Week Number Calculator
Calculate date and time values locally with visible assumptions and copy-ready results.
Use calendar math with the assumption visible
Use it for sprint planning, European business calendars, manufacturing schedules, payroll labels, and reports that refer to week numbers instead of date ranges. ISO weeks surprise people because they start on Monday and belong to the year containing the week Thursday.
ISO week numbers use Monday as the first day of the week and assign week 1 to the week containing the first Thursday of the year. July 8, 2026 falls in ISO week 28. The July 2026 example anchors the rule to a real date instead of merely displaying a week number.
Limits and local processing
ISO weeks can place early January dates in the previous ISO year or late December dates in the next ISO year. The selected date is converted locally into the ISO week-year calculation. Use a different standard if your company treats Sunday as week start or labels retail weeks differently.
Frequently Asked Questions
How does the week number calculator work?
ISO week numbers use Monday as the first day of the week and assign week 1 to the week containing the first Thursday of the year. July 8, 2026 falls in ISO week 28.
When should I use this tool?
Use it for sprint planning, European business calendars, manufacturing schedules, payroll labels, and reports that refer to week numbers instead of date ranges.
What should I watch out for?
ISO weeks can place early January dates in the previous ISO year or late December dates in the next ISO year.
Is this date tool private?
Yes. It runs entirely in your browser, so dates, schedules, and notes are not uploaded or stored.
More free browser utilities for planning, formatting, and technical cleanup.