Week Number Calculator
Find the ISO week number for any date, along with the day of year, year progress, and days remaining in the year.
How to Use This Calculator
- Select any date using the date picker.
- Click "Get Week Number" to find the ISO week number and related information.
- Review the ISO week number, day of year, year progress, and days remaining.
Formula
Day of Year: Number of days since January 1 (Jan 1 = day 1).
Year Progress: (Day of Year ÷ Days in Year) × 100%
Examples
ISO Week W01 — Most years, Jan 1 falls in week 1 of the ISO year.
A Friday. The week number depends on the year’s starting day and which year most of that week falls in.
Day 60 of 366 → 16.4% of the year complete. Year progress shows how far through the year you are.
Frequently Asked Questions
What is the ISO week date system?
ISO 8601 defines the week date system where weeks start on Monday and week 1 contains the year’s first Thursday. This is the international standard used by most countries.
Why does week 53 sometimes exist?
Week 53 exists when a year has 53 Thursdays (usually when January 1 is a Thursday in a leap year). This means the year spans 53 ISO weeks instead of the usual 52.
How is this used in business?
Many businesses use ISO week numbers for fiscal planning, project timelines, and manufacturing schedules. It provides a consistent weekly reference across different calendar systems.
What is the difference between ISO and US week numbering?
US weeks start on Sunday, and week 1 always contains January 1, regardless of the day of the week. ISO weeks start on Monday and week 1 contains the first Thursday.