Week Number Calculator
Calculate ISO 8601 week numbers for any date. Perfect for business planning, project management, and international scheduling with standardized week numbering.
Calculator Settings
Monday start; week 1 is the week containing 4 January (the week of the first Thursday). Equivalent to Excel WEEKNUM return type 21 and to ISOWEEKNUM.
Week Information
Week information will appear here
Select a date to calculate
Week Numbering Systems
- • ISO 8601: weeks start Monday; week 1 is the week containing 4 January (the week holding the year's first Thursday)
- • ISO week-year: a week belongs to the year owning its Thursday, so 29 Dec 2025 is week 1 of 2026 and 3 Jan 2021 is week 53 of 2020
- • 52 or 53: an ISO year has 53 weeks when 1 January is a Thursday, or a Wednesday in a leap year
- • US / North American: weeks start Sunday; week 1 is simply the week containing 1 January, so weeks 1 and 52/53 are usually partial
- • Excel WEEKNUM: return type 1 = Sunday start, 2 = Monday start, 11–17 set the start day Monday→Sunday, 21 = ISO 8601 (same as ISOWEEKNUM)
- • Simple systems can reach week 54: a leap year starting on the last day of week 1 (e.g. Sunday-start weeks in 2028) has 54 partial weeks
- • Quarter and leap year above follow the calendar year of the date, never the ISO week-year — those two can differ around New Year
- • Runs entirely in your browser: dates are never sent to a server, and all arithmetic is on plain calendar dates, so time zones and DST cannot shift a result
About Week Number Calculator
A week number calculator is a specialized utility that determines the ISO 8601 standardized week number for any given date, providing internationally recognized week numbering used in business and project management contexts. This tool accurately calculates which week of the year a specific date falls in, following the official ISO standard that ensures consistency across global organizations.
Why use a Week Number Calculator?
Week-based planning and reporting are essential in business, manufacturing, and project management, but manual week number calculations are complex due to varying year starts and ISO 8601 rules. This calculator eliminates confusion by providing accurate, standardized week numbers that ensure consistency in international business communications, scheduling, and reporting across different regions and industries.
Who is it for?
This tool is invaluable for project managers tracking weekly milestones, business analysts creating week-based reports, manufacturers following production schedules, logistics coordinators planning shipments, financial professionals preparing weekly analyses, and international teams needing standardized week references for coordination and communication.
How to use the tool
Pick a calculation mode: Date → Week Number to look up a date, or Week Number → Date to see which days a given week covers
Choose a week numbering system: ISO 8601 (Monday start, week 1 contains 4 January), US / North American (Sunday start, week 1 contains 1 January), or an Excel WEEKNUM return type — 2, 11 through 17, or 21 for ISO
In Date → Week Number mode, select a date with the picker or press Today to jump to the current local date
In Week Number → Date mode, enter a year and a week number; the tool returns that week's exact start and end dates and rejects week numbers the year does not have
Read the results panel: week number and week-numbering year, week start and end dates, day of week, day of year with days remaining, quarter and leap-year status for the calendar year
Compare conventions in the 'Same Date in Other Systems' block, which always shows the ISO 8601 and US week numbers and ranges for the same date
Open 'Show all weeks' for the full week table of that year, then copy or download the results as plain text for reports and documentation
Frequently Asked Questions
How do I find the week number for a date?
Select a date with the picker (or press Today) and the tool displays the ISO 8601 week number, the US / North American week number, the day of week, the day of year (1-366) with the days remaining, and the exact start and end dates of that week. A selector switches the headline result between ISO 8601, the US convention and the Excel WEEKNUM return types 2, 11-17 and 21. Everything runs in your browser on plain calendar dates — nothing is sent to a server, and no time zone or DST shift can move a result by a day.
What is ISO 8601 week numbering?
ISO 8601 is the international standard for week numbering. Weeks start on Monday. Week 1 is the week containing the first Thursday of the year (equivalently, the week containing January 4). Years can have 52 or 53 ISO weeks. Some early days of January may belong to week 52 or 53 of the previous year (if Mon-Thu fall in December); similarly, late December may belong to week 1 of the next year. ISO weeks are the standard in Europe, business reporting worldwide, and the ISO date format (e.g., '2026-W21-3' for Wed of week 21).
Why is the ISO week number different from US week numbers?
Two different conventions. ISO 8601 (most of the world, business): weeks start Monday, week 1 contains the first Thursday. US (common in spreadsheets and US wall calendars, and identical to Excel WEEKNUM return type 1): weeks start Sunday, week 1 is the week of January 1 regardless of which day January 1 falls on. The two can differ by one week, particularly in early January and late December. For business reporting and international work use ISO; for US consumer applications use the US convention. This tool shows both side by side for every date you enter.
What's special about week 53?
Most years have 52 weeks; some have 53 (about 71 of every 400 years). A year has 53 ISO weeks if Jan 1 falls on Thursday, OR if Jan 1 falls on Wednesday in a leap year. Examples: 2020 had 53 weeks; 2026 has 53 weeks; 2032 will have 53. Week 53 falls in late December. Year-on-year comparisons across 53-week years need adjustment (else you'd be comparing a 53-week year to a 52-week one). Financial calendars often use 4-4-5 or 13-week schemes to avoid this. The results panel always states the total weeks in the year you are looking at.
Which Excel WEEKNUM return types does this tool support?
All of the commonly used ones. Return type 1 (and 17) starts weeks on Sunday; 2 (and 11) on Monday; 12 Tuesday, 13 Wednesday, 14 Thursday, 15 Friday, 16 Saturday. For types 1-17, week 1 is simply the week containing January 1, so the first and last weeks are partial and a leap year can even reach week 54 — Sunday-start weeks in 2028 do. Return type 21 is the ISO 8601 rule, the same value Excel's ISOWEEKNUM returns. Pick the matching entry in the Week Numbering System selector and the headline week number reproduces the spreadsheet exactly.
How do I get the start and end dates of a given week?
Switch to Week Number → Date mode, enter the year and the week number, and the tool returns that week's exact start and end dates under the selected system (plus a year overview with the total number of weeks). In Date → Week Number mode the same range is shown for whichever date you entered. 'Show all weeks' lists every week of the year with its start and end dates. In code: Python `datetime.date.fromisocalendar(year, week, 1)` gives the ISO Monday, and date-fns exposes `setISOWeek()` / `startOfISOWeek()` in JavaScript.
What is the day-of-year (ordinal date)?
Day-of-year is the day number within the calendar year, from 1 (January 1) to 365 or 366 (December 31) — the ordinal date in ISO 8601, written 2026-140. May 20, 2026 is day 140. This tool shows it with the days remaining in the year, alongside the week number. It is used in scientific data, weather records, agriculture and aviation. Note the naming clash: 'Julian day' informally means this day-of-year, but the astronomical Julian Date is a continuous count of days since 4713 BC — a different number entirely, and not what this tool reports.
How do I do week-on-week or year-on-year comparisons?
For week-on-week: subtract 7 days from the current date and compare the same metric. For year-on-year: compare the same ISO week across years — but be careful with 53-week years, which have an extra week and shift everything after it. Some businesses use 4-4-5 fiscal calendars (quarters of 4-4-5 weeks = 52 weeks, with a 53rd week added periodically) to keep comparisons clean. Retail typically uses 4-4-5, tech uses calendar weeks, finance uses fiscal quarters. For the day counts between two specific dates, use the Date Calculator.
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