Week Number to Dates
Which Monday to Sunday a given ISO week number covers, and how many weeks that year holds.
Results
What this tool does
Factories, hospital rotas, school terms and shipping schedules all run on week numbers, and everyone else has to look them up. Give this page a year and a week and it returns the Monday it starts and the Sunday it ends, following ISO 8601 — the standard that puts week 1 wherever the year's first Thursday falls, which is why it sometimes begins in the previous December.
Formula
week 1 of a year is the one holding its first Thursday, and every week starts on a Monday (ISO 8601)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
year | Year | — |
week | Week number | — |
OUT | That week runs | — |
MO | Monday | — |
SU | Sunday | — |
WY | Weeks this year has | — |
VA | That week exists | — |
DY | Day of the year it starts on | — |
Worked example
- Year2026
- Week number38
- That week runs14–20/09/2026
- Monday14 Sep 2026
- Sunday20 Sep 2026
- Weeks this year has53
- That week existsYes
- Day of the year it starts on257
Frequently asked questions
Why does week 1 sometimes start in December?
Because ISO 8601 defines week 1 as the one containing the year's first Thursday, so that no week is split between two years. When 1 January falls on a Friday, Saturday or Sunday, those days belong to the last week of the year before, and week 1 begins on the Monday after — which can be as late as 4 January. When it falls on a Monday to Thursday, week 1 starts in the old year, as early as 29 December.
Which years have 53 weeks?
Those that start on a Thursday, and leap years that start on a Wednesday. It happens about every five or six years — 2020 and 2026 both have one — and it is the reason a payroll or a reporting cycle built on weeks occasionally has an extra period that nobody planned for. The line on this page tells you which kind of year you are looking at.