Weekday Calculator
Find the day of the week for any date, plus its day-of-year number and ISO week number.
What this calculator does
The Weekday Calculator estimates your result instantly from the values you enter, using standard date & time formulas. Find the day of the week for any date, plus its day-of-year number and ISO week number. Calculations run in your browser, so nothing you type is sent to a server, and the output is an estimate rather than a certified or official figure.
Result
About Weekday Calculator
The Weekday Calculator tells you which day of the week a date falls on, and also reports its day-of-year number (1 to 365, or 366 in a leap year) and its ISO-8601 week number.
Formula & Mathematical Calculation
d = new Date(raw + 'T00:00:00')
doy = Math.round((new Date(d.getFullYear(), d.getMonth(), d.getDate()) - startOfYear) ÷ 86400000) + 1
Derived directly from the verified calculation function in the tool engine.
Calculation Example
Input: Date 2024-07-04.
Result: Day of Week Thursday, Day of Year 186, ISO Week Number 27. The summary reads: July 4, 2024 falls on a Thursday. It is day 186 of the year and lies in ISO week 27.
Key Benefits
- Names the weekday for any date
- Shows the day-of-year number, counting from January 1
- Includes the ISO-8601 week number
- Handles leap years, so the day-of-year count reaches 366 when applicable
How to Use this Calculator
-
Pick a date
Select the date in the Date field. -
Calculate
Click Calculate. -
Read the results
Read the Day of Week, Day of Year, and ISO Week Number boxes.
Assumptions & Limitations
What the calculation assumes
- Every value you enter is treated as an exact, known amount.
- Any rate or interval you enter is assumed to hold for the whole period unless the form asks for it separately.
- Results use standard published formulas and account only for the values entered above.
- Rounding is applied for display; the underlying calculation uses your full precision.
- Results are returned in the same units you enter, unless the tool states the output unit.
What it does not cover
- Weekday Calculator produces a calculated result for reference and learning, not a certified or official measurement.
- Real-world outcomes differ when tolerances, rounding rules or local standards apply.
- Definitions and standards vary by region and change over time; verify the convention that applies to your case.
- Figures are only as accurate as the inputs you supply — check them before acting on the result.
- For work where the result carries real consequences, confirm it against an authoritative source.
Frequently Asked Questions
Free to use, no account needed. Calculations run in your browser, so nothing you type is sent to a server. Results are estimates — how we build and check our calculators.