Free Week Number Calculator — ISO 8601 + US (Sunday-start) Conventions

What This Calculator Does

The Week Number calculator answers a deceptively simple question: which week of the year is this?Drop in any Gregorian date and the tool returns the week number under both major conventions in parallel — ISO 8601 (the international standard, used by most of Europe, Asia, Australia, and much of Latin America) and the US Sunday-start convention (the default in Microsoft Excel, most American payroll systems, and the calendars hanging on most American refrigerators). It also gives you the day-of-year (a number from 1 to 365 or 1 to 366 in leap years) and the start-and-end dates of the week under each system.

Two systems exist because two different organising principles are both reasonable. ISO 8601 starts every week on Monday and defines Week 1 as the week containing the year’s first Thursday — equivalently, the week containing January 4th. The US convention starts every week on Sunday and defines Week 1 as whichever week contains January 1st, regardless of how few days of January it actually holds. Most days of the year, the two systems agree. Around the New Year boundary — late December and early January — they routinely disagree by one number, and occasionally disagree about which calendaryear a week belongs to. That boundary is where 90% of week-number bugs live.

The most common uses are exactly what you would guess: filing tickets in support systems that timestamp by ISO week (“reproduced in W17”), reading European delivery schedules quoted in week numbers (“ships in CW23”), reconciling a fiscal calendar against a Gregorian one, matching WEEKNUMoutput between an Excel file authored in the US and a colleague’s Excel set to a German locale, and figuring out why your project-management tool labels today as Week 17 while the spreadsheet next to it insists on Week 18. The worked examples below cover the three regimes the calculator handles — a simple agreement case, a hostile boundary case, and a mid-year case — plus the exact arithmetic behind each one.

The Two Systems Explained

Before the worked examples, it helps to lock in the rules. Both systems agree that a week is seven days and that the year contains either 52 or 53 of them. They disagree on everything else.

The Thursday rule in ISO 8601 is not arbitrary — it is the rule that produces a clean “majority of the week falls in the new year” result. A week running Monday through Sunday has its midpoint on Thursday; if Thursday is in January, the week is mostly in January, and ISO calls it Week 1. If Thursday is in December, the week is mostly in December, and ISO assigns it Week 52 or 53 of the prior year. The US rule is cruder but operationally simpler: whatever week January 1 falls in is Week 1, full stop. Both rules are internally consistent; they just answer slightly different questions about what “the first week of the year” means.

How to Use This Calculator

1. Enter the date in YYYY-MM-DDformat (or use your browser’s native date picker, which writes that format for you). Any valid Gregorian date works — past, present, or future.
2. Hit Calculate. The output gives you the ISO 8601 week number (with its ISO year, which may differ from the calendar year near year boundaries), the US week number, and the day of year (a 1–365 or 1–366 integer).
3. Read the week start and end datesunder each system. The ISO row shows a Monday→Sunday span; the US row shows a Sunday→Saturday span. The two spans are usually offset by one day.
4. Compare the two week numbers. Most of the year they will be equal or off by one. If they disagree by more than one — or if the ISO year disagrees with the calendar year — you are in a boundary case and should be deliberate about which convention you cite downstream.
5. Re-run the calculator for the start and end of any project window. If your project spans a New Year boundary, you will almost certainly need both week numbers in different conversations: ISO for European stakeholders, US for American payroll, and an honest note about which one you are quoting at any given moment.

Three Worked Examples

Specific dates — copy any of them into the calculator above to verify. These walk through the three regimes the tool handles: a clean agreement case, a hostile cross-boundary disagreement, and a mid-year case where everything lines up.

Example 1 — January 1, 2026 (the agreement case)

Today: 2026-01-01, a Thursday. Under ISO 8601, the week containing this date runs Monday December 29, 2025 through Sunday January 4, 2026— and because that week contains January 4 (and the first Thursday of 2026, which is January 1 itself), it is ISO Week 1 of 2026. Under the US convention, the week runs Sunday December 28, 2025 through Saturday January 3, 2026 — and because it contains January 1, it is also US Week 1 of 2026. Both systems agree: Week 1. The day of year is 1 of 365. This is the clean case — despite the two different week-start days and the two different boundary rules, the answer comes out the same. Most of the year looks like this. The two examples below are the trouble spots.

Example 2 — December 30, 2024 (the boundary disagreement)

Date: 2024-12-30, a Monday. Under ISO 8601, the week starting that Monday runs Monday December 30, 2024 through Sunday January 5, 2025. Because this week contains January 4 (it actually contains January 1 through 5), ISO assigns it Week 1 — but Week 1 of which year? Of 2025, because the majority of the week is in 2025. So this date in late December 2024 carries the ISO label 2025-W01: ISO year 2025, ISO week 1, even though the calendar year is unambiguously 2024. Under the US convention, the same date sits in the week running Sunday December 29, 2024 through Saturday January 4, 2025; because that week contains December 30 (well before the new year) and not January 1 of 2025, the US system labels it Week 53 of 2024. So the same physical Monday is ISO 2025-W01 and US 2024-W53simultaneously. Two systems, two years, two week numbers, one calendar day. This is the case that breaks spreadsheets, Jira filters, and timezone-blind “what week is it?” queries every December and January.

Example 3 — April 25, 2026 (today, mid-year)

Today: 2026-04-25, a Saturday. ISO 8601 puts this in the week running Monday April 20, 2026 through Sunday April 26, 2026, which is ISO Week 17 of 2026. The US convention puts it in the week running Sunday April 19, 2026 through Saturday April 25, 2026, which is also US Week 17 of 2026. Both systems agree on the number and the year — only the start-and-end dates of the week differ by one day. The day of year is 115 of 365 — about 31.5% of the way through 2026. This is what most of the year looks like once you are clear of the January boundary: minor cosmetic differences in week-start day, but identical week numbers. Note also that 2026 is one of the rare 53-week ISO years— the previous one was 2020, the next one is 2032, and the cadence is roughly every 5–6 years. So if you are building a yearly report that bins by week, allocate space for Week 53 in 2026 even though most years stop at 52.

Common Mistakes

• Assuming the two systems always agree.They agree on most days of the year — usually April through November. They diverge predictably around the New Year boundary, and they sometimes diverge by one across the rest of the year too. Never quote a week number to an international audience without specifying ISO or US; “Week 23” is ambiguous on its own.
• Expecting Week 1 to always start on January 1 in ISO.Under ISO 8601, Week 1 is the week containing January 4 (or equivalently the first Thursday). Depending on what day of the week January 1 falls on, ISO Week 1 can start as early as December 29 of the previous year and as late as January 4 of the new year. The first few days of January — January 1, 2, or 3 — sometimes belong to Week 52 or 53 of the previous ISO year, not Week 1 of the new one. This breaks intuitions that were built on US-style calendars.
• Not handling 53-week years. ISO 8601 occasionally has 53 weeks: 2004, 2009, 2015, 2020, 2026, 2032 are recent and upcoming examples. If your dashboard, BI tool, or fiscal-period table assumes a flat 52 weeks per year, every 53-week year produces an off-by-one error somewhere downstream — usually a missing row, a duplicated row, or a YTD figure that silently excludes the last week of December. Stress-test reports against 2020 and 2026 specifically.
• Using US weeks in international correspondence.Most of Europe, Asia, Australia, and Latin America defaults to ISO. SAP, Outlook in EU locales, German factories, Scandinavian logistics firms, and most enterprise software outside North America assume ISO weeks. Quoting a US Week 1 to a Berlin-based supplier in early January is a near-guaranteed scheduling miscommunication — their Week 1 may not even cover the same days. Always specify ISO when the recipient might not be in North America.
• Ignoring ISO year vs calendar year at boundaries. December 30, 2024 has ISO year 2025 and calendar year 2024. December 31, 2018 has ISO year 2019 and calendar year 2018. January 1, 2017 has ISO year 2016 and calendar year 2017. If you are building a key like YYYY-Wxx from one system but reading YEAR(date) from another, the few days around January 1 can produce keys that point to the wrong bucket. Always pair the ISO week with the ISO year, never with the calendar year.
• Treating week number as timezone-aware.The week number is a property of a calendar date, not a timestamp. A meeting at 11:00 PM Pacific on a Sunday is the same UTC moment as 7:00 AM Monday in London — but the week number you assign depends entirely on which date you anchor the moment to. For week-binning across time zones, decide upfront whether you are using local-time dates or UTC dates, and apply that decision consistently across every system.
• Confusing week-of-month with week-of-year.“The third week of April” usually means the third Monday-or-Sunday-starting week within April, which is a different concept entirely from the ISO or US week-of-year number. This calculator computes week-of-year only; week-of-month math is a separate (and locale-specific) calculation.

When This Calculator Decides For You

Week-number questions feel like trivia until they cost you a deadline, a delivery, or a reconciled report. The most common decisions the headline answers:

1. Filing international support tickets that reference ISO week.A vendor replies “the bug was reproduced in W17 and patched in W18.” You need to know exactly which seven calendar days each label covers to judge whether the patch predates or postdates your incident. Run the dates through the calculator under ISO (the vendor’s convention if they are non-US) and you have an exact mapping between the week label and the calendar window.
2. Reading European delivery schedules.German manufacturers, Dutch logistics firms, and most European wholesalers quote shipment dates as “CW23” or “KW23” (Kalenderwoche 23, ISO calendar week 23). Convert that to a Monday–Sunday range with this calculator and you can integrate the schedule into a US-style date-driven plan without manually counting weeks. The reverse case — quoting a delivery to a European supplier — is just as important; tell them the calendar dates and the ISO week number, and you remove an entire class of miscommunication.
3. Matching Excel WEEKNUM output between locales.The same spreadsheet opened on a US Excel install and a German Excel install can produce different week numbers from the same date column — not because of a bug, but because WEEKNUM’s default behaviour is locale-sensitive. The fix is to switch to ISOWEEKNUM(date) or WEEKNUM(date, 21)everywhere, run a baseline through this calculator, and confirm both files now produce identical numbers. Same trick works for Power BI (use WEEKNUM(date, 21) in DAX), PostgreSQL (TO_CHAR(date, ‘IW’) for ISO), and most other date-aware analytics tools.
4. Setting fiscal-quarter boundaries.Many corporate fiscal calendars align quarters to ISO week boundaries: a quarter starts on a specific Monday and ends 13 weeks later on a Sunday. Setting those boundaries by guessing dates is how you end up with a Q1 that has 12 weeks and a Q2 that has 14. Anchor the boundary to an ISO week number, look up the Monday with this calculator, and your fiscal periods come out clean every year — including the 53-week years where one quarter has to carry the extra week.
5. Aligning with project-management tools like Linear, Jira, or Asana. Many tools have a “current week” or “week view” that defaults to either Sunday-start or Monday-start without making the choice obvious. If your team’s burn-down chart and your manager’s status report disagree about which day a sprint started, this calculator is the tiebreaker: identify which convention each tool is using, pick one as the team standard, and configure the rest to match. The hidden cost of two tools using different week conventions is roughly one off-by-one bug per quarter; pinning the convention up front avoids the recurring tax.

What This Calculator Doesn’t Model

Honest scope keeps the tool useful. Five things this calculator deliberately does not do:

• Fiscal-year week numbering.Many retailers and large enterprises run a 4-4-5 or 4-5-4 fiscal calendar with weeks numbered relative to the fiscal year start (often the Sunday closest to February 1, or the first Monday of the fiscal year). This calculator does not know your fiscal calendar — it only computes ISO and US week-of-Gregorian-year. For fiscal week numbers, anchor your fiscal-year start to a specific Monday (using this calculator’s ISO output to find it), then count forward by 7-day increments yourself.
• Non-Gregorian calendars. Hijri (Islamic), Hebrew, Persian Solar Hijri, Buddhist, and Lunar New Year-based calendars are not supported. The week-numbering math assumes a Gregorian year of 365 or 366 days starting on January 1. For week numbers in a Hijri context (where weeks are conventionally Saturday-start in Saudi Arabia and some Gulf states), you need a calendar-aware tool, not this one.
• Saturday-start week conventions.Some Islamic countries — historically Saudi Arabia, parts of the Gulf, traditional usage in Egypt and the Levant — treat Saturday as the first day of the week, with Friday as the rest day. The calculator only models Monday-start (ISO) and Sunday-start (US). For a Saturday-start week-of-year computation, convert your date to ISO, then mentally shift week starts by two days — or use a region-specific calendar tool.
• Week-of-month calculations.“The third week of April” or “the second Tuesday of October” (used in US federal holiday rules, recurring meeting schedules, and church calendars) is a different and locale-sensitive calculation. It depends on whether you count the first partial week as Week 1 or Week 0, and whether weeks start Sunday or Monday. This tool only handles week-of-year.
• Date arithmetic across DST or time-zone shifts.The week-number computation is purely a function of a calendar date — the day, month, and year. It does not consult time zones, daylight-saving transitions, or UTC offsets. If your source system stores instants (timestamps with offset), convert them to your target time zone’s local date first, then look up the week number. Trying to assign a week number to a UTC instant directly will produce off-by-one errors at midnight boundaries.

For other date and time math — counting calendar or business days between two dates, ticking down to a future deadline, computing an exact age in years/months/days, or adding-and-subtracting days from a fixed date — pair this calculator with the rest of the toolkit on the date and time calculators page. The days between dates calculator gives you the gap between two arbitrary dates measured seven ways at once; the days until calculator handles the single-target countdown case; the age calculator decomposes a span into the years/months/days form humans actually use; and the date add/subtract calculator lets you walk forward or backward from a fixed anchor by an arbitrary number of days, weeks, months, or years. Use the week-number tool when the question is “which week?” and one of the others when the question is “how long?” or “what date?” instead.