Week Number Calculator
Compute ISO 8601 week number + US week number for any date. Plus day-of-year, day-of-week, week-of-month, quarter, and ISO week date format (YYYY-Www).
Week Number Calculator
How to use the Week Number Calculator
Pick any date
Default: today. Click the date field to use your browser\'s native date picker. ISO 8601 format used internally; output shows multiple formats simultaneously.
Read ISO 8601 + US week numbers
ISO 8601: Mon-Sun weeks; week 1 = week containing year\'s first Thursday (or 4 January). Used in Europe, Australia, business worldwide. US system: Sun-Sat weeks; week 1 = week containing Jan 1. Used in the US + American-headquartered firms.
Check additional date metadata
Day of year (1-366), day of week, week of month, quarter, ISO 8601 week date format (e.g., 2025-W14), and the start/end dates of the ISO week. All useful for different scheduling, reporting, and date-arithmetic contexts.
Use ISO 8601 week format for technical contexts
"2025-W14" is unambiguous + sortable + universal. Use this when entering dates into spreadsheets, databases, APIs, or technical documents. For human communication, use the friendly format ("Week 14 of 2025, ending April 6").
Week numbering — the everyday standard most people don't know exists
Week numbers ("Week 14 of 2025") are widely used in European business + global software but largely unknown to American + Asian consumer audiences. The ISO 8601 standard defines week numbering precisely: weeks run Monday-Sunday; week 1 of each year is the week containing the year\'s first Thursday (equivalent: the week containing 4 January). This rule produces 52 or 53 weeks per year depending on the calendar arrangement. Some years have an extra "week 53" — 2026 is one of them. American business sometimes uses an alternative system where weeks run Sunday-Saturday and week 1 starts January 1. Knowing both conventions is essential for cross-border business work.
The 53-week year mystery
Most years have 52 weeks; some have 53. The extra week occurs in ISO 8601 when: (a) the year starts on a Thursday, or (b) it\'s a leap year that starts on Wednesday. Notable 53-week ISO years: 2004, 2009, 2015, 2020, 2026, 2032. Implications: retail + finance fiscal calendars often need an "extra week" adjustment every 5-6 years; payroll cycles paid weekly need 53 pay periods in a 53-week year; sprint counts in agile dev teams need a "Sprint 27" for the year. For systems that assume exactly 52 weeks per year (a common shortcut), the 53rd week causes bugs every few years.
Some years have 53 ISO weeks instead of 52. Famous "Sprint 53" bugs occur in agile dev teams every 5-6 years. 2026 is one of these years.
Year-boundary edge cases
ISO 8601 weeks can cross Gregorian year boundaries. Example: December 31, 2024 belongs to ISO week 1 of 2025 (it\'s a Tuesday, and the ISO week starts Monday Dec 30, 2024). Conversely, January 1, 2023 belonged to ISO week 52 of 2022 (it was a Sunday, ending the week that began Monday Dec 26, 2022). This is why ISO 8601 week dates always include the year explicitly: "2025-W01" is unambiguous; "Week 1" alone is not. For business reporting: always quote ISO year + week together ("Q1 2025, ISO Week 1"). For US-system reporting: "Week 1 of 2025" still works because US week 1 always starts January 1 — but cross-team confusion can occur if some readers assume ISO.
The ASEAN business calendar reality
Most ASEAN markets use Monday-start weeks in business + government contexts, aligning with ISO 8601 conventions. Singapore + Malaysia + Thailand + Vietnam + Indonesia + Philippines: all observe Mon-Fri working week with Sat-Sun weekend (some Muslim-majority states in Malaysia, particularly Kelantan + Terengganu, historically used Friday-Saturday weekend but most business operates Mon-Fri now). Cross-border ASEAN business: stick with ISO 8601 week notation in cross-border documents to avoid confusion with US-system reporting from American-HQ firms operating in the region. Fiscal year considerations: most ASEAN firms use Jan-Dec calendar year; some Japanese subsidiaries use Apr-Mar; some American-HQ subsidiaries use US fiscal calendars (Oct-Sep US government, various others). Fiscal week numbers within each firm\'s calendar may differ from ISO week numbers.
10 Things to Know About Week Numbers
ISO 8601: weeks run Mon-Sun. Week 1 = week containing year\'s first Thursday (or 4 January).
US system: weeks run Sun-Sat. Week 1 = week containing January 1. Different from ISO 8601.
Some years have 53 ISO weeks instead of 52. 2004, 2009, 2015, 2020, 2026, 2032 are 53-week ISO years.
ISO 8601 week format: "YYYY-Www" (e.g., 2025-W14). Sortable + unambiguous globally.
ISO weeks can cross Gregorian year boundaries: Dec 31, 2024 = ISO Week 1 of 2025.
European + Australian + Asian business commonly uses ISO 8601 week numbers; US business uses the Sunday-start variant.
Sprint planning in agile development: "Sprint 14 ends in week 28" — week numbers anchor planning across teams.
Retail + finance fiscal calendars often use 4-4-5 or 4-5-4 week patterns rather than calendar months for consistency.
Most ASEAN business uses ISO 8601 Monday-start weeks. Cross-border with US firms: clarify which system applies.
Excel + Google Sheets both support ISO + US week numbers via WEEKNUM() function with different mode parameters.
Frequently Asked Questions
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The ISO 8601 calendar must allocate 365 (or 366) days into complete Monday-Sunday weeks while keeping Week 1 containing the year\'s first Thursday. 52 weeks × 7 days = 364 days; leaves 1-2 days that need to belong to either the previous or next year\'s ISO week count. When the math works out a certain way, the year has a "53rd week." 53-week ISO years: 2004, 2009, 2015, 2020, 2026, 2032. Software that assumes exactly 52 weeks/year produces bugs every 5-6 years; 53-week-aware code is standard for fiscal + payroll + sprint-planning systems.
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Depends on context. ISO 8601 for: European + Australian + Asian + cross-border business; software + databases; agile sprint planning; international standards. US system for: American-HQ firms operating with US-based teams; US retail + finance fiscal reporting. Both work — but cross-system confusion occurs. Best practice for cross-border: state which system explicitly ("ISO Week 14" vs "US Week 14"). When in doubt, use ISO 8601 + write out "YYYY-Www" format.
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The tool shows the ISO week\'s start (Monday) + end (Sunday) for any date you pick. For sprint planning: enter the sprint review date, get the week\'s Mon-Sun bounds. For weekly reports: enter the reporting date, get the period\'s actual calendar range. The "week boundaries" row shows ISO week 1 start + end dates explicitly.
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ISO 8601 weeks can cross Gregorian year boundaries. If December 31 falls on a Monday-Wednesday, it belongs to ISO Week 1 of the NEXT year (because that week\'s Thursday is in the new year). Conversely, if January 1-3 fall on Friday-Sunday, they belong to ISO Week 52 or 53 of the PREVIOUS year. Why: ISO weeks anchor to Thursdays, not to calendar year boundaries. This is the whole point of using ISO year + week together (2025-W01 is unambiguous; "Week 1" alone could refer to two different periods near year boundaries).
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For ISO-aligned fiscal calendars, yes. For custom fiscal calendars (4-4-5, 13-period accounting, broadcast calendar), no — these use proprietary period definitions that don\'t match ISO weeks. Retail firms often use a 4-4-5 calendar (each quarter = 4 weeks + 4 weeks + 5 weeks). US broadcast calendar uses 52 weeks of 7 days starting Monday. Custom fiscal calendars require company-specific lookups; this tool covers standard ISO + US systems only.
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Excel\'s
WEEKNUM(date, return_type)takes a "return_type" parameter that selects week-numbering systems. Return_type 1 or omitted: Sunday-start, week 1 contains Jan 1 (US system, this tool\'s "US Week"). Return_type 21: ISO 8601 (this tool\'s "ISO Week"). Other return_types: 2 = Monday-start with week 1 containing Jan 1 (not ISO 8601); 11-17 = various Monday-Sunday variants. For ISO 8601, always use Excel\'s ISOWEEKNUM() function or WEEKNUM(date, 21). -
US TV + radio broadcasting use a 52-week calendar starting Monday, where each broadcast month is 4 or 5 weeks. Broadcast Week 1 always starts on the Monday on or before January 1. Different from ISO 8601 (which uses Thursday rule) + US system (Sunday-start). Used primarily in US media advertising contexts; rarely relevant for ASEAN audiences. If you encounter "broadcast week" terminology, it\'s a specific industry convention separate from this tool.
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Calendar months vary 28-31 days — comparing March to February revenue is unfair (March has more days). Retail + finance fiscal calendars use 4-4-5 weeks (each quarter = 4 + 4 + 5 weeks = 91 days) for consistency. Three of four months are 4 weeks (28 days), one is 5 weeks (35 days). This makes period-over-period comparisons mathematically clean. Quarterly: 91 + 91 + 91 + 92 (or 91 + 91 + 91 + 91 + 1 extra in some calendars) = 365-366 days. The "extra week" every few years (53-week year) is the 53rd week added to one of the quarters.
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No. All calculations run in your browser via JavaScript. Open DevTools → Network and confirm zero outbound requests. Date stays on your device. Safe for confidential project + business date calculations.
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Pair with: Date Difference Calculator (RT-CNV-085) for between-dates math; Working Days Calculator (RT-CNV-086) for business-day arithmetic; Date Add/Subtract Calculator (RT-CNV-087) for date arithmetic; Time Duration Calculator (RT-CNV-089) for hour/minute math. External: Excel\'s ISOWEEKNUM() function; ISO 8601 standard documentation; broadcast calendar resources for US media contexts.
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