Week Number Calculator
Week number calculator
Get ISO, US, or fiscal week numbers with week-date range context.
InputsDate & time2 fieldsLive
Result
2026-W27
ISO week 27 range: Jun 29, 2026 - Jul 5, 2026 (Mon-Sun).
Live update
Formula
Week index = floor((dayIndex + leadingPartialDays - 1) / 7) + 1 W_{ISO} = 1 + \left\lfloor \frac{d_{thursday} - d_{first\ thursday}}{7} \right\rfloor Symbol legend
| Symbol | Meaning | Unit | Copy |
|---|---|---|---|
W | Week number in selected system | week index | |
d_{index} | Day index from period start (1-based) | days | |
d_{lead} | Leading partial days before first full week boundary | days |
- Choose week system: ISO, US, or fiscal.
- Resolve the period start (calendar year or fiscal-year start) and the week-start day rule.
- Compute week index and show clamped date range for that week.
Example
Worked example: 2027-01-01
- 1 ISO mode returns 2026-W53 because ISO week-year can differ near January boundaries.
- 2 US mode returns 2027-US-W01 (Sunday start, week 1 begins Jan 1).
- 3 Fiscal mode (July 1 start) returns a fiscal week inside FY2027 with fiscal date-range context.
Same date can map to different week codes depending on ISO, US, or fiscal rules.
How
- Select the target date.
- Choose week system (ISO, US, or fiscal).
- If fiscal mode is selected, choose fiscal-year start month and fiscal week-start day.
- Read week code and the week date range.
Avoid
- Assuming ISO and US week numbers are interchangeable.
- Forgetting fiscal-year start month when comparing enterprise reports.
- Ignoring year-boundary behavior where week-year can differ from calendar year.
FAQ
Why do ISO and US week numbers differ?
ISO uses Monday-start weeks and week 1 as the week with the first Thursday, while US mode uses Sunday-start weeks with week 1 beginning on January 1.
Can week numbers reach 53?
Yes. ISO, US, and fiscal systems can all reach week 53 depending on year boundaries and week-start alignment.
How is fiscal week calculated?
Fiscal mode anchors the period to your chosen fiscal-year start (Jan/Apr/Jul/Oct preset), then computes week number using your selected fiscal week-start day.
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