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ISO week calculator

Find the ISO week number, week year, weekday and Monday–Sunday boundaries, or convert a week year and number to dates.

ISO weeks start on Monday; week 1 contains January 4. The week year can differ from the calendar year. Week 53 exists only in some years. Both Monday and Sunday are shown; weeks extending outside years 0001–9999 are rejected. Hidden inputs do not participate.

Digits 0–9, ٠–٩ and ۰–۹ are accepted. Oversized input is rejected without silent truncation.

Calculation stays in your browser without uploads or automatic storage. Operations are bounded and synchronous; there is no background job or Cancel button. Only page assets are loaded from the site.

How to use

Choose date-to-week or week-to-dates. Fill the visible fields and calculate. Review the week year, number, Monday start and Sunday end. Date mode also shows weekday Monday=1 through Sunday=7.

Try an example

2021-01-01 is weekday 5 in 2020-W53, from 2020-12-28 through 2021-01-03. Week 53 of 2021 does not exist and is rejected.

Support & limits

Week 1 contains January 4, equivalently the year’s first Thursday. Early January can therefore belong to the preceding week year. The local first-weekday setting is irrelevant. Week dates are not clipped at year 9999: if Sunday falls in 10000 the whole week is explicitly rejected. Week mode returns its boundaries without inventing a selected date inside it.

Common questions

Does changing input mode use hidden fields?

No. Date mode uses only the date; week mode uses the year and week number. Old field values are retained for switching back, but hidden fields do not participate.

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