New Year on a Thursday
Input 2026-01-01
Result Week 1 of 2026, weekday 4, running from 2025-12-29 to 2026-01-04. That year has 53 weeks.
ISO Week Number Calculator
Everyday LifeEnter a date to get its ISO 8601 week number, week year, weekday and the Monday-to-Sunday range, plus the total weeks in that year.
Your inputs
Result
ISO week number
—
- Week year —
- Weekday, 1 = Monday —
- Week starts —
- Week ends —
- Day of the year —
- Weeks in that year —
Change any input and the result updates instantly.
Formula and rules
- ISO 8601 counts weeks from Monday to Sunday, and week 1 is the week that contains 4 January.
- Week number = floor((day of year − weekday + 10) ÷ 7), then corrected when the result is 0 or 53.
- The week year is not always the calendar year: 31 December 2025 belongs to week 1 of 2026.
- Weekday 1 is Monday and 7 is Sunday, so the week ends six days after it starts.
- A year has 53 weeks when 1 January is a Thursday, or a Wednesday in a leap year; every other year has 52.
Worked examples
Last day of the calendar year
Input 2025-12-31
Result Week 1 of 2026, weekday 3, because the week that holds 4 January 2026 started on 29 December 2025.
A mid-year Monday
Input 2026-06-15
Result Week 25 of 2026, weekday 1, running from 2026-06-15 to 2026-06-21. It is day 166 of the year.
FAQ
Why does this week number differ from the one on my phone?
Many calendars start the week on Sunday and call the week containing 1 January week 1. ISO 8601 starts on Monday and counts the week containing 4 January as week 1, which is the rule used in Europe for school terms, shift plans and payroll periods.
How can 31 December belong to the next year?
A week is never split across two years. If the days around New Year fall inside the week that contains 4 January, they join week 1 of the new year. The same happens in reverse at the other end of the year.
Does every year have the same number of weeks?
No. A year has 52 weeks plus one day, or two in a leap year. When that extra day lands on a Thursday, the year gains a 53rd week, which happens roughly every five to six years.
Where are ISO week numbers actually used?
In Norway, Sweden, Denmark, Finland, Germany, the Netherlands and much of Europe, week numbers drive school holidays, shift plans, delivery slots and payroll periods, so people look them up far more often than they look up plain dates.
Disclaimer: Results follow the formula shown on this page and are for reference only. Real-world quantities vary with materials, technique and conditions, so allow a margin before ordering.