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Quickly and accurately calculate what week of the year it is today.
ISO 8601 is an international standard where Monday is the first day of the week. Local standards, often used in American or lunar calendars, typically start on Sunday.
Overview
Understand what the tool solves, how it works, and the boundaries of its data.
Enter a calendar date to see its week number, the week’s start and end dates, weekday, day-of-year progress, and week count information. You can choose ISO or the Sunday-start option. ISO weeks run Monday through Sunday and are numbered by the ISO week-date convention; around New Year, a Monday-to-Sunday week can begin in December or end in January. Its week-year can therefore differ from the selected date’s calendar year.
The Sunday-start option uses Sunday-to-Saturday ranges and treats the week containing January 1 as week 1. It is a calendar numbering choice, not a definition of a company’s workweek or pay period. The page begins with the ISO option selected and lets you switch conventions or reset to today.
Alongside the week range, the results include the selected date’s ordinal day within its calendar year, days remaining after that date, the selected week number and the year’s final week number under the chosen convention. Around year boundaries, read the actual week range as well as the displayed calendar year: the page does not show a separate ISO week-year field.
Sources: ISO, ISO 8601 — Date and time format describes the international date/time representation standard; standard context; checked 2026-09-28. date-fns, getISOWeek documents the ISO week-number operation used for the ISO selection; checked 2026-09-28.
Guide
Follow the workflow and verify inputs and outputs with practical examples.
The page starts with today. Select another calendar date to check a past or future week.
Use the same convention as the calendar, report, or dataset you need to compare. ISO uses Monday-based weeks; the other option uses Sunday-based week ranges with January 1 in week 1.
Read the start and end dates alongside the number. Near New Year, the range may cross into another calendar year. Copy the week number if you only need that value, or share the selected date and convention with the result.
Example: if two calendars disagree on a January date, first compare whether they use ISO or Sunday-start numbering. The difference can come from the rule, not an incorrect date entry.
Use cases
See how the tool fits into real work and everyday tasks.
Choose a convention agreed by the team, then use the week range to check that a milestone belongs to the intended seven-day span.
Students can see why a date close to January 1 may have different week labels in ISO and Sunday-start calendars.
Check a report’s week reference against its stated convention before comparing dates across years.
Q&A
Find concise answers to common questions and confusing cases.
The selected rules differ in both week start and which days define week 1. Compare the chosen convention and the displayed date range before treating values as inconsistent.
Yes. ISO weeks are complete Monday-to-Sunday blocks, so the first week associated with an ISO week-year may begin in late December. The result shows the selected date’s calendar year, not a separate ISO week-year label.
No. It reports one of two calendar week-numbering conventions. Employer schedules, fiscal calendars, and pay periods can follow different rules.
It is the number of calendar days after the selected date through December 31 of that date’s calendar year. The selected day itself is not counted as remaining.
Notes
Review scope, result limitations, and important precautions before use.
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