it is day 366 because 29 February adds one. The calculator shows both the day number and the total days in that year.

Why does early January sometimes show the previous year’s week?

Under ISO 8601, week 1 is the week with the year’s first Thursday. Dates in early January that fall before that Thursday belong to the last week of the previous year, so 1 January can read as, say, week 52 or 53 of the year before. The calculator applies this rule exactly.

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