# Week Number Calculator Find the ISO 8601 week number and US week number for any date, with the full Monday-Sunday range and 53-week-year detection. --- - **Canonical URL:** https://dothecalculation.com/calculators/week-number-calculator - **Category:** Everyday utilities - **Publisher:** Do The Calculation (https://dothecalculation.com) - **Cost:** Free, no account or sign-up required - **Privacy:** Runs entirely in the browser; inputs are never sent to a server - **Methodology:** https://dothecalculation.com/methodology --- ## Find the ISO and US week number for any date See both the ISO 8601 week number (Monday-start, business standard) and the US week number (Sunday-start) side by side for any date. - ISO 8601 and US week numbers together - Full Monday-Sunday date range shown - Flags rare 53-week years automatically ## Two different systems share the same name 'Week number' means two different things depending on who's asking. The ISO 8601 international standard — used by most of Europe, project management tools, and software systems — starts weeks on Monday and defines Week 1 as the week containing the year's first Thursday (equivalently, the week containing January 4th). The US convention most American calendars and spreadsheets default to starts weeks on Sunday and simply counts Week 1 as the week containing January 1st, no matter which weekday that falls on. The two systems can disagree on the week number for the same date by exactly 1, and disagree on which year a late-December or early-January date belongs to — which is the source of most 'why does my spreadsheet show a different week number' confusion. ## Why ISO 8601 anchors to the first Thursday Anchoring Week 1 to the first Thursday (not simply January 1st) guarantees that most of Week 1's days fall within the new year — since a week containing a Thursday has at least 4 of its 7 days in the same month as that Thursday. This keeps the ISO week-numbering internally consistent every year without needing a special exception for years where January 1st falls near the end of a week. The practical effect: if January 1st falls on a Friday, Saturday, or Sunday, that day actually belongs to the LAST ISO week of the previous year, not Week 1 of the new year. ## Most years have 52 weeks, some have 53 A year has 52 weeks of 7 days each (364 days), leaving 1 or 2 extra days depending on leap years — so calendars can't divide evenly into 52 whole weeks every year. Under ISO 8601, a year gets a 53rd week specifically when January 1st falls on a Thursday, or when it's a leap year and January 1st falls on a Wednesday. That pattern makes roughly 71 years out of every 400-year cycle a 53-week year (about 1 in 5-6 years), not a fixed schedule — which is why this calculator checks the specific year rather than assuming 52. ## Worked example: the year-boundary edge case 2026 is a real example of the edge case above. January 1, 2026 falls on a Thursday, which means 2026 is a 53-week ISO year, and its ISO Week 53 runs from December 28, 2026 through January 3, 2027. That means January 1, 2027 — a date most people would assume is 'Week 1 of 2027' — is actually still ISO Week 53 of 2026. Anyone hard-coding 'week number resets to 1 every January 1st' into a report or spreadsheet formula would get this specific date wrong. ## When each system actually matters ISO 8601 week numbers show up in manufacturing date codes, European business reporting, ISO-compliant project timelines, and most programming language date libraries' built-in week functions. US-style Sunday-start week numbers show up in US payroll and timesheet systems, US retail 4-4-5 calendars (a variant), and default spreadsheet WEEKNUM() functions in some regional settings. If a report, contract, or system doesn't specify which standard it's using, cross-check both — they only agree exactly when January 1st falls on a Monday. ## Related calculators Need the day-of-year ordinal number instead of the week? Try the [Day of Year Calculator](/calculators/day-of-year-calculator). Comparing two dates directly, including full weeks and business days between them? See the [Date Calculator](/calculators/date-calculator) and [Business Days Between Dates Calculator](/calculators/date-difference-business-calculator). Counting down to a deadline that falls in a specific week? The [Days Until Calculator](/calculators/days-until-calculator) handles that from either direction. ## Frequently asked questions ### What week number is today? Enter today's date to see both the ISO 8601 week number (Monday-start) and the US week number (Sunday-start) — they're usually the same number but can differ by 1 depending on what day January 1st fell on. ### What's the difference between ISO and US week numbers? ISO 8601 weeks start on Monday and Week 1 is defined by the year's first Thursday. US-convention weeks start on Sunday and Week 1 is simply the week containing January 1st, whichever weekday that is. ### Can a date in January belong to the previous year's week number? Yes, under ISO 8601. If January 1st falls on a Friday, Saturday, or Sunday, that date is actually counted as the final week of the previous ISO year, not Week 1 of the new year. ### Do all years have 52 weeks? No. Most years have 52 ISO weeks, but years where January 1st falls on a Thursday (or on a Wednesday in a leap year) have 53 weeks — roughly 1 in every 5-6 years. ### Why does my spreadsheet show a different week number than this calculator? Spreadsheet WEEKNUM() functions often default to the US Sunday-start convention unless you specify the ISO mode explicitly, which can be off by 1 from the ISO 8601 number this calculator also shows. ### What date range is ISO week 1? ISO week 1 is always the week (Monday to Sunday) that contains January 4th of that year, which is also always the week containing the year's first Thursday. ### How many days are in one week number? Always 7 days, Monday through Sunday under ISO 8601, or Sunday through Saturday under the US convention — except the calendar year's first and last week numbers, which can be partial if the year doesn't start on the week's first day. ## Related concepts - **ISO 8601 week** — A Monday-start week where Week 1 is defined as the week containing the year's first Thursday (equivalently, containing January 4th). - **US week number** — A Sunday-start week convention where Week 1 is simply the week containing January 1st, regardless of weekday. - **53-week year** — An ISO 8601 year that gets an extra week because January 1st falls on a Thursday, or on a Wednesday in a leap year. ## Related guides - [How to Use Do The Calculation Calculators: A Practical Step-by-Step Guide](https://dothecalculation.com/blog/site-guides/how-to-use-calculators) — Learn the fastest reliable workflow for using Do The Calculation calculators, reading results, checking formulas, and using save, print, share, and export actions correctly. - [Understanding Calculator Formulas: How DTC Turns Inputs into Results](https://dothecalculation.com/blog/site-guides/understanding-calculator-formulas) — Understand how Do The Calculation formulas are presented, what the explanation blocks mean, and how to verify calculator logic before using a result in a real decision. ## Related calculators - [Day of the Year & Julian Date Calculator](https://dothecalculation.com/calculators/day-of-year-calculator) — Find the ordinal day number for any calendar date, compute Julian Date (JD), calculate remaining days in the year, and check if it is a leap year. - [Days Until Calculator](https://dothecalculation.com/calculators/days-until-calculator) — Find exactly how many days until or since any date, with a weeks-and-days total, a full calendar breakdown, and a weekday-versus-weekend split. - [Leap Year Calculator](https://dothecalculation.com/calculators/leap-year-calculator) — Check whether any year is a leap year under the full Gregorian rule, and find the next and previous leap years instantly. - [Age Calculator](https://dothecalculation.com/calculators/age-calculator) — Calculate your exact age in years, months, weeks, and days instantly by entering your birth date with this free online tool. - [Bill Split Calculator](https://dothecalculation.com/calculators/bill-split-calculator) — Split a bill evenly or by share among any number of people, including tip percentage and service charges, instantly for free. - [Binary to Text & ASCII Translator](https://dothecalculation.com/calculators/binary-translator-calculator) — Translate binary code to readable ASCII text and vice versa, with a full bit-grid visualization showing each character encoding. --- _Week numbers are calculated using the standard ISO 8601 algorithm (Monday-start, first-Thursday rule) and a simple US Sunday-start count. Some organizations use their own fiscal or retail calendars (like 4-4-5) that don't match either standard._ --- _Source: [Do The Calculation](https://dothecalculation.com/calculators/week-number-calculator). Quote freely with attribution and a link to this page._