ISO Week Date Converter
Convert any Gregorian calendar date to its ISO 8601 week date (YYYY-Www-D) and back, with the exact week number and weekday.
Direction
Enter a valid number
How does this calculator work?
This tool converts a Gregorian date to an ISO 8601 week date (YYYY-Www-D) and back. ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday, so the ISO year can differ from the calendar year near January and December. Weekdays run Monday = 1 to Sunday = 7.
Formula
How this is calculated
Pick a direction. "Gregorian date to ISO week date" takes a calendar date as YYYY-MM-DD and returns its ISO 8601 week date. "ISO week date to Gregorian date" takes an ISO year, week number (1–53) and weekday (Monday = 1 … Sunday = 7) and returns the corresponding calendar date.
The ISO 8601 week date system is anchored on Thursdays: each ISO week runs Monday through Sunday, and the week containing the year's first Thursday is week 1. To find the week number we compute the day-of-week index dayNr = (getUTCDay() + 6) mod 7 (Monday = 0), shift the date to the Thursday of its week (date − dayNr + 3), and count whole weeks from that year's first Thursday. The ISO weekday is simply dayNr + 1. For the inverse, we start at January 4 (always in week 1), step back to the Monday of week 1, then add (week − 1) × 7 + (weekday − 1) days.
All arithmetic uses UTC dates to avoid daylight-saving and timezone shifts. The ISO year can differ from the Gregorian year near January 1 and December 31 — for example, 2027-01-01 belongs to ISO week 2026-W53. Inputs are validated: impossible calendar dates and out-of-range weeks (such as week 53 in a 52-week year) are rejected via a round-trip check.
Frequently asked questions
ISO weeks never split: a week belongs entirely to the year that holds its Thursday. So the last days of December can fall in week 1 of the next ISO year, and early January can fall in the final week of the previous ISO year.
Most ISO years have 52 weeks, but some have 53. A year has 53 weeks when January 1 is a Thursday, or when it is a Wednesday in a leap year.
ISO 8601 numbers weekdays Monday = 1 through Sunday = 7, unlike the common Sunday = 0 convention used by many programming languages.
Also known as
TG we-Calculate Editorial Team. (2026). ISO Week Date Converter [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/iso-week-date-converter
TG we-Calculate Editorial Team. "ISO Week Date Converter." TG we-Calculate. 2026. https://we-calculate.com/calculator/iso-week-date-converter.
TG we-Calculate Editorial Team, "ISO Week Date Converter," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/iso-week-date-converter
@misc{wecalculate_iso_week_date_converter, title = {ISO Week Date Converter}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/iso-week-date-converter}}, year = {2026}, note = {TG we-Calculate} }
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