Week Number Calculator
ISO 8601 week number, week year and day of year for any date.
Quick answer
- Formula: Shift the date to the Thursday of its week; Week = 1 + whole weeks between that Thursday and the first Thursday of the ISO year
- Worked example — ISO week number: 1 (full example below).
Week Number Calculator
| ISO week number | 1 |
| ISO week year | 2,026 |
| Day of year | 1 |
January 1, 2026 is a Thursday, so it falls in ISO week 1 of 2026 — the first day of the year is also day 1 of week 1.
Find the ISO 8601 week number of any date, along with its ISO week year (which can differ from the calendar year near New Year) and its day of the year.
ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday — the system used across Europe and in international business planning.
The formula
Worked example
January 1, 2026 is a Thursday, so it falls in ISO week 1 of 2026 — the first day of the year is also day 1 of week 1.
The example is pre-loaded in the result panel — press Calculate with your own numbers to replace it.
Frequently asked questions
Why can the ISO week year differ from the calendar year?
Because weeks are assigned to the year containing their Thursday. December 29–31 can belong to week 1 of the next year, and January 1–3 can belong to the last week (52 or 53) of the previous year.
Do all years have 52 weeks?
Most do, but an ISO year has 53 weeks when it starts on a Thursday (or on a Wednesday in a leap year) — for example 2020 and 2026.
Is this the same week numbering used in the US?
Not always. US calendars often number weeks from the first Sunday or from January 1. This calculator follows ISO 8601 strictly.
Category: Date & Time calculators
Sources & methodology
This calculator applies the formula shown on this page exactly as written, using the standard definitions for its inputs. Results are computed from the values you enter — no data is stored — and are rounded only for display.
Content last reviewed by CalcHub Editorial.
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