DIY Cost Tools
ISO Week Number Calculator
Convert a calendar date into its ISO 8601 week-year, week number, and Monday-based weekday number.
How to use this calculator
- Replace the example values with your own measurements or planning assumptions.
- Keep every value in the unit printed beside its field.
- Select Calculate and review the result together with the formula and limitations below.
Formula and method
ISO weeks start Monday. Week 1 is the week containing the year’s first Thursday, so early January dates can belong to the previous ISO week-year.
Results are hidden whenever an input changes or validation fails, so an old answer cannot be mistaken for a current calculation.
Worked example
Calendar date = 2026-10-04. The calculator returns ISO week-year = 2026; ISO week number = 40; ISO weekday = 7; Week label = 2026-W40-7.
The worked example uses the same default values displayed in the calculator.
Limitations and safety
Frequently asked questions
Why can the ISO week-year differ from the calendar year?
Days near New Year can belong to the adjacent ISO week-year because ISO week 1 is defined by the first Thursday.
Why might the real result differ?
This reports the ISO 8601 system. Some businesses and calendars use Sunday-based numbering or define week 1 differently.
How should I round the answer?
Keep extra precision while planning, then round up when purchasing whole containers, bags, pieces, or stock lengths. Follow the output labels and supplier packaging.
Review notes and reference
Reviewed October 4, 2026. The formula was checked against the displayed worked example, and the script was tested for defaults, boundaries, invalid formats, condition-specific errors, and recovery. Reference: NIST Time and Frequency Division. Explanations and examples are original.