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Date & Timedates

Week Number Calculator

Week number under both the ISO and US conventions, which frequently disagree. ISO weeks start Monday and can put early January in the previous year; the US convention starts Sunday and never does.

Also called: iso week number, what week is it.

ISO week number
34

Week 34 of 2,026, running 2026-08-17 to 2026-08-23. The US convention numbers it week 34, because it starts weeks on Sunday.

ISO week-numbering year
2,026
Week starts
2026-08-17
Week ends
2026-08-23
US week number
34
Weeks in this ISO year
53
How this is calculated
Method and background

How this is calculated

ISO 8601 defines week one as the week containing the first Thursday, with weeks running Monday to Sunday. That produces a week-numbering year that can differ from the calendar year at either end, and years with either 52 or 53 weeks. The US convention starts weeks on Sunday and simply calls the week containing 1 January week one, which is simpler and does not line up with ISO. Reporting that mixes the two silently shifts numbers by one.

the US convention instead starts weeks on Sunday and calls the week containing 1 January week 1
d
The date

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

mid August

Date
2026-08-18

ISO week number34

The ISO week starts on the Monday

Open this example

the fourth of January is always in week 1

Date
2026-01-04

ISO week number1

boundary: the anchor date in the ISO definition

Open this example

Method and limits

What it assumes

  • UTC.

What it deliberately does not model

  • Other conventions exist, including a Saturday start used in some retail calendars.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Why do the two week numbers differ?
Different start days and different rules for week one. ISO starts Monday and anchors on the first Thursday; the US starts Sunday and anchors on 1 January. They agree for much of the year and diverge at the edges.