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Universal date, business-day and time calculations · Guide + calculator Without the guesswork.

Week Number Calculator

Find the ISO 8601 week number or use a Sunday- or Monday-start calendar convention with the complete week range shown. Inputs stay in your browser.

Reference toolWeek Number CalculatorFind the ISO 8601 week number or use a Sunday- or Monday-start calendar convention with the complete week range shown. Inputs stay in your browser.
Reference calculation

Enter your details.

Calculated locally in your browser. Nothing is uploaded to CalcSpan.

Mon, Aug 24, 2026

Plain-date semantics · start date is not counted unless the result explicitly says inclusive.

Your result
ISO week 35 of 2026

Aug 24, 2026 falls in the Monday-through-Sunday ISO week shown below.

DateAug 24, 2026
ISO weekW35 · 2026
Week rangeAug 24, 2026 – Aug 30, 2026
ISO ruleWeek 1 contains January 4
Timeline
Aug 24, 2026ISO week 35 starts
Aug 30, 2026ISO week 35 ends
Calendar options

Add to calendar

Choose a calendar service, then confirm the event. Downloading an .ics file does not add it until you open or import it.

Read before relying

Context matters.

    Rule set 1.1.0

    Sources & scope

    Source & calculation QA reviewed

    The calculation guide and this source trail show what the result means, where the rule came from and when it was last reviewed. This is source-and-calculation QA, not a claim of medical, legal, tax or other professional review; time-sensitive decisions still belong with the linked authority.

    International Organization for Standardization · SRC-ISO-8601-001ISO 8601 date and time formatSource retrieved 2026-08-22
    JurisdictionISO 8601 or user-selected calendar-week numbering
    Last reviewed2026-08-22
    Next review due2027-02-22
    Review scopeCalcSpan source & calculation QA
    PrivacyNo input upload
    How this calculation works

    A visible path from input to answer.

    Find the ISO 8601 week number or use a Sunday- or Monday-start calendar convention with the complete week range shown. The page keeps the calculation scope and its limits beside the result so the output can be checked, copied or revisited.

    Calculation flow
    1. Read the record. The calculator uses Date and Week-number system supplied in the form.
    2. Apply the scope. It applies the ISO 8601 or user-selected calendar-week numbering reference and the explicit date, unit and counting conventions shown in the result.
    3. Expose the checkpoints. Intermediate rows, warnings and timeline events stay visible; the emphasized row identifies the primary checkpoint when one exists.
    Illustrative default example ISO week 35 of 2026

    Aug 24, 2026 falls in the Monday-through-Sunday ISO week shown below.

    • ISO weekW35 · 2026
    Defaults are an illustration only. Replace them with the record that applies to your situation.
    Assumptions and scope

    Calculation purpose: Find the ISO 8601 week number or use a Sunday- or Monday-start calendar convention with the complete week range shown. This is a high-sensitivity reference output; the linked authority or qualified professional controls the final decision.

    Boundary behavior

    Calendar inputs use plain year-month-day semantics. Leap days, month ends, date-pair order and unsupported dates are checked before the result is shown.

    Rule trail

    Rule set 1.1.0 · Last reviewed 2026-08-22 · Next review due 2027-02-22. The source panel below contains the linked evidence.

    Questions answered

    What to know before using the result.

    What week number is it?

    The default result uses ISO 8601: weeks start Monday and week 1 is the week containing January 4.

    Why can week numbers differ?

    Some calendars start weeks Sunday or place January 1 in week 1. Select the convention that matches the system you are using.

    Can an ISO week belong to another year?

    Yes. Dates near New Year can belong to the final ISO week of the prior ISO year or week 1 of the next ISO year.

    Keep going

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