Formula
ISO weeks begin Monday; week 1 contains January 4 (the first Thursday).
- Use the units shown beside each field.
- Results update automatically whenever you change an input.
- Displayed values use up to four decimal places, or scientific notation for very small or large numbers. Calculations use standard floating-point arithmetic; exact integer and fraction tools identify their own precision rules.
- Gregorian date-only calculation. ISO week-year can differ from calendar year near New Year.
Example Calculation
Example inputs
- Calendar date: 2021-01-01
1 January 2021 belongs to ISO week 53 of week-year 2020, Friday.
Frequently asked questions
How do I use this calculator?
Enter calendar date. The result updates automatically. Use Reset to restore the example values.
What method does it use?
ISO weeks begin Monday; week 1 contains January 4 (the first Thursday).. Gregorian date-only calculation. ISO week-year can differ from calendar year near New Year.
Why might a rounded result differ?
The calculation keeps full numeric precision internally, then rounds the displayed result. Rounding intermediate steps by hand can produce a slightly different answer.
Are my inputs saved online?
No. This calculator processes your inputs in your browser. A recently used list stores only calculator names on this device, not your entered values.
Check the assumptions and units before using this result. Read about calculation methods.