What Is the ISO Week Date & Day of Year Calculator?
The ISO week date system is an international standard time format specified in ISO 8601.
It assigns every week in a calendar year an ordinal number from W01 through W52 or W53.
How Does the ISO Week Date & Day of Year Calculator Work?
The calculator computes midnight UTC boundaries to prevent local daylight saving time shift distortions.
It finds the Thursday of the target week to establish the canonical ISO year and computes week count.
ISO Week Date & Day of Year Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
According to ISO-8601, weeks begin on Monday and week 1 is the week that contains January 4th (or the first Thursday).
How to Use the ISO Week Date & Day of Year Calculator
- Select any date using the calendar picker.
- Read the ISO week code (e.g. 2026-W41-2) and ordinal day of year.
Step-by-Step Example Calculation
ISO Week for October 6, 2026
Input Values:
Understanding Your Result
Week number indicates the business week of the year.
Day of year shows sequential count from January 1st (Day 1).
Factors That Affect the Result
- Leap year additions (February 29th adds 1 to all subsequent ordinal days).
When Should You Use This Calculator?
- Supply chain shipping windows, manufacturing lot codes, fiscal week accounting, and project milestones.
Assumptions & Limitations
- Based on the proleptic Gregorian calendar standard.
Frequently Asked Questions
Calculation Accuracy & Reference Note
Strict ISO 8601:2019 compliance.