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Week number calculator

Find the ISO week number for any date of the year.

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Enter Date and the Week number calculator works out ISO week number straight away. For instance, with Date = 1985-03-20 it returns ISO week number = 12.

How to use it

  1. Enter your values: Date.
  2. Read the result instantly: ISO week number.

Frequently asked questions

What does the Week number calculator actually compute?

It takes Date and derives ISO week number from them. The calculation is live as you type, so the result updates on every change.

What information do I need to provide?

A single value: Date. Nothing else is required — no account, no file upload.

Can you show a worked example?

With Date = 1985-03-20, the calculator returns ISO week number = 12. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

What happens if I enter larger values?

It moves a lot. Using Date = 1990-06-15 instead, ISO week number goes from 12 to 24 — which is why it is worth testing a few scenarios rather than trusting a single figure.

When would I actually use this?

Answering a scheduling question exactly: how many working days until a deadline, what date falls sixty days out, and what time that is for a colleague elsewhere.

What is the most common mistake?

Counting days by subtracting dates and forgetting whether the endpoints are included. Inclusive and exclusive counts differ by one, which is the difference between meeting a deadline and missing it.

What is the difference between the Week number calculator and the Day of the week calculator?

This one returns ISO week number; the Day of the week calculator returns Day of the week. That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

Birth Day of the Week Calculator is the closest one after this: Find the weekday you were born on, the weekday of your next birthdays, which weekdays repeat most, and the Doomsday rule behind it.

What else is worth having open alongside it?

Screen time calculator and Sick day believability calculator — they come up in the same task often enough to be worth a second tab.

Where do the figures come from, and how current are they?

Calendar arithmetic follows the proleptic Gregorian calendar and the IANA time-zone database, so daylight-saving transitions and leap years are handled. Public holidays vary by country and region and are not assumed.

Further reading

All guides →
ExplainerWhat Week Number Is It?Week numbers run from 1 to 52 or 53. Here's how the ISO 8601 system numbers them, why some years have 53 weeks, and where businesses use them.How-toHow to Calculate Your AgeSubtract your birth year from this year, then take one off if your birthday hasn't come yet. Here's the rule, the common mistake, and how to get an exact age.How-toHow Many Days Between Two DatesSubtract the earlier date from the later, minding month lengths and leap years. Here's how to count it, and whether to include both endpoints.How-toHow to Count Business DaysBusiness days are weekdays minus holidays. Here's what counts, how to work them out between two dates, why holidays complicate it, and where it matters.ExplainerWriting a Date as ISO 8601, and Why It Is the Only Unambiguous Format01/02/2026 is 1 February in most of Europe and 2 January in the United States, and nothing in the string says which. ISO 8601 fixes that, sorts as text, and stops being ISO 8601 in four specific places that RFC 3339 rejects.How-toWhat Day of the Week Was I Born? Find Out in SecondsFind the weekday of any birth date. Learn the quick calculator method and a short introduction to the Doomsday rule for working it out in your head.