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Week Number Calculator

ISO 8601 week number and day of year for any date.

Best for: Project and payroll scheduling, planning, and any system that references weeks of the year.

Complete guide

Input

How it's calculated & sources
The Week Number Calculator uses the standard, deterministic formula, so the same inputs always produce the same result. It runs entirely in your browser, so your inputs are never uploaded. Last reviewed June 2026.

Free & no sign-up · runs entirely in your browser. Results are estimates for general information, not professional advice — verify important decisions with a qualified expert. Last reviewed June 2026.

How it works

ISO 8601: weeks start Monday. Week 1 contains the first Thursday of the year.

Example

2024-01-01 → ISO Week 1, 2024.

Frequently asked questions

Why ISO?+

ISO 8601 is the international standard used in business and software for week numbering.

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Complete guide

Quick answers

Short, sourced answers to the questions people (and AI assistants) ask most.

What is Week Number?
A week number calculator finds which week of the year a date falls in — for example week 32 — using the ISO 8601 standard by default.
Why does Week Number matter?
The week number under the chosen convention. Always confirm whether a system uses ISO weeks or a local scheme, because they can differ around the year boundary.
How is Week Number calculated?
Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday (equivalently, the week that includes January 4th). This means a date in very early January can belong to week 52 or 53 of the previous year. Some regions, including parts of the US, instead start weeks on Sunday and count week 1 as the one containing January 1st.
What are common week number mistakes?
Assuming everyone uses ISO weeks when some systems start weeks on Sunday.
When should you use the Week Number Calculator?
Project and payroll scheduling, planning, and any system that references weeks of the year.

What is the Week Number Calculator?

A week number calculator finds which week of the year a date falls in — for example week 32 — using the ISO 8601 standard by default.

How the Week Number Calculator works

Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday (equivalently, the week that includes January 4th). This means a date in very early January can belong to week 52 or 53 of the previous year. Some regions, including parts of the US, instead start weeks on Sunday and count week 1 as the one containing January 1st.

Key terms

ISO 8601 week:
Monday-start weeks; week 1 contains the year's first Thursday.

Common mistakes to avoid

  • Assuming everyone uses ISO weeks when some systems start weeks on Sunday.
  • Being caught out at the year boundary, where early-January dates can be week 52/53 of the prior year.
  • Mixing Monday-start and Sunday-start conventions.

Keywords: week, number, iso.

Sources: ISO 8601 — Date and time format

Reviewed by the Free Tools Galaxy editorial team · Updated June 2026 · Calculated privately in your browser.

Frequently asked questions

Is the Week Number Calculator free to use?+

Yes. Every tool on Free Tools Galaxy is 100% free, runs in your browser and requires no signup.

How accurate is the Week Number Calculator?+

Week Number Calculator uses the standard week number formula in double-precision arithmetic, so the same inputs always produce the same result and you can verify any figure by hand. It is an educational estimate — real-world outcomes depend on your actual rates, rules and assumptions.

Do you store my inputs?+

No. The Week Number Calculator runs entirely in your browser. Nothing is uploaded or saved to a server.

Can I use the Week Number Calculator on mobile?+

Yes — the interface is fully responsive and works on phones, tablets and desktops.

What are common mistakes to avoid?+

The most frequent mistake is mixing units. Double-check your inputs use a single, consistent unit before clicking Calculate.

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Week Number Calculator on Free Tools Galaxy — ISO 8601 week number and day of year for any date. https://freetoolsgalaxy.com/tools/week-number-calculator
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# Week Number Calculator — the free tool I keep coming back to

ISO 8601 week number and day of year for any date.

Try it: https://freetoolsgalaxy.com/tools/week-number-calculator
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The fastest way is to use a free in-browser week number calculator: https://freetoolsgalaxy.com/tools/week-number-calculator. ISO 8601 week number and day of year for any date.

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