Week Number Calculator
Find the week number for any date — or look up the exact dates for any week — using ISO 8601, US, or Simple systems.
Updated
What is the Week Number Calculator?
Week numbers give you a precise, universally understood shorthand for any seven-day period — no month-boundary confusion, no ambiguous date ranges. Whether a colleague says "ship it by W23" or a spreadsheet groups data by calendar week, this calculator instantly tells you what that means in real dates, and vice versa.
Paste in a date and get back its week number, the full Monday-to-Sunday (or Sunday-to-Saturday) range it falls in, what quarter it belongs to, how many weeks remain in the year, and the day of the year — all at once. Flip to Week → Dates mode to do the reverse: enter any year and week number and get the exact start and end dates.
Three numbering systems are supported side by side:
| System | Starts on | Week 1 definition | Used in |
|---|---|---|---|
| ISO 8601 | Monday | First week with a Thursday (= contains Jan 4) | Europe, international business, most software |
| US / Canada | Sunday | Week containing January 1 | North America, payroll, retail |
| Simple Monday | Monday | Week containing January 1 | Casual use, some Asian markets |
The year mini-calendar at the bottom lists every week in the year alongside its exact date range — useful when you need to scan a full quarter or hand a reference table to a colleague.
How it works
Date → Week Number
- Pick a date using the date picker, or hit Today to pre-fill the current date.
- Choose a system — ISO 8601 (default), US/Canada, or Simple Monday.
- Results appear instantly:
- Week number and week-year (ISO week-year can differ from the calendar year near New Year).
- Week start and end dates — the exact Monday–Sunday or Sunday–Saturday span.
- Day of year, quarter, and week within that quarter.
- Weeks remaining in the year.
- A plain-English summary line you can copy into a message or document.
ISO boundary tip: Under ISO 8601, Dec 29–31 can belong to Week 1 of the next year, and Jan 1–3 can belong to Week 52/53 of the previous year. The calculator detects this automatically and shows a warning so you're never caught off-guard.
Week Number → Dates
- Switch to the Week Number → Dates tab.
- Enter the year and week number. The hint beneath the week field shows the valid range for that year (52 or 53 weeks).
- Results show the start date, end date, and total weeks in that year.
Year Mini-Calendar
Click Show year calendar to see every week in the selected year as a scannable table — week number, start date, and end date. The current week is highlighted with a subtle tint; the selected week is highlighted in bold. Useful for printing a reference sheet or quickly spotting quarter boundaries.
How the maths works
ISO 8601 uses the Thursday rule: a week belongs to the year that contains its Thursday. Concretely:
Find the Thursday of the target week.
Week 1 of that Thursday's year starts on the Monday of the week containing Jan 4.
Week number = floor((target_date − week_1_monday) / 7) + 1
US / Canada counts from the Sunday before or on January 1:
jan1_day = day-of-week of Jan 1 (0 = Sun)
week = floor((day_of_year + jan1_day) / 7) + 1
Simple Monday is identical to US but shifts the anchor to Monday and always keeps the year as the calendar year — no cross-year edge cases.
Examples
What week number is July 4, 2025?
Find the ISO week number for a US holiday.
What dates does Week 1 of 2026 cover?
Reverse lookup — enter a week number to get the exact date range.
Does December 30, 2024 belong to 2024 or 2025?
ISO edge case — late-December dates can fall in the next week-year.
Week number using the US Sunday system
Switch to the US system for North American payroll or reporting.
Frequently asked questions
What week number is it today?
What week number is it today?
The calculator pre-fills today's date automatically — just open the tool and the current week number is shown instantly. Today is always highlighted in the year mini-calendar as well, so you can see at a glance where the current week sits relative to the rest of the year.
Why does my week number differ between systems (ISO vs US vs Simple)?
Why does my week number differ between systems (ISO vs US vs Simple)?
Different regions and industries adopted different rules for which day starts a week and what counts as "Week 1." ISO 8601 uses Monday as the first day and defines Week 1 as the first week containing a Thursday. The US/Canada system starts on Sunday and anchors Week 1 to whichever week contains January 1. These rules produce different numbers for the same date — especially in early January and late December. The tool lets you switch systems instantly so you can match whichever standard your organization uses.
Why does December 30 show as Week 1 of the next year under ISO?
Why does December 30 show as Week 1 of the next year under ISO?
Under ISO 8601, a week's year is determined by where its Thursday falls. If Dec 29–31 land on a Monday, Tuesday, or Wednesday, that week's Thursday is already in January of the new year — so the whole week belongs to the new year's Week 1. For example, December 30, 2024 (a Monday) is in ISO Week 1 of 2025 because that week's Thursday is January 2, 2025. The calculator detects this automatically and shows a warning.
Can a year have 53 weeks?
Can a year have 53 weeks?
Yes — under ISO 8601, a year has 53 weeks when January 1 is a Thursday, or when it's a leap year and January 1 is a Wednesday. This happens roughly 71 times every 400 years. The US and Simple systems can also produce a Week 53 depending on what day January 1 falls on. The tool always shows the correct total and validates your input against it.
What is the difference between ISO week-year and calendar year?
What is the difference between ISO week-year and calendar year?
The calendar year is simply the year on the Gregorian calendar (2025, 2026, etc.). The ISO week-year is the year that owns a given week under ISO 8601 rules. For dates near New Year, these two can differ by one. A date like January 1, 2016 (a Friday) belongs to ISO Week 53 of 2015, not Week 1 of 2016, because that week's Thursday was December 31, 2015. Always use the ISO week-year when writing ISO week notation — e.g. 2015-W53-5, not 2016-W53-5.
How do I find the start and end dates of a specific week number?
How do I find the start and end dates of a specific week number?
Switch to Week Number → Dates mode, enter the year and week number, and choose your system. The tool returns the exact start and end dates for that week. The valid range (52 or 53) is shown as a hint so you don't accidentally enter a week that doesn't exist in that year.
How do I calculate week numbers in Excel or Google Sheets?
How do I calculate week numbers in Excel or Google Sheets?
- ISO 8601: Use
=ISOWEEKNUM(A1)in Excel 2013+ or Google Sheets. This matches the ISO result from this calculator. - US system: Use
=WEEKNUM(A1, 1)(Sunday start) or=WEEKNUM(A1, 2)(Monday start, different from ISO). - Avoid using
WEEKNUMwhen you need ISO compliance — it does not follow the Thursday rule.
Why do project management tools sometimes show different week numbers than my calendar app?
Why do project management tools sometimes show different week numbers than my calendar app?
Most European calendar apps and project tools (Jira, SAP, many ERPs) follow ISO 8601. Most American apps default to the US Sunday system. A date in early January can be Week 1 in one system and Week 52 or 53 in another. Use this calculator to confirm which system your tool is using by checking a known date, then stick to that system consistently across your team.
What is week numbering used for in business?
What is week numbering used for in business?
Week numbers are widely used in manufacturing (production scheduling, delivery windows), retail (weekly sales reporting, promotional planning), logistics (shipment tracking, lead-time quoting), finance (fiscal week reporting, payroll cycles), and software development (sprint planning, release cadences). ISO 8601 week numbers are the international default; the US system is common in North American HR and retail.
How many weeks are in a year?
How many weeks are in a year?
Most years have 52 weeks plus one or two extra days. ISO 8601 years have either 52 or 53 complete weeks. The US and Simple systems can similarly produce a 53rd partial or full week depending on what day January 1 falls on. The calculator always shows the correct total for the selected year and system.
Can I use this for fiscal week numbers?
Can I use this for fiscal week numbers?
If your fiscal year aligns with the calendar year and uses ISO or US week numbering, yes — the results map directly. If your fiscal year starts on a different date (e.g. a company fiscal year starting in October), you would need to offset the result by the number of weeks between January 1 and your fiscal year start. That calculation is beyond the scope of this tool, but the date-range output gives you the raw building blocks to do it manually.
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