Week Calculator
Calculate the ISO week number for a given day of the year and find how many weeks are in a year.
This free online week calculator provides instant results with no signup required. All calculations run directly in your browser — your data is never sent to a server. Enter your values below and see results update in real time as you type. Perfect for everyday calculations, homework, or professional use.
Ordinal day number within the year
Results
Approximate Week Number
14
Day Within Week
7
Weeks Remaining
38
How to Use This Calculator
Enter your input values
Fill in all required input fields for the Week Calculator. Most fields include unit selectors so you can work in your preferred unit system — metric or imperial, whichever matches your problem.
Review your inputs
Double-check that all values are correct and that you have selected the right units for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.
Read the results
The Week Calculator instantly computes the output and displays results with units clearly labeled. All calculations happen in your browser — no loading time and no data sent to a server.
Explore parameter sensitivity
Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.
When to Use This Calculator
- •Use the Week Calculator when you need accurate results quickly without the risk of manual computation errors or unit conversion mistakes.
- •Use it to verify calculations made by hand or in spreadsheets — an independent check can catch errors before they lead to costly decisions.
- •Use it to explore how changing input parameters affects the output — a quick way to develop intuition and identify the most influential variables.
- •Use it when collaborating with others to ensure everyone is working from the same numbers and applying the same assumptions.
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About Week Calculator
The Week Calculator determines the approximate week number for a given day of the year. Week numbering is essential for business planning, project scheduling, manufacturing cycles, and ISO-standard date references. Many industries plan operations on a weekly basis, and knowing the current week number provides a common reference for scheduling and reporting.
The Math Behind It
Formula Reference
Week Number
Week = ceil(dayOfYear / 7)
Variables: dayOfYear = ordinal day of the year (1-366)
Worked Examples
Example 1: Week number for day 98
Find the week number for the 98th day of the year.
Day 98 falls in approximately week 14, the 7th day of that week, with 38 weeks remaining.
Common Mistakes & Tips
- !Assuming every year has exactly 52 weeks (most have 52 weeks plus 1-2 days).
- !Confusing ISO week numbering (starts Monday, based on first Thursday) with simple division-based week numbers.
- !Forgetting that the first and last weeks of the year may span two calendar years under ISO rules.
Related Concepts
Used in These Calculators
Calculators that build on or apply the concepts from this page:
Frequently Asked Questions
How many weeks are in a year?
There are 52 complete weeks plus 1 extra day in a standard year (2 extra in a leap year). Under ISO 8601, some years have 53 numbered weeks when January 1 falls on a Thursday.
What is the ISO week date format?
ISO 8601 defines the format YYYY-Www-D, where YYYY is the ISO year, ww is the two-digit week number, and D is the day of the week (1=Monday, 7=Sunday). For example, Monday of week 14 in 2026 is 2026-W14-1.
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