Week Number Calculator
Convert dates and ISO week dates with the correct week-year, Monday–Sunday range and batch exports.
This tool runs entirely in your browser. Your data is never uploaded, never stored, and never leaves your device.
Convert Gregorian dates and ISO week dates, including the correct week-year, weekday and Monday–Sunday range.
How to use it
- 1Choose date-to-week or week-to-date.
- 2Enter YYYY-MM-DD or YYYY-Www-D values, one per line.
- 3Review the week-year, range and any invalid week-53 errors.
- 4Download the complete batch as CSV or JSON.
Example
- Input
- 2016-01-01
- Output
- 2015-W53-5; 2015-12-28 to 2016-01-03
ISO week-year 2015 differs from calendar year 2016.
What happens to your data
Date arithmetic stays in this browser, without uploads, location lookup, persistent history or URL inputs. Exports remain optional local files.
Last updated October 2026
A date near New Year can belong to the previous or following ISO week-year. This week number calculator converts an ordinary date into YYYY-Www-D and reverses that notation back into a calendar date. It also shows the week's Monday and Sunday, making a week-based schedule easier to check.
The rule is deliberately ISO-only rather than an unlabeled local convention. Paste dates or week dates in batches, retain errors such as a nonexistent week 53 and download the reviewed results. No calendar account, location lookup or uploaded file is required.
How it works
ISO weeks begin on Monday, numbered weekday 1, and finish on Sunday, weekday 7. Week 1 is the week containing January 4, equivalently the first Thursday of the year. This definition means that the week-year is not always the calendar year written in the date. January 1, 2016 belongs to 2015-W53-5, while January 4 belongs to 2016-W01-1.
For date to week, enter an exact YYYY-MM-DD value. The tool validates the Gregorian calendar date rather than letting invalid dates roll into another month. It locates the week's Thursday to establish the ISO week-year, then counts whole weeks from the Monday of the week containing January 4. Weekday numbering is independent of your browser's locale or first-day-of-week preference.
For week to date, enter YYYY-Www-D: a four-digit week-year, uppercase W and two-digit week number, followed by a weekday digit 1–7. For example, 2015-W53-5 returns 2016-01-01. A year has either 52 or 53 ISO weeks, not automatically 53. The reverse conversion checks the computed date back against the requested year and week, so 2021-W53-1 receives an error rather than being accepted as a date in 2022.
Each successful row includes the Monday–Sunday range and explicitly names the ISO week-year. This is useful when two systems group the same late-December or early-January date differently. It is not a United States Sunday-start week rule, a financial week, a retail 4-4-5 calendar, a school week or an arbitrary fiscal-year start. If a receiving system uses another convention, its matching week number must be established separately.
The converter supports Gregorian calendar years and ISO week-years 0001–9999. At the far supported boundary, a week's range can extend beyond those Gregorian years; that endpoint is labeled outside the range rather than fabricated. A requested date or resulting week-year outside the supported scope is rejected. Gregorian rules apply even before their historical local adoption. Dates are whole calendar days, not timestamps, so timezone or daylight-saving conversion is neither needed nor inferred.
You may paste 500 nonblank values, with a 50,000-character total limit. Blank lines are ignored while original line numbers stay visible. The first 25 results are previewed; CSV and JSON downloads contain every processed row, including individual errors. CSV includes a formula-leading-cell apostrophe safeguard; JSON retains exact string values. All work stays in browser memory without uploads, persistent storage or query-string inputs. Changing the direction or data removes prior results and exports, preventing accidental reuse of a stale week number.
Worked example: New Year belongs to the previous ISO week-year
- Input
- 2016-01-01
- Result
- 2015-W53-5; Monday 2015-12-28 to Sunday 2016-01-03
The calendar year is 2016, but the ISO week-year is 2015.
Common use cases
- Resolve a New Year mismatch between a calendar date and an ISO week-year.
- Translate a supplier's YYYY-Www-D schedule into a readable date.
- Check whether a week 53 actually exists in a specified year.
- Export date-to-week mappings with explicit Monday–Sunday ranges.
Frequently asked questions
Why is January 1 sometimes in the previous week-year?＋
ISO week 1 contains January 4 and starts Monday. A few early-January dates can still fall in the last ISO week of the previous year. For example, 2016-01-01 is 2015-W53-5.
Does every year have week 53?＋
No. Most ISO week-years have 52 weeks. A week 53 request is checked against the actual year; 2020-W53-1 is valid, but 2021-W53-1 is not.
What does the final digit in an ISO week date mean?＋
It is weekday 1–7: Monday is 1 and Sunday is 7. The output 2015-W53-5 therefore identifies Friday of ISO week 53 in week-year 2015.
Can I use a Sunday-start or fiscal week convention?＋
Not on this page. It intentionally uses one explicit ISO convention. Another system's week number can differ even for the same date.
Do daylight saving or my local timezone change the answer?＋
No. These inputs are Gregorian calendar dates without clock times. The arithmetic does not convert an instant across timezones.
Related tools
Date Calculator
Add or count dates with custom weekends, excluded dates, endpoint rules and a CSV breakdown.
Printable Calendar Maker
Make year or month calendars for 1900–2200, choose Monday or Sunday starts and print or save as PDF.
Excel Date Converter
Convert spreadsheet serial dates and civil date-times with explicit date systems and row-by-row errors.
UK Bank Holidays
Find GOV.UK bank holidays for England and Wales, Scotland or Northern Ireland; export CSV or ICS.