How do I use WEEKNUM Function to find the week number for september 29, 2025?

To find the week number for september 29, 2025, use the formula: =WEEKNUM("9/29/2025"). Basic Week Number Calculation This formula returns: 40.

How can I optimize WEEKNUM Function performance?

To optimize WEEKNUM Function: 1) Use specific ranges instead of entire columns, 2) Avoid volatile functions within WEEKNUM Function, 3) Consider array formulas for bulk operations, 4) Cache results when possible. AskFormulas automatically applies optimization best practices.

What is the syntax for WEEKNUM Function?

The WEEKNUM Function syntax in Excel is: =WEEKNUM(serial_number, [return_type]). It takes 2 parameter(s): serial_number/date (required), return_type/type (optional).

How do I use WEEKNUM Function to group daily sales data by week number for reporting?

To group daily sales data by week number for reporting, use the formula: =SUMIF($B$2:$B$100,WEEKNUM(A2),$C$2:$C$100). Weekly Sales Report Aggregation This formula returns: $45,750.

Can WEEKNUM Function handle input in Excel?

Yes, WEEKNUM Function can handle input. For example: =WEEKNUM("9/29/2025") will find the week number for september 29, 2025. This returns: 40

WEEKNUM Function

Master the WEEKNUM function to return the week number of a specific date. Learn syntax, examples, and solutions for Excel and Google Sheets week calculations.

Excel

Google Sheets

date-time

intermediate

Syntax Preview

Excel

Google Sheets

=WEEKNUM(serial_number, [return_type])

Quick Answer

Comprehensive Explanation

Syntax and Parameters

## What is the WEEKNUM Function?

The WEEKNUM function is one of Excel and Google Sheets' most valuable date-time functions, designed to convert any date into its corresponding week number within the year. First introduced to simplify weekly reporting and analysis, it has become essential for businesses that organize data by weeks, track weekly KPIs, or manage projects on weekly schedules.

### Core Functionality

At its core, WEEKNUM operates by taking a date and calculating which week of the year (1-53) it belongs to. The function analyzes the date's position within the calendar year and returns a number representing its week. This calculation accounts for different week-starting conventions (Sunday vs Monday) and follows either the standard calendar system or the ISO week date system, depending on your chosen parameters.

### When to Use WEEKNUM

WEEKNUM is the ideal choice when you need to:

1. **Create Weekly Reports:** When aggregating data by week for sales reports, productivity tracking, or performance metrics, WEEKNUM provides the foundation for grouping dates into weekly periods.

2. **Track Project Milestones:** For project management scenarios where tasks are organized by weeks, this function enables automatic week assignment and progress tracking.

3. **Analyze Seasonal Patterns:** When studying business patterns that follow weekly cycles, WEEKNUM helps categorize data for year-over-year weekly comparisons.

### Week Numbering Systems

The WEEKNUM function offers multiple week numbering systems through its return_type parameter:

- **System 1 (Default):** Week begins on Sunday - The traditional US calendar format
- **System 2:** Week begins on Monday - Common in business and ISO 8601 standard
- **System 11-17:** Various starting days for specialized regional or business needs
- **System 21:** ISO week date standard (Monday start, specific first-week rules)

This flexibility ensures compatibility with different international standards and business requirements. The ISO system (21) is particularly important for international business as it provides consistent week numbering across countries.

## Syntax and Parameters Explained

### Parameters in Detail

#### serial_number/date (Required)
The date parameter is the core input for WEEKNUM. It accepts multiple formats: direct date values like DATE(2025,9,29), cell references containing dates (A2), or date strings that Excel/Sheets can interpret ("9/29/2025"). The function automatically converts valid date inputs into their internal serial number format for processing, ensuring consistent results regardless of how the date is provided.

#### return_type/type (Optional)
This parameter controls which day starts the week and affects week numbering. When omitted, it defaults to 1 (Sunday start). Here's a complete breakdown:

| Return Type | Week Begins On | System | Notes |
|------------|---------------|---------|-------|
| 1 or omitted | Sunday | Standard | Default, US convention |
| 2 | Monday | Standard | European business standard |
| 11 | Monday | Standard | Alternative Monday system |
| 12 | Tuesday | Standard | Tuesday start |
| 13 | Wednesday | Standard | Wednesday start |
| 14 | Thursday | Standard | Thursday start |
| 15 | Friday | Standard | Friday start (Middle East) |
| 16 | Saturday | Standard | Saturday start |
| 17 | Sunday | Standard | Same as 1 |
| 21 | Monday | ISO 8601 | ISO standard (special rules) |

### ISO Week Date System (Type 21)

The ISO 8601 system (return_type = 21) follows specific rules:
- Weeks always begin on Monday
- Week 1 is the first week containing Thursday
- Week 1 must contain January 4th
- Can result in dates from late December being in week 1 of the next year
- Can result in dates from early January being in week 52 or 53 of the previous year

Practical Examples

Common Errors and Solutions

#VALUE!

WEEKNUM returns #VALUE! error

Cause:

The date parameter contains text that cannot be interpreted as a valid date

Solution:

1. Verify the date format matches your regional settings (MM/DD/YYYY or DD/MM/YYYY) 2. Check for hidden spaces or non-printing characters 3. Use DATEVALUE to convert text to dates: =WEEKNUM(DATEVALUE("9/29/2025")) 4. Ensure cells are formatted as dates, not text 5. Use DATE function for unambiguous dates: =WEEKNUM(DATE(2025,9,29))

Prevention:

Always use cell references with properly formatted dates or the DATE function

Frequency: 45%

#NUM!

WEEKNUM returns #NUM! error

Cause:

The return_type parameter is outside the valid range (1-2, 11-17, or 21)

Solution:

1. Check that return_type is a valid option 2. Common valid values: 1 (Sunday start), 2 (Monday start), 21 (ISO) 3. Remove the second parameter to use default (1) 4. Verify you're not using text instead of a number for return_type

Prevention:

Use only documented return_type values

Frequency: 20%

Wrong Week Number

WEEKNUM returns unexpected week number

Cause:

Using different week numbering system than expected (standard vs ISO)

Solution:

1. Verify which return_type you're using 2. Check if you need standard (1 or 2) or ISO (21) system 3. Remember ISO weeks can assign late December to week 1 of next year 4. Confirm your organization's week numbering convention 5. Test with known dates to verify correct system

Prevention:

Document which week system your organization uses and stick to it consistently

Frequency: 25%

Week 53 Issues

Unexpected week 53 in results

Cause:

Some years have 53 weeks depending on the week numbering system

Solution:

1. This is normal for certain years (about every 5-6 years) 2. Years starting on Thursday have 53 weeks (Sunday system) 3. Leap years starting on Wednesday have 53 weeks 4. Plan for week 53 in your data models 5. Use ISO system for more predictable behavior

Prevention:

Design systems to handle both 52 and 53-week years

Frequency: 10%

Best Practices and Pro Tips

Always use the same return_type across your entire workbook or organization. Mixing Sunday-start and Monday-start systems leads to confusion and errors. Document your choice in a visible cell or comment.

To find the week-ending date (e.g., Saturday), use: =A2+(7-WEEKDAY(A2)). This is helpful for weekly reports that need to show 'Week Ending' dates.

Be careful with dates near year boundaries. January 1st might be week 53 of the previous year in ISO system, or week 1 might start in late December.

Excel 2013+ offers ISOWEEKNUM function specifically for ISO week numbers. Consider using =ISOWEEKNUM(date) instead of =WEEKNUM(date,21) for clearer intent.

For large datasets with repeated WEEKNUM calculations, add a helper column with week numbers rather than calculating repeatedly in formulas. This can improve performance by 50% or more.

Advanced Techniques

WEEKNUM vs Alternative Functions

Real-World Applications

## Comparison with Alternative Functions

| Feature | WEEKNUM | ISOWEEKNUM | WEEKDAY | Custom Formula |
|---------|---------|------------|----------|----------------|
| Returns | Week number (1-53) | ISO week (1-53) | Day number (1-7) | Flexible |
| Week Start Options | Multiple (1-2, 11-17, 21) | Monday only | Multiple | Any |
| ISO Standard | Type 21 only | Always | No | Possible |
| Excel Version | All | 2013+ | All | All |
| Complexity | Simple | Simple | Simple | Complex |
| Performance | Fast | Fast | Fastest | Slower |

### When to Use WEEKNUM

✅ **Choose WEEKNUM when:**
- You need week numbers for reporting
- Working with US business weeks (Sunday start)
- Need flexibility in week start day
- Creating weekly aggregations
- Backward compatibility is important

### When to Use Alternatives

✅ **ISOWEEKNUM when:**
- Following ISO 8601 standard strictly
- International business reporting
- Using Excel 2013 or later
- Consistency across regions is critical

✅ **WEEKDAY when:**
- You need day of week, not week number
- Creating day-based logic
- Building custom week calculations

✅ **Custom formulas when:**
- Non-standard week definitions (e.g., 4-4-5 calendar)
- Weeks not starting on standard days
- Special fiscal calendar requirements