Week Number Calculator — ISO 8601 & US Week Numbers, Both Ways

Real-world use cases

• Name a sprint by its ISO week for a global agile team

  Your team labels every two-week sprint "W22–W23" so a developer in Berlin and a PM in São Paulo refer to the same fortnight without arguing about dates. Open the tool, pick the sprint start date, and read the ISO week number off the cyan card — ISO is the right system here because the rest of your Jira / Linear tooling and most European calendars already use it. Steps: (1) pick the Monday the sprint starts; (2) copy the ISO week number; (3) if you need the closing week, add one. The "this week" badge confirms whether the current sprint is live.

• Reconcile a weekly finance report to calendar dates

  Finance hands you a report keyed "Week 6, FY2026" and you need the actual dates to match it against bank statements. Switch to reverse mode, enter the year and the week number, and read the Monday-to-Sunday range. If finance is using the Sunday-start US numbering instead of ISO, check the forward mode first with a known date from the report to confirm which system gives Week 6, then trust the range. The tool refuses impossible weeks, so a typo like "Week 60" surfaces immediately instead of producing a garbage range.

• Set a CI cron that fires only on even ISO weeks

  You want a heavy integration job to run biweekly, on even ISO week numbers, but cron has no native "every other week" expression. First confirm what week the schedule should start on: pick today's date, read the ISO week, and decide whether this week is the "on" or "off" week. Then your job's guard becomes a one-liner that computes the ISO week and skips odd ones — and you can sanity-check its output against this tool for any date for the next few months.

• Plan a content calendar by week instead of by date

  A marketing calendar that says "ship the launch post in W37" reads more cleanly than juggling specific dates that slip. Use reverse mode to turn each planned week into its real date range so writers know their actual deadlines, and use the total-weeks readout to see whether your year stretches to W52 or W53 before you over-commit the back half of December. Steps: list your target weeks, expand each to a date range, drop the ranges into the shared calendar.

• Check whether a year you're scheduling against has 53 weeks

  Annual planning templates that hardcode 52 rows quietly break in a 53-week year — the last week's data has nowhere to go. Before you clone last year's spreadsheet, type any date in the target year and read the "weeks this year" line: if it says 53 (like 2026 does), add the extra row now. This is the same edge case that causes "missing week 53" bugs in BI dashboards every few years; catching it at planning time is far cheaper than at year-end close.

Common pitfalls

• Assuming ISO and US week numbers always match. They agree most of the year but can differ by one in early January because ISO weeks start Monday and require the first week to hold a Thursday, while the US numbering starts Sunday and counts the January-1 week as week 1. Pick the system your counterpart uses before you quote a number.

• Treating the ISO week-year as the calendar year. An ISO week belongs to the year that holds its Thursday, so January 1 can be in week 52 or 53 of the previous year, and late December can be in week 1 of the next year. Always read the week-year line, not just the calendar year, when a date is within a few days of January 1.

• Hardcoding 52 weeks. Years where January 1 is a Thursday (or a leap year with January 1 on a Wednesday) have 53 ISO weeks. Spreadsheets and dashboards built for 52 rows silently drop week 53's data. Check the year's total before you build the template.

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