Week NumberFree online date & time calculator — instant results, no sign-up
Find the ISO week number of any date.
About the Week Number
The Week Number calculator finds the ISO week number, ISO week year, and day of the week for a selected date. It uses ISO 8601 rules: weeks start on Monday, and week 1 is the week containing January 4. For the default date, 2026-10-23, the results are week 43, ISO week year 2026, and Friday.
- Date
- The calendar date to evaluate, entered as year-month-day (YYYY-MM-DD), with a default of 2026-10-23 and precision of one calendar day.
Formula and method
t = d + (4 − u) calendar days; Y = year(t); n = dayOfYear(t); W = 1 + floor((n − 1) / 7). The weekday output is the name corresponding to u. For d = 2026-10-23, these rules give W = 43, Y = 2026, and Friday.
- dThe input calendar date.
- uThe ISO weekday number of d, with Monday = 1 through Sunday = 7.
- tThe Thursday in the same ISO week as d, found by shifting d by 4 − u calendar days.
- YThe ISO week year, equal to the calendar year containing t.
- nThe ordinal day of t within Y, counting January 1 as day 1.
- WThe ISO week number; floor means round down to the nearest integer.
Determine the selected date's weekday using the Gregorian calendar. Shift to the Thursday of that same Monday-to-Sunday week, because Thursday determines the ISO week year. Use that Thursday's position within its year to calculate the week number with the formula above. Report the weekday of the original input date, not the shifted date.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Date1970-01-01
Results
- ISO week number1
- ISO week year1970
- Day of weekThursday
How to use the Week Number
- 1Enter or select the date in the Date field; keep 2026-10-23 to follow the worked example.
- 2Read the ISO week number result: the default date belongs to week 43.
- 3Read the ISO week year result: for the default date, it is 2026.
- 4Read the Day of week result: 2026-10-23 falls on Friday.
- 5Use the week number together with the ISO week year when recording or comparing weekly dates.
Reading your result
ISO week number identifies the date's Monday-to-Sunday week within its ISO week year; 43 means the default date is in the 43rd ISO week. ISO week year identifies the year to which that week belongs; the default result is 2026. Day of week names the weekday of the selected date; the default result is Friday. Near New Year, the ISO week year can differ from the date's calendar year.
Limitations
This tool uses ISO 8601 week numbering and the Gregorian calendar, not Sunday-start, fiscal, retail, or organization-specific week systems. It works with calendar dates rather than timestamps, so it does not perform time-zone conversion; convert a timestamp to the intended local date before entering it. Historical dates may differ from dates recorded under calendars used locally at the time. The supported date range depends on the page's date input and implementation.
Common questions
Why is 2026-10-23 in ISO week 43?+
ISO weeks run from Monday through Sunday, starting with the week containing January 4. Applying those rules places 2026-10-23 in week 43 of ISO week year 2026, on Friday.
Why does the calculator show an ISO week year separately?+
A week can cross the calendar-year boundary. ISO assigns the entire week to the year containing its Thursday, so dates in early January can belong to the previous ISO week year, and dates in late December can belong to the next.
Can this calculator return week 53 or week 0?+
An ISO week year has either 52 or 53 weeks. Week 53 is valid in years that start on Thursday, or leap years that start on Wednesday. ISO numbering never uses week 0.
Why might my calendar display a different week number?+
Some calendars start weeks on Sunday or define week 1 as the week containing January 1. This calculator always uses Monday-start ISO weeks and the week containing January 4 as week 1.
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.
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Results are estimates for informational purposes and are not financial, medical or legal advice.