Week Number Calculator

Current week

-- ----

—

This week --

This year --

Pick a date

Week Number settings

Default week standard

Free Online Week Number Calculator

This week number calculator shows the current week at a glance, converts any date into its week number and back again, and lays out a full visual calendar with a week-number column built right in — so you never have to count weeks by hand or dig through a spreadsheet.

ISO 8601, US, or your own custom week

Not every calendar counts weeks the same way. ISO 8601 weeks start on Monday and Week 1 is the week containing the year's first Thursday — the standard used across Europe, in business planning, and in most project-management tools. US-style weeks start on Sunday, with Week 1 simply being the week that contains January 1st. If neither fits, switch to Custom and pick any day of the week as your start day — every number on the page recalculates instantly, and your choice is remembered for next time.

ISO 8601 — Monday-start, week 1 contains January 4th
US — Sunday-start, week 1 contains January 1st
Custom — any start day you choose

Look up a date's week, or a week's dates — in one place

Use Date → Week to pick any date, past or future, and see its week number instantly. Flip to Week → Date to type a week number and year and get the exact start and end date back. Planning something across time zones? Check the world clock to line up the days first.

A real visual calendar, not just a table

Instead of a plain list of dates, the built-in calendar renders an actual month grid — with the week number running down the left edge of every row, the current week and today highlighted, and a one-click switch to a full-year overview strip when you want to see all 52 or 53 weeks at once.

Month view with an integrated week-number column
Full-year strip for a bird's-eye view of every week
Click any day or week to open its details

Week details, calendar export, and shareable links

Click into any week to see its exact date range plus its position in the month and quarter (useful for reporting and sprint planning), then use Add to calendar to download a ready-to-import .ics file for that week, or Copy link to send that exact week straight to someone else.

Week-of-month and week-of-quarter position
One-click .ics export for calendar apps
Shareable link to any specific week

Why some years have 53 weeks

A year is about 52.14 weeks long, so every few years an extra partial week gets rounded into a 53rd week — see the FAQ below for exactly when this happens under each standard. Need to count down to a specific week instead? Try the online countdown tool.

Fast, free, and runs entirely in your browser

No installation, no account, and no server round-trip — every calculation happens instantly on your device, and your preferred week standard is saved locally for your next visit.

Online week number calculator help & questions

1. What is a week number?

➥ A week number labels each week of the year with a single number (1 through 52 or 53) instead of a date range, which makes it easy to reference a period in planning, payroll, agile sprints, or school schedules without spelling out the exact dates.

2. What's the difference between ISO 8601 and US week numbering?

➥ ISO 8601 weeks start on Monday and Week 1 is the week containing the year's first Thursday (equivalently, the week containing January 4th), which can place the very end of December in Week 1 of the following year. US-style weeks start on Sunday and Week 1 is simply the week that contains January 1st, so week numbers always stay within their own calendar year.

3. Can I choose a different day for the week to start on?

➥ Yes. Besides the ISO 8601 and US presets, you can pick Custom and choose any day of the week as the start day — the hero display, lookup results, and calendar grid all recalculate instantly, and your choice is remembered for your next visit.

4. Why do some years have 53 weeks instead of 52?

➥ A 365-day year is about 52.14 weeks, so the extra partial week is rounded into a 53rd week roughly every 5-6 years (more often in leap years). Under ISO 8601 this happens whenever January 1st falls on a Thursday, or on a Wednesday in a leap year.

5. Can I look up the week number for any date, past or future?

➥ Yes. Use the Date → Week tab and pick any date — past, present, or future — to instantly see which week number it falls in, its exact date range, and its position in the month and quarter.

6. Can I find the date range for a specific week number?

➥ Yes. Use the Week → Date tab, enter a week number and a year, and you'll get the exact start and end date of that week under your selected standard.

7. Can I add a week to my calendar app?

➥ Yes. Open any week's detail panel and use Add to calendar to download a standard .ics file spanning that week's date range, which opens directly in Google Calendar, Outlook, Apple Calendar, and most other calendar apps.

8. Can I share a link to a specific week?

➥ Yes. Every week — whether from the calendar, a lookup result, or the current-week hero — has a Copy link button that generates a URL. Opening that link takes anyone straight to that week's detail panel, with no need to look it up again.

9. Does the site remember my week-numbering standard?

➥ Yes. Your chosen standard (ISO 8601, US, or Custom with a specific start day) is saved in your browser and applied automatically the next time you open the Week Number tool — no account required.