What is the formula for the Week Number Calculator?
Week Number Calculator: ISO week = ceil((ThursdayOfWeek − Jan1 + 1 day) / 7 days). Input definitions: date = Calendar date. Use the selected convention throughout the calculation.
Dates & time
Week Number Calculator
Look up a Gregorian calendar weekday or ISO 8601 week number. ISO weeks begin on Monday, and the first week contains January 4; the ISO week-year can differ from the calendar year.
- Explicit formulas and worked examples
- Check your inputs, then calculate
- Data is calculated in this browser only
Check the stated input format, calculation convention and limits before using the result.
Check the example inputs, then calculate. Your values stay in this browser.
This tool uses the proleptic Gregorian calendar, not historical local calendar reforms.
Calculation notes
Understand the Week Number Calculator
Week Number Calculator validates calendar date before applying the displayed equation. This tool uses the proleptic Gregorian calendar, not historical local calendar reforms.
How can I reproduce the Week Number Calculator example?
Week Number Calculator baseline input: Calendar date: 2026-01-01. Output: ISO week number = 1; ISO week-year = 2,026. Substitute these values into the displayed equation to reproduce the result.
How should I interpret the Week Number Calculator result?
- Read iso week number with its stated unit and convention. The default example and the independent worked scenario use different inputs, so compare the assumptions before comparing the numbers.
- Change one input at a time and use the equation to explain the result; do not mix unit, grade-scale, time-zone or rounding conventions across comparisons.
What limits apply to the Week Number Calculator?
- This tool uses the proleptic Gregorian calendar, not historical local calendar reforms.
- ISO weeks are not Sunday-start or locale-specific week numbering.
- Calendar-date inputs and calculated calendar dates must stay within years 0001–9999.
Content and calculation reviewed:
Understand the calculation
How to use it and the math behind it
Four steps to check your inputs and results
- Look up a Gregorian calendar weekday or ISO 8601 week number. ISO weeks begin on Monday, and the first week contains January 4; the ISO week-year can differ from the calendar year.
- Enter Calendar date in the stated format. Baseline values are for reproduction.
- Check the output unit and convention: Read iso week number with its stated unit and convention. The default example and the independent worked scenario use different inputs, so compare the assumptions before comparing the numbers.
- Change one input and compare the result. This tool uses the proleptic Gregorian calendar, not historical local calendar reforms. ISO weeks are not Sunday-start or locale-specific week numbering. Calendar-date inputs and calculated calendar dates must stay within years 0001–9999.
Week Number Calculator: calendar conventions
ISO weeks start on Monday, and week 1 contains January 4. January 1, 2021 therefore belongs to week 53 of ISO week-year 2020, even though its calendar year is 2021. Shifting to the Thursday of the same week identifies the ISO year. Keep week number and week-year together when naming reports or grouping dates.
ISO week = ceil((ThursdayOfWeek − Jan1 + 1 day) / 7 days)
The detailed guide adds variable definitions, a reproducible example, a practice question and model limitations.
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