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Week Calculator

Find which week of the year a day falls in, counting 7-day weeks from January 1, with its day in that week and how many of the year's 52 weeks come after it.

By Christopher FloiedPublished Updated

Range: 1 – 366. Whole numbers only

Ordinal day number within the year

Results

Approximate Week Number

14

Day Within Week

7

Weeks Remaining

38

How to Use This Calculator

1

Enter your input values

Fill in all required input fields for the Week Calculator. Follow the units and input format printed next to each field. Where a unit selector is available, choose the units that match your data.

2

Review your inputs

Double-check that all values are correct and match the units or format shown 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.

3

Read the results

The Week Calculator computes the output in your browser and displays the results, with units clearly labeled, as soon as they are ready — no data sent to a server.

4

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, and manufacturing cycles. Many industries plan operations on a weekly basis, and knowing the current week number provides a common reference for scheduling and reporting. This calculator counts 7-day weeks from January 1, so week 1 is January 1 to 7; that is not the ISO 8601 week number, whose weeks start on Mondays.

The Math Behind It

Week numbering provides a coarser time resolution than daily dates, useful for planning and reporting at the weekly level. A standard year has 52 complete weeks plus 1 or 2 extra days. A leap year has 52 weeks plus 2 days. The ISO 8601 standard defines week numbering rules: the first week of the year is the week containing the first Thursday of January. Weeks start on Monday and are numbered 01 through 52 (or 53 in some years). This means January 1 is not always in week 1, and some days at the end of December may belong to week 1 of the following year. The simpler approach used here divides the day of the year by 7 and rounds up, so week 1 is always January 1 to 7, whatever weekday the year starts on. It is not the ISO week number. The two agree all year only in a year that starts on a Monday, apart from the last day or two of December, which belong to ISO week 1 of the next year. In any other year they differ by one week on one to three days in every seven, depending on the weekday the year starts on, and near the start and end of the year the ISO week can belong to the year before or after. Week-based planning is standard in many industries. Retail businesses track sales by week. Manufacturing schedules production runs on weekly cycles. Academic institutions organize semesters into numbered weeks. Software development sprints are typically one or two weeks long. An ISO year has 53 weeks when it starts on a Thursday, or on a Wednesday in a leap year. This happens roughly every 5-7 years. ISO years with 53 weeks include 2004, 2009, 2015, 2020, and 2026.

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.

Step 1:Week number: ceil(98 / 7) = ceil(14.0) = 14
Step 2:Day within week: ((98-1) % 7) + 1 = (97 % 7) + 1 = 6 + 1 = 7
Step 3:Weeks remaining: 52 - 14 = 38 of the year's 52 weeks come after week 14

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

Day of the Year Calculator

Find the ordinal day number needed as input for the week calculator.

Birthday Calculator

Calculate age in various time units including weeks.

Used in These Calculators

Calculators that build on or apply the concepts from this page:

Day of the Year Calculator

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, a year has 53 numbered weeks when it starts on a Thursday, or on a Wednesday in a leap year.

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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