Time & Date Tool
Week Calculator
Find the week number of any date, calculate the start/end dates of a week, or add/subtract weeks from a date.
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Interactive Week Calculator Workspace
ISO Week Solver: Resolve numeric calendar coordinates. Instantly identify annual ISO 8601 week counts, evaluate Monday-Sunday boundary limits, and shift dates in increments of full weeks.
Calculated Week coordinates
ISO Week NumberWeek 29Calendar Year 2026
Week Start (Monday)Monday, Jul 13, 2026
Week End (Sunday)Sunday, Jul 19, 2026
Mathematical Formula & Variables
Equation Model
ISO Week = Floor((Day of Year - Day of Week + 10) / 7)Calculates the official ISO 8601 week number, where week 1 of a calendar year is defined as the week containing the first Thursday of January.
Variable Definitions
| Symbol | Description |
|---|---|
| ISO Week | The numeric week sequence of the year |
| Day of Year | Elapsed days since January 1 |
| Day of Week | Indexed day position (Monday = 1, Sunday = 7) |
How to Use the Week Calculator
- Enter any calendar date to find its corresponding Week Number of the Year.
- Inspect the starting (Monday) and ending (Sunday) boundaries of that week.
- Use the add/subtract weeks offset control to project forward or backward in time.
Practical Example Calculation
Scenario Context: Determining the ISO week number for July 19, 2026.
Step 1: Identify day of year: July 19 is the 200th day of 2026.
Step 2: Identify day of the week: July 19, 2026 is a Sunday (7).
Step 3: Apply ISO formula: Floor((200 - 7 + 10) / 7) = Floor(203 / 7) = 29.
July 19, 2026 is in Week 29 of the year.
Frequently Asked Questions
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