Compare week systems
The same calendar date can have different week numbers depending on which convention is used.
OneFreePlace · Tool #122
Find the week number—and understand which week system produced it.
Convert any date to ISO 8601 or other week numbering, reverse a week number back to its seven dates, compare systems side by side, inspect year-boundary surprises, navigate every week of a year and export a complete week table.
✓ ISO 8601 week-year aware✓ Reverse week lookup
✓ Custom first-day/minimum-days rules✓ 52/53-week inspector
✓ CSV export
Week Number Calculator
Date → week · week → dates · compare · export
Date to week
Which week does this date belong to?
Week numbering systemSwitch instantly and compare below.
ISO 8601: Monday-start weeks; week 1 has at least four days in the new year.
Reverse lookupEnter a week-year and week number.
—Enter a valid week-year and week.
Week number
ISO 8601—
week
Choose a date.
Week-year—can differ from calendar year
Week range——
Day of week—position inside this week
Weeks in week-year—52 or 53 under common systems
Weeks remaining—after the selected week
ISO week date—always shown for interoperability
Selected seven-day week—
Week-year boundary check
—
Spreadsheet-ready formulas
Use the same convention in your spreadsheet so reports and dashboards agree with the calculator.
ISO 8601 — Excel / Sheets
=ISOWEEKNUM(A1)ISO alternative
=WEEKNUM(A1,21)Sunday + Jan 1
=WEEKNUM(A1,1)Monday + Jan 1
=WEEKNUM(A1,2)Complete week-year table
Every numbered week under the selected system, including dates that cross calendar-year boundaries.
| Week | Start | End | Crosses year? | Action |
|---|
Why week numbers disagree
ISO 8601 starts weeks on Monday and requires week 1 to contain at least four days of the new year. A Sunday-start/Jan-1 system starts weeks on Sunday and makes the week containing January 1 week 1. Monday/Jan-1 keeps Monday starts but still assigns January 1 to week 1.
Custom mode lets you choose the first weekday and the minimum number of new-year days required in week 1—useful for matching a specific organization or software configuration.
Boundary intelligence
Dates around New Year are where week-number bugs usually hide. This calculator keeps the week-year separate from the ordinary calendar year, shows the exact seven-day range and flags when the selected week crosses into another calendar year.
The full table also exposes whether the selected week-year has 52 or 53 numbered weeks.
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There is no single universal week-number convention.When exchanging week numbers with another person or system, include the convention and week-year—for example ISO 2026-W39—rather than sending only “week 39.”