Date & Time

Week Number Calculator

Find the ISO-8601 week number and ISO week year for any date, along with the weekday it falls on.

What this calculator does

The Week Number Calculator estimates your result instantly from the values you enter, using standard date & time formulas. Find the ISO-8601 week number and ISO week year for any date, along with the weekday it falls on. 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.

Week Number Calculator

Result

0
ISO Week Number
0
ISO Week Year
0
Weekday

About Week Number Calculator

The Week Number Calculator returns the ISO-8601 week for a date. ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday, so it also reports the ISO week year, which can differ from the calendar year around the turn of January.

Formula & Mathematical Calculation

d = new Date(raw + 'T00:00:00')

dayNum = (t.getUTCDay() + 6) % 7

Derived directly from the verified calculation function in the tool engine.

Calculation Example

Input: Date 2024-01-01.

Result: ISO Week Number W01, ISO Week Year 2024, Weekday Monday. The summary reads: That date falls in ISO week 2024-W01 and is a Monday.

Key Benefits

  • Uses the ISO-8601 standard, where weeks begin on Monday
  • Reports the ISO week year, which can differ from the calendar year in early January or late December
  • Names the weekday of the date
  • Pads single-digit weeks to two digits, such as W01

How to Use this Calculator

  1. Pick a date
    Select the date in the Date field.
  2. Calculate
    Click Calculate.
  3. Read the week
    Read the ISO Week Number, ISO Week Year, and Weekday 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

  • Week Number 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

ISO-8601. Weeks run Monday to Sunday, and week 1 is the week containing the first Thursday of the year.

Days in late December can belong to week 1 of the next year, and early January days can belong to the last week of the previous year. The ISO Week Year reflects the week's year, not the date's calendar year.

Not always, but for 2024-01-01 it does: that date is a Monday and lands in ISO week 2024-W01.

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.