Week Number Calculator – Find the ISO Week Number for Any Date
Invoices labeled "Week 32," timesheets that reference pay periods by week, sprint planning boards, and manufacturing date codes all rely on a simple idea: numbering the weeks of a year from 1 to 52 or 53. The Week Number Calculator converts any calendar date into its week number using the ISO 8601 standard by default, with support for US, Simple, and custom Fiscal numbering systems, plus reverse lookup and a full year table generator.
How ISO 8601 Week Numbering Works
Under ISO 8601, weeks always start on Monday, and Week 1 is defined as the week containing the year's first Thursday. Equivalently, Week 1 is the week that contains January 4th. This rule means Week 1 can begin in late December of the previous calendar year, and it explains why a date like January 1 sometimes belongs to Week 1 of the new year and sometimes to the last week of the previous year, depending on which weekday it falls on.
52 vs. 53 Week Years
Most years have 52 ISO weeks, but a year has 53 weeks whenever January 1 falls on a Thursday, or the year is a leap year and January 1 falls on a Wednesday. The calculator detects this automatically and shows the correct maximum week number so reverse lookups and year tables never suggest an invalid Week 53 for a 52-week year.
US, Simple, and Fiscal Alternatives
Not every system numbers weeks the same way. The US convention starts weeks on Sunday, with Week 1 beginning on the Sunday-starting week that contains January 1. The Simple system ignores weekdays entirely and counts consecutive 7-day blocks starting from January 1. The Fiscal system shifts week numbering to a custom fiscal year start month — for example, choosing April produces fiscal weeks labeled against a year span like FY2026-2027, which is common in accounting, retail, and government budgeting.
Forward and Reverse Lookup
Use Date to Week mode to enter any date and instantly see its week number, the week-numbering year, and the exact start and end dates of that week. Use Week to Date mode to work backwards: enter a year and a week number to get that week's precise date range, which is useful when a document references "Week 29 of 2026" and you need to know exactly which days that covers.
Full Year Table and Weeks Remaining
The Full Year Table mode lists every week in a chosen year alongside its start and end dates, automatically highlighting today's row when it falls within the selected year, and can be exported as a CSV file for payroll systems, academic calendars, or project planning spreadsheets. Alongside the main result, the calculator also reports how many full weeks remain until the end of the year from any selected date.
Who Uses Week Numbers
Payroll administrators use week numbers to align pay periods across a fiscal calendar. Project managers and agile teams reference sprint weeks by number instead of dates. Manufacturers and shippers encode production week numbers into batch and date codes. Students and academic staff cross-reference syllabus week numbers against the actual calendar.
Limitations
The calculator supports years 1 through 9999 under the proleptic Gregorian calendar. Fiscal week numbering assumes Monday-start weeks anchored to the first day of the chosen fiscal start month; organizations using a different fiscal week convention (such as a fixed "4-4-5" retail calendar) may see slightly different week boundaries.