Week Number Calculator

There is a single input, a date picker. Choose a date and every result updates at once. The page opens on August 31, 2026, so that is the example walked through here.

The highlighted result is the ISO 8601 week number together with its ISO year: Week 36, 2026. The ISO year is shown on purpose, because for a few days around New Year it differs from the calendar year. The next row gives the ISO week range, Aug 31, 2026 - Sep 6, 2026, always a Monday through the following Sunday.

The second block shows the US week number, Week 36, and its Sunday-to-Saturday range, Aug 30, 2026 - Sep 5, 2026. If a colleague's calendar shows Sunday, August 30 as part of week 36 and yours shows it in week 35, this is why: in ISO terms that Sunday closes week 35.

The last block adds context. Day of year is 243, the ordinal day counted from January 1. Total ISO weeks this year is 53, because 2026 is one of the less common 53-week years, and ISO weeks remaining is 17, the number of full ISO weeks still to come after week 36. The bar chart plots those same two figures, 36 weeks elapsed against 17 remaining.

ISO 8601 week number. The calculator works in UTC so that time zones and daylight saving cannot move a date across midnight. For a date D:

• Weekday index d = (JavaScript weekday + 6) mod 7, which makes Monday 0 and Sunday 6.
• Week range: Monday = D − d days; Sunday = Monday + 6 days.
• Thursday of the same week: T = D − d + 3 days. The ISO year is the calendar year of T. This single step is what places December 29-31 in the next ISO year, or January 1-3 in the previous one, whenever their Thursday falls on the other side of New Year.
• First Thursday of the ISO year: take January 4 of T's year and move to the Thursday of its week, F = Jan 4 − d(Jan 4) + 3 days.
• Week number = 1 + round((T − F) ÷ 7 days).
• Weeks in an ISO year = the ISO week number of December 28, which always falls in the year's last week. The result is 52 or 53.

A year has 53 ISO weeks exactly when January 1 is a Thursday, or when it is a leap year and January 1 is a Wednesday. Across any 400-year Gregorian cycle, 71 years qualify; after 2026 the next are 2032, 2037, 2043 and 2048.

US week number. With weekday numbered Sunday = 0 to Saturday = 6: US week = ceil((day of year + weekday of January 1) ÷ 7). The range shown is the Sunday on or before the date through the following Saturday. Because week 1 can be as short as a single day (when January 1 is a Saturday), the count reaches 53 in most years and 54 in a leap year that starts on a Saturday, such as 2000 and 2028. This matches Microsoft Excel's WEEKNUM function with its default return type, which Microsoft documents as "System 1"; Excel's ISOWEEKNUM returns the ISO number.

Change the date to Friday, January 1, 2027. The ISO result becomes Week 53, 2026, with a range of Dec 28, 2026 - Jan 3, 2027. The Thursday of that week is December 31, 2026, so the whole week belongs to ISO year 2026. Total ISO weeks shows 53, the count for 2026 rather than 2027, and ISO weeks remaining is 0.

The US side reads Week 1, running Dec 27, 2026 - Jan 2, 2027, and day of year resets to 1. So the same date is the last week of one year in one system and the first week of the next year in the other. A report that labels weeks as "2027-W01" without saying which system it uses will put this date in the wrong bucket half the time.

Step forward to Sunday, January 3, 2027 and the ISO answer is still week 53 of 2026, while the US count has already moved to week 2 (Jan 3 - Jan 9, 2027). ISO week 1 of 2027 does not begin until Monday, January 4, 2027, which is still US week 2.

The boundary can also cut the other way. Monday, December 30, 2024 is ISO week 1 of 2025, because the Thursday of that week is January 2, 2025, while the US count calls it week 53 of 2024. Either way, the ISO year is not always the calendar year.

The two systems do not drift randomly. Running both formulas over every date from 1900 through 2300 shows a fixed pattern that depends only on the weekday of January 1:

• January 1 falls Sunday to Thursday: for Monday-to-Saturday dates the two numbers are identical. Every Sunday is one higher in the US count, because the US week has already rolled over while the ISO week is closing.
• January 1 falls on Friday or Saturday: ISO does not count those first days as week 1, so the US number runs one ahead for every Monday-to-Saturday date all year, and two ahead on Sundays. 2027 is such a year: Wednesday, June 16, 2027 is ISO week 24 and US week 25.
• The last few days of December can differ by 52, because ISO has already moved into week 1 of the next year while the US count is on week 53.

In practice, that means a shared deadline written as "week 25" can be a full week apart between a European supplier using ISO numbering and a US team using a Sunday-start calendar. When a document or system does not name its convention, state the date range, not just the week number.

The calculator implements exactly two conventions: ISO 8601 and the simple US Sunday-start count. It does not model fiscal and retail calendars, such as 4-4-5 or 52-53-week retail years, where week 1 starts on a company-chosen date, and it does not model Middle Eastern calendars where the working week starts on Saturday. Excel's other WEEKNUM return types, which start weeks on other weekdays, are not reproduced either.

Dates are treated as calendar dates in UTC with no time of day, so the result is the week for the date you pick, not for a moment in a particular time zone. Week numbers for dates before the Gregorian reform of 1582 are computed on the proleptic Gregorian calendar and will not match historical records kept under the Julian calendar.

The week number is one way of locating a date within its year. The Day of Year Calculator gives the ordinal day instead, the figure this page shows as 243 for August 31, 2026. To move from a week to an actual deadline, the Date Calculator adds or subtracts weeks and days, and the Business Days Between Dates Calculator counts working days across a week-numbered sprint or payroll period. For a countdown to a date that sits in a particular week, use the Days Until Calculator.