Week Number Calculator
Find the ISO week number and day of the year for a date. This free online time & date calculator runs entirely in your browser — no signup, no data sent anywhere.
Inputs
Results
How It Works (Formula & Method)
Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday. The tool also reports the ordinal day of the year (1–365/366) and the calendar quarter.
Worked Example
Below is a worked example using the calculator's default values. The same numbers are pre-filled in the form above so you can press Calculate and see the result without typing anything.
Inputs used:
- Date: 2026-07-21
With these inputs, the calculator computes the metrics shown in the Results panel. Change any value and press Calculate again to see how the result responds — the live widget and the chart both update instantly.
Frequently Asked Questions
What does the Week Number Calculator compute?
The Week Number Calculator takes 1 input value and returns 3 results. Find the ISO 8601 week number for any date, along with the day of the year and the quarter.
Is my data sent to a server?
No. The Week Number Calculator runs entirely in your browser using static JavaScript. Your inputs are never transmitted to CalculatorHive or any third party, and nothing is stored after you close the page.
Is this calculator free to use?
Yes. Every calculator on CalculatorHive is completely free. There is no signup, no paywall, and no usage limit. The site is supported by display advertising and affiliate partnerships, which are clearly labeled.
How accurate is the result?
The math is mathematically exact for the inputs you provide. Real-world accuracy depends on how accurately your inputs reflect the real situation — defaults are typical values, not guarantees.