How to Calculate Week Number
What is Week Number?
ISO week number identifies which week of the year a date falls in. ISO 8601 defines week 1 as containing the first Thursday of the year, with weeks starting on Monday. Week numbers run from 1 to 52 or 53.
Formula
Step-by-Step Guide
- 1Week 1 contains the first Thursday of the year
- 2Weeks always start on Monday
- 3Year can have 52 or 53 weeks
- 4Days in late December may belong to week 1 of the next year
Worked Examples
Frequently Asked Questions
What is Week Number?
ISO week number identifies which week of the year a date falls in. ISO 8601 defines week 1 as containing the first Thursday of the year, with weeks starting on Monday
How accurate is the Week Number calculator?
The calculator uses the standard published formula for week number. Results are accurate to the precision of the inputs you provide. For financial, medical, or legal decisions, always verify with a qualified professional.
What units does the Week Number calculator use?
This calculator works with inches, watts. You can enter values in the units shown — the calculator handles all conversions internally.
What formula does the Week Number calculator use?
The calculator applies the standard formula for this type of calculation. See the 'How It Works' steps above for the detailed formula breakdown.
Ready to calculate? Try the free Week Number Calculator
Try it yourself →