Purpose and steps
Week Number Calculator validates calendar date before applying the displayed equation. This tool uses the proleptic Gregorian calendar, not historical local calendar reforms.
- 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.
Input reference and units
These values reproduce the model’s demonstration. Replace them with your own records or measurements; they are not recommended targets.
On a small screen, swipe the table horizontally to see all columns.
| Parameter | Demonstration value | How to enter it |
|---|---|---|
| Calendar date | 2026-01-01 | A real Gregorian date in years 0001–9999; calculations use UTC calendar dates. |
Model-specific method
How this calculator produces its result
Week Number Calculator: ISO week = ceil((ThursdayOfWeek − Jan1 + 1 day) / 7 days). Input definitions: date = Calendar date. Use the selected convention throughout the calculation.
Week Number Calculator: calendar conventions
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. 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.
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)
- date: Calendar date. A real Gregorian date in years 0001–9999; calculations use UTC calendar dates.
- Result units: stated beside each output
- Calculation relationship: ISO week = ceil((ThursdayOfWeek − Jan1 + 1 day) / 7 days)
- Supported domain: see the limits below
Worked 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.
Reproduce the default scenario
This reproduces the default inputs in the reference table above; it is not an additional independent example. Displayed rounding may differ from intermediate precision.
- ISO week number
- 1
- ISO week-year
- 2,026
Reading results without overstating them
- 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.
Assumptions and exclusions
- 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.
Sources and content notes
Toolify describes the implemented algorithm and its assumptions. Sources below support the topic or applicable rules; they do not endorse this calculator. Examples are illustrative, and published rules take precedence over simplified estimates.
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