Find which week of the year a date is in (ISO weeks)

"Week 41" on a supplier schedule means nothing until you can map it to real dates. How ISO week numbers work and how to look them up.

Tools used in this guide

The job

Manufacturing plans, logistics schedules, and corporate calendars often say "W40" instead of a date. You need the actual Monday–Sunday range — or the reverse: which week a given date lands in.

How ISO weeks work

ISO 8601 weeks always start on Monday and end on Sunday. Week 1 is the week containing January 4 — so January 1–3 can belong to the previous ISO year. A year has 52 or 53 weeks; 2026 has 53.

The week-numbering year can differ from the calendar year: December 29, 2025 is already ISO 2026-W01, while January 1, 2026 is still in ISO week 2026-W01 but December 29, 2026 belongs to 2027-W01.

Date → week: enter the date to get something like 2026-W40-3 (week 40, Wednesday).
Week → dates: enter the ISO year and week to get the Monday–Sunday range.

Worked example

A shipment note says "delivery in W41". Enter year 2026 and week 41: the week runs October 5–11, 2026. For a meeting on December 30, the date lookup returns ISO 2027-W01 — the year flips because that Monday–Sunday block is week 1 of the next ISO year.

Limits

This uses ISO 8601 only. The US convention (weeks start Sunday, week 1 contains January 1) and other regional systems give different numbers — check which system your counterparty means when the stakes matter. Years 1–9999; the device's time zone applies.