Week Number Calculator
ISO-8601 week number and day of year.
Amortization schedule
| # | Payment | Principal | Interest | Balance |
|---|---|---|---|---|
Showing first 12 of months.
Growth over time
| Year | Invested | Value |
|---|---|---|
Introduction
Week Number Calculator — ISO-8601 week number and day of year. Enter Date to get an instant, accurate result.
Formula
ISO-8601 week number (weeks start Monday, week 1 contains the year's first Thursday) and day-of-year count.
Step-by-step
- Enter the Date.
- Click Calculate to see your result instantly.
Real-world example
Example: With Date = 2026-10-09, the Week Number Calculator gives Iso Week: 41, Iso Year: 2026, Day Of Year: 282.
Frequently Asked Questions
Why does the ISO week number sometimes seem off by one?
ISO-8601 weeks always start on Monday and the first week of the year is defined as the one containing the first Thursday, which can shift week 1 slightly compared to a naive Jan 1-based count.
What is "day of year"?
A running count from 1 (Jan 1) to 365 or 366 (Dec 31) for the entered date.
What inputs does the Week Number Calculator need?
You'll need to provide: Date. All fields use sensible defaults, so you can see a working example immediately and then adjust the values to match your own numbers.
How accurate is the Week Number Calculator?
The Week Number Calculator applies the formula exactly as calculated — ISO-8601 week number (weeks start Monday, week 1 contains the year's first Thursday) and day-of-year count. — so results are precise for the inputs you provide. Accuracy depends on entering correct, realistic input values.
Is the Week Number Calculator free to use?
Yes, the Week Number Calculator is completely free, requires no signup, and runs instantly in your browser.
About the Week Number Calculator
The Week Number Calculator uses a real, verifiable formula — ISO-8601 week number (weeks start Monday, week 1 contains the year's first Thursday) and day-of-year count. — so results are accurate every time, not an approximation.