📅 Date & Time

Calendar Week Number

Find the ISO week number for any date, or look up which dates fall in a given week. Current week number shown live. Full-year calendar with all week numbers. Europe (ISO) vs US comparison. ISO 8601 week date format output.

Live current week number ISO 8601 & US week systems Full-year week calendar ISO week date format (YYYY-Www-D)

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ISO WeekEurope / intl

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US WeekSun start

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Week datesMon – Sun

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ISO 8601YYYY-Www-D

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Day of yearordinal date

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QuarterQ1–Q4

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Weeks leftin year

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Try:

Enter a year and week number to find the dates of that week.

Year

Week #

Try:

2026

Week	Start date	End date	ISO code	Days

Today: Year:

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Features

Full-year week calendar, ISO 8601 format, Europe vs US — what free tools skip

Most week number tools give you a single number for a date. This tool adds the full-year calendar showing every week with its dates, the ISO 8601 week date format used in logistics and supply chain, a live current week display, and a direct comparison of ISO (Monday start) vs US (Sunday start) numbering.

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Full-year week calendar

The Full Year tab shows every week of any year with start date, end date and ISO code. Current week highlighted. Toggle between ISO (Monday start) and US (Sunday start). Switch years with navigation arrows. 53-week years flagged. Absent on all major free tools.

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ISO 8601 week date format

Every date result includes the ISO 8601 week date notation: YYYY-Www-D. For example, 12 April 2026 = 2026-W15-7. This format is used in logistics, supply chain, manufacturing and fiscal planning where “week 15” is more useful than a calendar date.

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Europe vs US week comparison

ISO weeks start Monday and Week 1 is the week containing the first Thursday. US weeks start Sunday and Week 1 contains January 1. Both are shown simultaneously. Select “Show both” to see where they differ — especially important for dates near year boundaries.

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Live current week display

The top of the Date → Week tab shows the current ISO week number, today’s date, the full Mon–Sun span of the current week, and the ISO 8601 week date. Updates automatically on page load without any user action.

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Week → Dates lookup

Enter a year and week number to find the start and end dates of that week. Useful for interpreting sprint numbers (“when is sprint week 23?”), logistics schedules and fiscal calendars. Validates that the requested week exists in the year (53-week check).

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Year progress & weeks remaining

For any date, shows the day of year (e.g. day 102 of 365), the quarter (Q1–Q4), weeks remaining in the year, and a visual progress bar showing how far through the calendar year the date falls. Useful for project planning and year-end deadlines.

How to use

How to find a week number

Date → Week: pick any date

Use the date picker in the Date → Week tab. The ISO week number, US week number, Mon–Sun date span, ISO 8601 week date format, day of year, quarter and weeks remaining all update instantly. Use the quick-try buttons for year-boundary dates like 1 Jan 2026 where ISO and calendar year differ.

Choose ISO or US system

Select “ISO 8601 (Mon start — Europe)” for international use. Select “US (Sun start — Americas)” for North American calendars. Select “Show both” to see where the two systems give different numbers — particularly important for Sundays and dates near 1 January.

Week → Dates: look up a week number

Switch to the Week → Dates tab. Enter the year and week number (e.g. 2026, W15). Click Find dates to see the Monday and Sunday of that week, the ISO date range, and a note on whether the year has 52 or 53 weeks. Try “2026 W53” — 2026 is a 53-week year.

Full Year tab: browse all weeks

The Full Year tab shows a complete table of every week in any year. Use the ‹ › arrows to change year. The current week is highlighted in indigo. 53-week years show a badge. Use this to plan sprint schedules, fiscal calendars or to check which dates fall in which week across a whole year.

📊 Comparison

LazyTools vs other week number tools

Most week number tools show one result for one date. The full-year calendar, Week → Dates reverse lookup and ISO 8601 week date format are the three differentiators absent from all major free tools.

Feature	⭐ LazyTools	EpochConverter	whatweekisit.org	weeknumber.net
Current week shown live	✔	✔	✔	✔
Date → Week number	✔	✔	✔	✔
ISO 8601 week date format (YYYY-Www-D)	✔	✘	✘	✘
Europe (ISO) vs US comparison	✔ Side-by-side	⚠ Separate pages	✘	⚠ Separate tools
Week → Dates reverse lookup	✔	✘	✘	✔
Full-year calendar (all weeks)	✔ Interactive	⚠ Static page only	✘	✘
53-week year detection	✔	✔	✘	✔
Year progress bar & weeks remaining	✔	✘	✘	✘
No ads / no signup	✔	⚠ Ads	✔	⚠ Ads

Comparison based on publicly available features as of April 2026.

Quick reference

ISO weeks for key dates — 2025–2027

Date	ISO week	ISO year	ISO 8601 code	Note
29 Dec 2025	W01	2026	2026-W01-1	First Monday of ISO year 2026 — still December 2025
1 Jan 2026	W01	2026	2026-W01-4	Thursday → anchors Week 1 of 2026
4 Jan 2026	W01	2026	2026-W01-7	Sunday → last day of Week 1
5 Jan 2026	W02	2026	2026-W02-1	Monday → Week 2 begins
27 Dec 2026	W53	2026	2026-W53-7	2026 has 53 ISO weeks — last day of W53
28 Dec 2026	W53	2026	2026-W53-1	Monday but still ISO year 2026 (W53)
4 Jan 2027	W01	2027	2027-W01-1	First Monday of ISO year 2027
1 Jan 2027	W53	2026	2026-W53-5	Friday → belongs to previous ISO year’s W53

ISO vs US week numbering — key differences

Property	ISO 8601 (Europe/International)	US System
Week starts	Monday	Sunday
Week 1 definition	Week containing first Thursday of year	Week containing 1 January
Partial weeks	Never — every week is Mon–Sun	First/last weeks may be partial
53-week years	When Jan 1 is Thursday (or leap year Wed)	When Jan 1 is Sunday
Year boundary	ISO year can differ from calendar year	Always matches calendar year
Used in	EU, UK, most of Asia, international business	USA, Canada, most of Latin America

Complete guide

ISO Week Numbers — A Complete Guide to Calendar Weeks, ISO 8601 and Europe vs US Systems

A week number is a number from 1 to 52 (or 53) that identifies a specific seven-day period in a calendar year. Week numbers are used extensively in business, manufacturing, logistics, project management and government reporting — anywhere it is more convenient to say “Week 14” than to name the exact dates. The two main week numbering systems in use globally are ISO 8601 (dominant in Europe and most of the world) and the US system (used in North and Latin America).

What ISO week number is today free

The ISO week number for today is shown live in the banner at the top of the Date → Week tab. ISO weeks are defined by the ISO 8601 international standard: a week always starts on Monday and ends on Sunday, and Week 1 is the week that contains the year’s first Thursday. Equivalently, Week 1 always contains January 4th. This means some days at the very start or end of a calendar year have an ISO week number that belongs to a different year — for example, December 29, 2025 is the start of ISO Week 1 of 2026.

Calendar week number for any date online

To find the ISO week number for any date: enter the date in the date picker above. The tool returns the ISO week number, the ISO year (which may differ from the calendar year for dates near January 1), the Monday–Sunday span of the week, and the ISO 8601 week date format. The ISO 8601 week date format is written as YYYY-Www-D, where Www is the two-digit week number prefixed by W, and D is the weekday number (1=Monday, 7=Sunday). For example, Sunday 12 April 2026 is 2026-W15-7.

Full year week number calendar 2025 2026

The Full Year tab shows a complete calendar of all weeks in any year. 2025 has 52 ISO weeks. 2026 has 53 ISO weeks — the extra week occurs because January 1, 2026 is a Thursday, which anchors Week 1, and the year is long enough for a 53rd week to appear at the end. 53-week years occur approximately every 5–6 years. Recent examples: 2015, 2020, 2026, 2032. ISO Week 53 of 2026 starts on Monday December 28, 2026 and ends on Sunday January 3, 2027.

Week number Europe vs US

The key difference between European (ISO) and US week numbering is the start day and the definition of Week 1. ISO weeks start on Monday; US weeks start on Sunday. In the ISO system, Week 1 is the week containing the first Thursday of the year. In the US system, Week 1 is simply the week containing January 1. This means January 1 is always in US Week 1, but it may be in ISO Week 52 or 53 of the previous year if it falls on a Friday, Saturday or Sunday. For most dates in the middle of the year, ISO and US week numbers are either identical or differ by one.

Week 1 of 2026 start date

ISO Week 1 of 2026 starts on Monday, December 29, 2025 and ends on Sunday, January 4, 2026. This is because January 1, 2026 is a Thursday — the Thursday anchors Week 1, and the week always starts on the preceding Monday. The ISO date for the first day of Week 1 of 2026 is 2026-W01-1. For the US system, Week 1 of 2026 starts on Sunday, January 4, 2026 (the Sunday on or before January 1) and ends Saturday, January 10, 2026.

52 or 53 weeks in a year

Most years have 52 ISO weeks. A year has 53 ISO weeks when January 1 falls on a Thursday, or when January 1 falls on a Wednesday in a leap year. The technical reason: with 52 complete weeks, 52×7 = 364 days, but a Gregorian year has 365 (or 366) days, so one or two days “spill over” into a 53rd week. Whether those extra days create a 53rd week or fall into Week 1 of the following year depends on the day of the week January 1 falls on. Years with 53 ISO weeks in the 2020s: 2020, 2026, 2032.

Frequently asked questions

What week number is it this week?

The current ISO week number is shown live in the purple banner at the top of the Date → Week tab above. It updates automatically when the page loads. The current week as of today (Tuesday, 14 April 2026) is ISO Week 16 of 2026, running from Monday 13 April to Sunday 19 April 2026.

What is an ISO week number?

An ISO week number is a number from 1 to 52 (or 53) assigned to each week of the year according to the ISO 8601 international standard. ISO weeks always start on Monday and end on Sunday. Week 1 is defined as the week that contains the year’s first Thursday — equivalently, the week that contains January 4th. The standard is used in most of Europe, the UK, Asia and international business contexts.

Why does December 29 belong to Week 1 of the next year?

Because ISO weeks always start on Monday and run to Sunday, and Week 1 is anchored to the first Thursday of the new year. If January 1 is a Thursday (as in 2026), then Monday December 29 is the first day of that week — which is Week 1 of the new year, even though December 29 is still in the old calendar year. The ISO year number can be one ahead of the calendar year for dates at the very end of December.

What is the ISO 8601 week date format?

The ISO 8601 week date format expresses a date as YYYY-Www-D in extended form, or YYYYWwwD in compact form. YYYY is the ISO week year (which may differ from the calendar year). Www is the two-digit week number, always prefixed by “W”. D is the weekday number: 1=Monday, 2=Tuesday, … 7=Sunday. Example: Sunday 12 April 2026 = 2026-W15-7. This format is used in logistics, supply chain, sprint planning and date/time libraries (Python’s isocalendar(), SQL’s EXTRACT(WEEK)).

How many weeks are in 2026?

2026 has 53 ISO weeks — Week 53 runs from Monday December 28, 2026 to Sunday January 3, 2027. A year has 53 ISO weeks when January 1 falls on a Thursday (as in 2026). Most years have 52 weeks. Other recent 53-week years: 2015, 2020, 2032.

What is the difference between ISO week and US week?

ISO weeks start on Monday; US weeks start on Sunday. ISO Week 1 is the week containing the first Thursday; US Week 1 contains January 1. The ISO year can differ from the calendar year for dates near January 1; the US system always uses the calendar year. For most dates mid-year the numbers match or differ by one. Use the “Show both” option in the Date → Week tab to see both values for any date.

When does Week 1 of 2027 start?

ISO Week 1 of 2027 starts on Monday, January 4, 2027 and ends on Sunday, January 10, 2027. January 1, 2027 is a Friday — it belongs to ISO Week 53 of 2026, not Week 1 of 2027. The week containing January 7 (the first Thursday of 2027) anchors Week 1. Enter “1 Jan 2027” in the tool above to see this immediately.

How are week numbers used in business?

Week numbers are used heavily in agile development (sprint planning by week), logistics and supply chain (delivery schedules, inventory cycles), manufacturing (production batch codes), accounting and fiscal reporting (fiscal week calendars), EU labour law (holiday entitlements calculated in weeks), and construction project management (Gantt charts by week number). The ISO 8601 standard ensures international teams can communicate unambiguously about “Week 23” without needing to specify exact dates.

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