Day of Year & Week Number Calculator
Pick any date to see its ordinal day-of-year (1–366), ISO 8601 week number, weekday, quarter, and how many days are left until year end — handy for reports, project scheduling and forms.
Please enter a valid number
Day of year
135
Days left in year
230
ISO 8601 week
Quarter
Week of year (Sun-start)
Leap year
Formula
dayOfYear = Σ daysInMonth(1..month−1) + day leap = (year mod 4 = 0 AND year mod 100 ≠ 0) OR (year mod 400 = 0) ISO week: the week containing the year's first Thursday = week 1
- · Ordinal day-of-year starts at 1 on Jan 1 — max 365 in common years, 366 in leap years.
- · ISO 8601 weeks start on Monday; week 1 is the week containing the year's first Thursday.
- · Late-December or early-January dates can therefore have an ISO week-year different from the calendar year (e.g. 2024-12-30 is 2025-W01).
- · A simple Sunday-start "week of year" (US-style, with week 1 containing Jan 1) is shown alongside as a complement.
- · Gregorian leap rule: divisible by 4, except centuries unless also divisible by 400 (so 2000 = leap, 1900 = not).
- · Pure date arithmetic — runs entirely in your browser; nothing is sent anywhere.
Frequently asked
Why does the ISO week sometimes differ from the "week 1" I learned at school?
School calendars often use "the week that contains Jan 1 is week 1" (Sunday- or Monday-start). ISO 8601 instead defines week 1 as the week containing the year's first Thursday, so the first few days of January may belong to the previous year's last week. Business, banking and ERP systems typically use the ISO standard.
What is day-of-year actually used for?
It compresses a date into a single number: meteorology, agriculture, satellite tracking, lab notebooks, camera file names and many official forms (e.g. birth certificates, tax filings) use "year + ordinal day" instead of year-month-day. It is also useful for tracking "how far through the year" you are.
Was the year 2000 a leap year?
Yes. Centurial years are normally not leap (e.g. 1900, 2100), but the rule makes an exception for years divisible by 400. Since 2000 ÷ 400 = 5, the year 2000 had 366 days.
Can I share a result with someone?
Yes — picking a date updates the URL, so you can use the “Copy link” button at the bottom-right to share the exact same date and result.
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