Week Number Calculator

Week Number Calculator features

• 01

  Today's week, front and centre

  The current week number and its year are shown large at the top, so you get an instant answer to "which week are we in?" without digging through a calendar.

• 02

  Date ↔ week, both ways

  Convert a specific date to its week number and year, or enter a year and week to get the exact date range it covers — useful for planning sprints, payroll, or reports.

• 03

  ISO or US, with a year grid

  Toggle between ISO 8601 and US week numbering — they can differ — and scroll a full-year table listing every week and its date span for the year you choose.

Week Number Calculator FAQ

What week number is it right now?

The current week is displayed at the top of the tool as soon as it loads, using ISO 8601 by default. Switch to US numbering if your calendar or organisation counts weeks from Sunday and January 1.

How is the ISO 8601 week number defined?

ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week with January 4). This means early January can fall in the last week of the previous year, which the calculator handles correctly.

Why do ISO and US week numbers sometimes differ?

US numbering starts weeks on Sunday and puts January 1 in week 1, while ISO starts on Monday and anchors week 1 to the first Thursday. Around the new year the two systems can give different numbers for the same date.

How do I find the dates for a given week number?

Enter the year and week number in the week-to-date section and the tool shows the start and end dates of that week, following whichever numbering standard you've selected.

Can a year have 53 weeks?

Yes. Under ISO 8601 a year has 53 weeks when January 1 (or December 31 in a leap year) falls on a Thursday. The calculator accounts for this, so the year grid shows 52 or 53 weeks as appropriate.