ISO Week Date Converter

Convert a calendar date into its ISO 8601 week date — the YYYY-Www-D form like 2026-W22-5 — and convert a week date back to a calendar date. The tool computes the ISO week number, the weekday, and the week-numbering year, which can differ from the calendar year in early January and late December. Everything runs live in your browser.

How to use the ISO Week Date Converter

Pick a date in the calendar date field and the ISO week date updates instantly, or type an ISO week date such as 2026-W22-5 and the calendar date fills in. The ISO form is YYYY-Www-D: a week-numbering year, a two-digit week from 01 to 52 or 53, and a weekday from 1 (Monday) to 7 (Sunday). You can also write it without the dashes, like 2026W225.

The cards below show the full breakdown — the ISO week-numbering year, the week number, the weekday name, and the matching calendar date — and they flag the cases where the week-numbering year differs from the calendar year. For example, 1 January 2021 belongs to 2020-W53-5 because that day falls in the last ISO week of 2020. The conversion follows the ISO 8601 rules exactly, including the rule that week 1 is the week containing the first Thursday of the year.

It all runs in your browser, so conversions are instant, work offline, and nothing you enter is uploaded anywhere.

How ISO 8601 week dates work

The ISO 8601 week date is an alternative way of naming a day that is built around weeks rather than months. Instead of a month and day, it gives a week-numbering year, a week number, and a weekday, written 2026-W22-5. It exists because many businesses plan, report, and schedule by week — manufacturing, retail, payroll, and project tracking all think in weeks — and the month-based calendar makes "the fifth week of the quarter" awkward to express. The week date makes weekly comparisons trivial: every ISO week is exactly seven days, every week starts on Monday, and the week number increments cleanly.

Two rules define the system. First, the week starts on Monday and ends on Sunday, with weekdays numbered 1 through 7. Second, week 1 is the week that contains the year\'s first Thursday — equivalently, the week containing 4 January, or the first week with the majority of its days in the new year. This Thursday rule is what makes the system slightly surprising at the edges. Because a week belongs to whichever year holds most of it, the first few days of January can fall into the last week of the previous year, and the last days of December can fall into week 1 of the next year. That is why the converter reports a separate week-numbering year: 31 December 2018 is 2019-W01-1, and 1 January 2021 is 2020-W53-5. A year has 53 ISO weeks when it starts on a Thursday (or is a leap year starting on Wednesday); otherwise it has 52.

Converting in either direction is a small piece of date arithmetic. To find the week date of a given day, you locate the Thursday of that day\'s week — adding the offset that moves you to Thursday — and read its calendar year as the week-numbering year, then count weeks from that year\'s first Thursday. To go the other way, you start from 4 January (always in week 1), step back to the Monday of week 1, and add the requested number of weeks and days. The arithmetic is easy to get subtly wrong by an off-by-one at the boundaries, which is exactly why a dedicated converter — one that has the Thursday rule and the week-year handling baked in and tested against the tricky New Year cases — saves time when you are wiring up reports or parsing timestamps that arrive in week-date form.

Common use cases

• Business reporting. Translate a calendar date into the fiscal-style week number your reports use.
• Parsing timestamps. Decode an ISO week date like 2026-W22-5 into a real calendar day.
• Scheduling. Find which Monday-to-Sunday week any date belongs to, across year boundaries.
• Data pipelines. Confirm your code handles the W53 and cross-year edge cases correctly.