Week Number Calculator

What this tool does

The Week Number Calculator answers two related questions. In its forward mode, you give it a date and it tells you the ISO-8601 week number, the ISO week-year, which day of the year it is, and how many days are left in the year. In its reverse mode, you give it a week number and a year and it tells you the exact Monday-to-Sunday date range that week covers.

It loads showing today’s ISO week number in a banner, so you get an instant answer without entering anything.

When you would use it

Week numbers run quietly through a lot of professional life. Manufacturing, logistics, and retail planning frequently schedule by week — a delivery is promised “in week 32”, a production run is slotted into “week 6”. If you receive a date like that, the reverse mode turns it into real calendar dates you can act on. If you need to report progress against a weekly plan, the forward mode tells you which week today’s date falls into.

Project teams and agencies use week numbers in sprint planning and timelines. Payroll and finance teams sometimes run weekly cycles identified by ISO week. Spreadsheet users comparing data week by week need a consistent week number to group rows. Even outside work, anyone coordinating with colleagues in Europe will run into ISO week numbers, since they are part of everyday business vocabulary there.

How to use it

1. The banner at the top shows today’s ISO week as soon as the page loads.
2. To look up any date, keep the Date → week mode selected and pick a date. The tool shows its week number, week-year, day of the year, and the days remaining in the year.
3. To go the other way, switch to Week → dates, enter a week number and a year, and the tool shows the Monday and Sunday that bound that week.
4. Use the copy buttons to put a plain-language summary on your clipboard.

How it works

The ISO rule is precise: locate the Thursday of the week containing your date, and the year of that Thursday is the ISO week-year. Week 1 is the week holding the first Thursday of the year. The tool finds the Thursday, compares it to the first Thursday of the week-year, and divides the gap by seven to get the week number. For the reverse direction it starts from January 4th — which is always in week 1 — steps back to that week’s Monday, then jumps forward the right number of weeks.

The day-of-year figure counts from January 1st as day 1, and the days-remaining figure counts through to December 31st, both based on the calendar year of the date (leap years correctly show 366 days).

How to read the result

The week number is the headline figure. The week-year tells you which year that number belongs to — usually the same as the date’s calendar year, but not always near the year boundary. The day-of-year and days-remaining figures give you a sense of how far through the year a date sits, which is handy for progress tracking. In reverse mode, the range is always a full seven days from Monday to Sunday.

For other date arithmetic, the date difference calculator measures spans between dates, the age calculator counts from a birth date, and the timestamp to date converter handles epoch values.

Privacy

All calculations run in your browser. The dates and week numbers you enter are never uploaded, never stored, and never logged. Close the tab and nothing remains.