Week Number Calculator
Get the ISO-8601 week number for any date, plus the day of year, quarter and week range.
Week Number Calculator
Get the ISO-8601 week number for any date, plus the day of year, quarter and week range.
How to use the Week Number Calculator
- Enter any date.
- Click Calculate.
Formula
ISO-8601: Week 1 contains the first Thursday of the year.
Week starts on Monday, ends on Sunday.
Example
August 21, 2026: ISO week 33 of 2026. Day 233 of 365. Q3.
Step-by-step calculation
- Find the Thursday of the target week.
- Calculate the Thursday's week number from January 1.
- Determine the Monday (week start) and Sunday (week end).
- Result: Week 33, Aug 17–23.
Real-world uses
- Business reporting periods.
- Academic semester planning.
- Sprint and project cycle tracking.
Common mistakes
- Using weeks starting on Sunday (ISO weeks start on Monday).
- Confusing the year of the week with the calendar year (Dec 31 can be week 1 of the next year).
- Not understanding that week 1 contains January 4.
Frequently asked questions
Why can December 31 be week 1?
If Dec 31 falls on a Monday and the Thursday is in January, it belongs to ISO week 1 of the next year.
What is the day of year?
The sequential count from January 1. Jan 1 = day 1, Dec 31 = day 365 (or 366 in leap years).
Which quarter is my date in?
Q1: Jan–Mar, Q2: Apr–Jun, Q3: Jul–Sep, Q4: Oct–Dec.