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ISO Week Number

ISO-8601 week of the year (weeks start on Monday).

Formula

W=⌊do−wd+107⌋W = \left\lfloor \dfrac{d_o - w_d + 10}{7} \right\rfloor

Variables

d_oOrdinal day of year
w_dISO weekday (Mon=1…Sun=7)

Example

A year has 52 weeks, or 53 when it starts on a Thursday

Did You Know?

ISO week 1 is the one containing the year's first Thursday, so early-January days can belong to the previous year's week 52 or 53.

iso weekcalendarweek number