Week Number Calculator
Find the week number of a date and its position within the year.
About Week Number Calculator
Counting weeks from 1 January gives a simple week number where week one starts on the first of the year. This differs from the ISO definition, which anchors weeks to Mondays. Two systems both called "week number" can differ by one, which is why reports comparing weekly data across tools need the definition checked.
Formula
Week Number
Week number = ⌈day of year ÷ 7⌉
- day of year
- Position of the date counted from 1 January
Worked example
With the default values below, Week Number Calculator returns Week Number of 24. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Date yearaccepts 1,970–2,200
- 2,025
- Date monthaccepts 1–12
- 6
- Date dayaccepts 1–31
- 15
Results
- Week Number
- 24
- Day of the Year
- 166
- Day of the Week
- Sunday
- Weeks Remaining in the Year
- 29
- Leap Year
- No
Step by step
- 1Values used
- 2Week Number
- 3Week Number= 24
- 4Day of the Year= 166
- 5Day of the Week= Sunday
- 6Weeks Remaining in the Year= 29
- 7Leap Year= No
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Week Number calculated?
Week number = ⌈day of year ÷ 7⌉ Counting weeks from 1 January gives a simple week number where week one starts on the first of the year. This differs from the ISO definition, which anchors weeks to Mondays.
Why does Week Number matter?
Two systems both called "week number" can differ by one, which is why reports comparing weekly data across tools need the definition checked.
What values do I need to enter?
This calculator takes 3 inputs: Date year, Date month, Date day. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.