ISO Week Number CalculatorFree online other calculator — instant results, no sign-up
Find the ISO week and week-year for a date.
About the ISO Week Number Calculator
The ISO Week Number Calculator finds the ISO 8601 week number and week-year for a selected calendar date. ISO weeks run from Monday through Sunday, and week 1 is the week containing January 4. For the default date, 2026-01-05, the results are ISO week = 2 and Week-year = 2026.
- Date
- The calendar date to convert into an ISO week number and week-year, entered in YYYY-MM-DD format with day-level precision; the default is 2026-01-05.
Formula and method
T = D + (4 − d) calendar days; Y = year(T); W = ceil(n(T) / 7). For D = 2026-01-05, d = 1, so T = 2026-01-08 and n(T) = 8; therefore W = ceil(8 / 7) = 2 and Y = 2026.
- DThe input calendar date.
- dThe ISO weekday number of D, with Monday = 1 through Sunday = 7.
- TThe Thursday in the same Monday-to-Sunday week as D.
- YThe ISO week-year, equal to the calendar year containing T; year(T) extracts that year.
- n(T)The day-of-year number of T within Y, counting January 1 as day 1.
- WThe ISO week number; ceil rounds the quotient upward to the nearest integer.
Determine the input date's ISO weekday, then shift to the Thursday of that same week. The year containing that Thursday is the ISO week-year. Divide the Thursday's day-of-year number by 7 and round upward to obtain the ISO week number. This implements the ISO rule that week 1 contains the year's first Thursday, equivalently January 4.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Date2026-01-05
Results
- ISO week2
- Week-year2026
How to use the ISO Week Number Calculator
- 1Enter the date you want to classify in the Date field.
- 2For the worked example, leave Date set to 2026-01-05.
- 3Run the calculation to determine the date's ISO week and week-year.
- 4Read ISO week = 2 and Week-year = 2026, keeping both values together when identifying the week.
Reading your result
The ISO week row gives the date's numbered week within its ISO week-year, ranging from 1 to 52 or 53. The Week-year row identifies the year to which that ISO week belongs, which may differ from the calendar year near New Year. For 2026-01-05, ISO week = 2 and Week-year = 2026 mean the date belongs to 2026-W02.
Limitations
The calculation assumes a valid Gregorian calendar date and ISO 8601 week rules. It does not use Sunday-start weeks, fiscal calendars, or locale-specific week numbering. The input is a date rather than a timestamp, so no time-zone conversion is performed; if starting with a timestamp, first determine its calendar date in the intended time zone.
Common questions
Why is 2026-01-05 in ISO week 2 rather than week 1?+
ISO week 1 is the Monday-to-Sunday week containing January 4. The default date, 2026-01-05, is the Monday immediately after that week, so its ISO week is 2 and its week-year is 2026.
Why can the week-year differ from the date's calendar year?+
An ISO week belongs to the year containing its Thursday. A week crossing New Year can therefore place early-January dates in the previous week-year or late-December dates in the next week-year.
When does an ISO week-year have 53 weeks?+
An ISO week-year has 53 weeks when January 1 falls on a Thursday, or when it is a leap year beginning on a Wednesday. Otherwise, it has 52 weeks.
Will a calendar that starts weeks on Sunday show the same week number?+
Not necessarily. ISO numbering requires Monday-start weeks and defines week 1 using January 4. A Sunday-start calendar or one that always labels the week containing January 1 as week 1 can produce different results.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
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Results are estimates for informational purposes and are not financial, medical or legal advice.