Week Number Calculator
Find the ISO 8601 week number for any date
📅 Pick a date
🔢 ISO week number
📊 At a glance
🗓️ The seven days of week 25
| Day | Date | ISO weekday |
|---|---|---|
| Mon | Monday, June 15, 2026 | 1 |
| Tue | Tuesday, June 16, 2026 | 2 |
| Wed | Wednesday, June 17, 2026 | 3 |
| Thu | Thursday, June 18, 2026 | 4 |
| Fri | Friday, June 19, 2026 | 5 |
| Sat | Saturday, June 20, 2026 | 6 |
| Sun | Sunday, June 21, 2026 | 7 |
ISO 8601 weeks always run Monday (1) through Sunday (7). The highlighted row is your selected date.
Week numbers follow the ISO 8601 standard (weeks start Monday; week 1 contains the first Thursday of the year). This differs from the US convention, where weeks often start Sunday and week 1 is the week containing January 1.
Last updated June 2026
Method: Week numbers follow the ISO 8601 standard - weeks run Monday to Sunday, and week 1 is the week containing the year's first Thursday (equivalently, the week with January 4). Calculations use the browser's native date engine.
Included: ISO week number, the ISO week-year, the Monday-to-Sunday date range, the day of the week (1-7), day of the year, and the total weeks (52 or 53) in that year.
Not included: The US Sunday-start week convention, fiscal/retail (4-4-5) calendars, and time-zone shifting. Use the ISO result for business, ERP, and international scheduling.
Week number calculator: everything you need to know
If a colleague in Berlin asks you to ship something "by week 24" and you are sitting in Chicago wondering what that means, you have run into the gap this week number calculator closes. Most of the world - and almost every business, manufacturing, and ERP system outside the United States - schedules by ISO 8601 week numbers rather than calendar dates. This tool turns any date into its ISO week, tells you which week-year it belongs to, and shows the exact Monday-to-Sunday range so "week 24" stops being a guessing game.
By default the calculator loads today's date, so the moment the page opens you can answer the question "what week is it?" Change the date and every figure updates instantly: the headline week number, the week-year, the seven days of that week, and how many weeks remain in the year.
What is an ISO 8601 week number?
ISO 8601 is the international standard for representing dates and times. Its week-numbering rules are deliberately unambiguous so that two systems anywhere in the world agree on which week a date belongs to. There are only two rules to remember:
- Weeks start on Monday and end on Sunday. Every ISO week is exactly seven days.
- Week 1 is the week that contains the year's first Thursday. An equivalent way to say it: week 1 is the week that contains January 4, or the first week with the majority of its days in the new year.
Because the rule pins to Thursday (the midpoint of a Monday-Sunday week), the year that "owns" a week is the calendar year of its Thursday. That single idea explains every quirk below.
The formula
The standard way to compute the ISO week number for a date is:
week = 1 + floor( (thursdayOfWeek − firstThursdayOfYear) ÷ 7 days ) In plain steps: (1) find the Monday that starts the week, (2) add three days to reach Thursday, (3) the calendar year of that Thursday is the ISO week-year, (4) find the first Thursday of that week-year, and (5) count the seven-day gaps between the two Thursdays and add one. The day-of-week itself uses the ISO numbering Monday = 1 through Sunday = 7.
How to use this calculator
- Pick a date. The field defaults to today; use the date picker or type any date you like.
- Read the headline. The large "Week N" number is the ISO 8601 week for that date.
- Check the week-year. Just below the number, the ISO week-year tells you which year actually owns the week - useful near January and December.
- Use the date range. The "Week range" line shows the Monday start and Sunday end, so you can translate a week number back into real dates. To count the days between two of those dates, the Date Calculator does the arithmetic for you.
- Use the shortcuts. The Today, Last week, Next week, and Jan 1 buttons jump to common dates instantly.
Who this calculator is for
- Project and operations teams who plan sprints, releases, or production runs by week ("freeze in W38").
- Anyone working with European colleagues or clients, where week numbers are the default way to schedule.
- Manufacturing and supply chain staff reading date codes and delivery windows printed as week numbers.
- Analysts and developers who need to confirm the ISO week a timestamp maps to before building a report or query.
- Students and planners who simply want to know the current week of the year or how many weeks are left.
Worked example 1: a date in the middle of the year
Take Wednesday, June 11, 2025. The Monday that starts its week is June 9; add three days and you land on Thursday, June 12, which is in 2025, so the week-year is 2025. The first Thursday of 2025 is January 2, and June 12 is 23 full weeks later, so 1 + 23 = week 24. The range is June 9-15, 2025. Mid-year dates are the simple case: no year-boundary surprises.
Worked example 2: late December that belongs to next year
Now take Tuesday, December 30, 2025. Its week runs Monday December 29 to Sunday January 4, 2026, and the Thursday of that week is January 1, 2026. Because the Thursday is in 2026, this date sits in week 1 of week-year 2026 - even though the calendar still says 2025. This is exactly why a naive "divide the day of year by 7" approach gives wrong answers near the new year, and why the calculator reports the week-year separately.
Worked example 3: early January that belongs to last year
Consider Friday, January 1, 2027. Its week is Monday December 28, 2026 to Sunday January 3, 2027, and the Thursday is December 31, 2026. So January 1, 2027 falls in the last week (week 53) of week-year 2026. The mirror image of example 2: the first days of a calendar year can belong to the previous year's final ISO week.
ISO vs. US week numbering
The most common confusion is treating an ISO week number as if it were the US convention. They differ in two ways, and the table below summarizes them:
| Rule | ISO 8601 (this tool) | US / Gregorian |
|---|---|---|
| Week starts on | Monday | Sunday |
| Week 1 is | Week with the first Thursday (contains Jan 4) | Week containing January 1 |
| Weeks per year | 52 or 53 | 53 or 54 (partial first/last weeks) |
| First/last week | Always 7 days | Can be a partial week |
| Used by | Europe, ERP/business, ISO systems | US calendars, some payroll |
If you need the Sunday-start US number, this is not the tool for it - but for any international or business context, ISO is what people mean by "week number."
Why 53-week years exist
Because 52 weeks only cover 364 days, the extra day (or two, in a leap year) accumulates until a year needs a 53rd week to stay aligned. Precisely, a year has 53 ISO weeks when January 1 falls on a Thursday, or when it is a leap year and January 1 falls on a Wednesday. The calculator shows the total number of weeks for whatever year you select, so you can confirm whether you are in a 52- or 53-week year before planning a year-end schedule.
Key terms explained
- ISO week: a Monday-to-Sunday period numbered 1-53 under ISO 8601.
- Week-year (ISO year): the year that owns a week - the calendar year of that week's Thursday. It can differ from the date's own year at the boundaries.
- Ordinal date / day of year: the position of a date within its calendar year, 1 to 365 or 366.
- ISO weekday: the day numbered Monday = 1 through Sunday = 7, rather than the common Sunday = 0.
- Proleptic Gregorian calendar: the Gregorian calendar projected backward in time, which ISO 8601 uses so the rules apply consistently to historical dates.
Tips for working with week numbers
- Always pair a week number with its year when it matters - "W01 2026" is unambiguous, "week 1" alone may not be.
- Remember Thursday is the anchor. When in doubt about which year a boundary week belongs to, find its Thursday.
- Spreadsheets vary. Some functions default to a Sunday-start system; use the ISO-specific option if you need to match this tool.
- Don't divide the day of year by 7. That shortcut breaks at year boundaries and on 53-week years.
Related concepts and calculators
This page answers "which ISO week is this date in?" If your question is different, a sister tool fits better: use the Date Calculator to add or subtract days and measure spans between dates, the Age Calculator to count exact years, months, and days, the Time Calculator to add and convert hours and minutes, and the Hours Calculator to total the time worked between two clock times.
Sources
- International Organization for Standardization - ISO 8601 date and time format (the standard that defines week numbering).
- National Institute of Standards and Technology (NIST) - Time and Frequency Division on calendar and timekeeping conventions.
- ISO 8601-1:2019, "Date and time - Representations for information interchange," which specifies the Monday-start week and the first-Thursday week-1 rule.
⚠️ Common mistakes & edge cases
Assuming the week starts on Sunday
The US calendar often starts weeks on Sunday, but ISO 8601 weeks always start on Monday. Mixing the two shifts every result by a day at the edges and can change the week number entirely near the new year.
Forgetting the week-year near January
December 29-31 can fall in week 1 of the next year, and January 1-3 can fall in week 52 or 53 of the previous year. Always read the week-year, not just the week number, for boundary dates.
Dividing the day of year by 7
A quick "(day of year) / 7" estimate ignores the first-Thursday rule and 53-week years, so it drifts and gives the wrong week around year boundaries. Use the ISO method instead.
Expecting every year to have 52 weeks
About every five to six years has 53 ISO weeks. If you hard-code 52 into a schedule or report, a 53-week year will silently drop or misalign a week's worth of data.
❓ Frequently asked questions
What week number is it right now?
Open this page and the calculator defaults to today's date, so the big number at the top is the current ISO 8601 week. ISO weeks start on Monday and the first week of the year is the one that contains the first Thursday, which is why early-January and late-December dates can belong to a neighboring year's week.
How is the ISO 8601 week number calculated?
Find the Thursday of the week you are in (weeks run Monday to Sunday). The calendar year of that Thursday is the ISO week-year. Then count how many weeks have passed since the first Thursday of that year, and add one. The week containing the first Thursday is week 1.
Why does January 1 sometimes fall in week 52 or 53?
Because ISO weeks belong to the year of their Thursday. If January 1 is a Friday, Saturday, or Sunday, that week's Thursday is still in December, so January 1, 2, and 3 are counted in the last week (52 or 53) of the previous year. The reverse also happens: late-December dates can fall in week 1 of the next year.
Can a year have 53 weeks?
Yes. A year has 53 ISO weeks when January 1 falls on a Thursday, or when it is a leap year and January 1 falls on a Wednesday. Otherwise the year has 52 weeks. Most years have 52; a 53-week year arrives roughly every five to six years.
Do ISO weeks start on Sunday or Monday?
ISO 8601 weeks always start on Monday and end on Sunday. This is the standard used across Europe and in most business, manufacturing, and ERP systems. The common US calendar convention instead starts the week on Sunday and numbers week 1 as the week containing January 1, so the two systems can disagree by a week near year boundaries.
What is the difference between the ISO week number and the US week number?
Two things differ: the start day and the week-1 rule. ISO weeks start Monday and define week 1 as the week with the first Thursday (equivalently, the week containing January 4). The US convention starts weeks on Sunday and treats the week containing January 1 as week 1. For the same date you can get different numbers, especially in the first and last days of the year. This calculator uses the ISO method.
What does 'week-year' mean?
The week-year (or ISO year) is the year that owns a given ISO week, which is not always the same as the calendar year of the date. For example, December 30, 2024 falls in ISO week 1 of week-year 2025. Reporting tools that aggregate by week use the week-year to avoid splitting a single week across two annual buckets.
Is week 1 always the first full week of January?
Not necessarily. Week 1 is the week containing the year's first Thursday, which is the same as the week containing January 4. If January 1 is a Monday, week 1 starts on January 1; if January 1 is a Friday, week 1 does not start until Monday, January 4, and the first days of January (the 1st through the 3rd) belong to the prior year's final week.
How do I find the date range of a week number?
Pick any date inside that week in the calculator and read the 'Week range' line, which shows the Monday start and Sunday end. Because ISO weeks are always seven days Monday to Sunday, the range is fixed once you know the week and week-year.
Does this calculator handle leap years and old or future dates?
Yes. It uses the browser's built-in date engine, which accounts for leap years and the proleptic Gregorian calendar, so it returns correct ISO weeks for past and future dates alike. It is meant for planning and reference, not for legal or contractual date determinations.
💡 Good to know
"Week 24" usually means ISO week 24
When a European partner, supplier, or ERP system gives you a week number, it is almost always the ISO 8601 number shown here. Confirm the year too, since the same week number repeats annually.
Thursday decides the year
The single trick that resolves every boundary puzzle: a week belongs to the year of its Thursday. If the Thursday is in January, it is week 1 of the new year, no matter what the surrounding days say.
Spreadsheets have an ISO option
Most spreadsheet apps offer an ISO week function (often ISOWEEKNUM) alongside a default Sunday-start WEEKNUM. If your spreadsheet disagrees with this tool, you are probably using the non-ISO version.
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