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Week Number Calculator

Get the ISO-8601 week number for any date, plus the quarter and day of year — or list every week of a year with start dates.

ISO week
W43
ISO year
2026
Quarter
Q4
Day of year
293 / 366

Tuesday, week 43 of 2026 (ISO-8601: weeks start Monday; week 1 contains the first Thursday).

ISO 8601 week rules are applied exactly — including 53-week years and week-1 starting in December.

Three Week Numbering Systems Coexist

The ISO 8601 system (weeks start Monday, week 1 contains the first Thursday, 52 or 53 weeks a year) is the European and business standard — but the US convention numbers weeks from January 1 with Sunday starts, and some industries count fiscal weeks differently. A date that is "week 2" in ISO can be "week 1" or "week 3" in US reckoning. Always state the system when the number drives schedules or reporting, and remember the January boundary: December 30 can belong to week 1 of the following year under ISO rules.

How the Dates Are Calculated

  1. Pick a date to see its ISO week number (1–53), quarter, and day of the year.
  2. The ISO standard is used: weeks start Monday, week 1 contains the first Thursday of the year.
  3. Switch to year mode to list all weeks of a year with their Monday start and Sunday end dates.
  4. Copy any figure, or use the table for sprint and production planning.
Formula & Technical Standard
ISO 8601: week = 1 + floor((ordinalDate − weekdayISO + 10)/7), with boundary correction for weeks 0 and 53.

Worked Example

Sprint planning
Input: 2026-09-14
Output: ISO week 38 of 2026, Q3, day 257
Teams that plan by ISO week numbers can anchor any date to a shared calendar reference.
Weeks of a year
Input: Year 2026
Output: 53 weeks, W1 starts Mon 2025-12-29
ISO week 1 can start in the previous December — a common source of off-by-one errors the tool makes visible.

Common Questions

Why do some years have 53 weeks?

Because 52 × 7 = 364 < 365. When January 1 falls on a Thursday (or a leap year Wednesday), the year gets a 53rd ISO week.

Is this the same as the US week numbering?

No. The US convention starts weeks on Sunday and week 1 contains January 1, which differs from ISO most years.

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