ArcGIS Field Calculator: Display Percentages

Generate precise expressions to format numbers as percentages directly within your attribute table. This tool helps you create the correct syntax for Python 3 and VBScript, saving you time and preventing common errors.

Percentage Expression Generator

What is ‘arcgis use field calculator to display percentages’?

In ArcGIS, the Field Calculator is a versatile tool that allows you to perform calculations on the attribute data of your geographic features. One of the most common tasks for GIS analysts is to calculate percentages—for example, determining the percentage of a certain land use type within a county, or the percentage of a population subgroup within a census tract. The phrase “arcgis use field calculator to display percentages” refers to the process of creating a new field and populating it with values formatted as a percentage string (e.g., “75.25%”) derived from two other fields. This usually involves dividing a ‘part’ value by a ‘total’ value, multiplying by 100, and concatenating the ‘%’ symbol.

This process is crucial for data normalization, thematic mapping (choropleth maps), and statistical analysis. Users often need to generate expressions in either Python or VBScript, the two main scripting languages supported by the Field Calculator, to accomplish this. Knowing the correct syntax, like using !FieldName! for Python and [FieldName] for VBScript, is essential. This calculator and guide simplifies that entire workflow. For more complex calculations you might want to learn about advanced field calculations.

The Formula for Calculating Percentages in ArcGIS

The core formula is straightforward, but its implementation depends on the chosen scripting language (Expression Type). The fundamental mathematical formula is:

Percentage = (Part Value / Total Value) * 100

This result is then typically rounded and formatted as a text string. The Field Calculator expression must handle data types correctly (e.g., ensuring floating-point division) and concatenate the ‘%’ sign.

Variables in Percentage Calculation

Variable	Meaning	Unit	Typical Range
Numerator Field	The field representing the partial amount or subset of data.	Varies (e.g., count, area)	0 to Total Value
Denominator	The field or constant value representing the total amount.	Varies (must match Numerator)	Greater than 0
Resulting Percentage	The calculated percentage formatted as text.	%	“0.00%” to “100.00%” or higher

Understanding the data is key. You can learn more about geospatial data analysis to improve your results.

Practical Examples

Example 1: Calculating Percentage of Vacant Housing Units

Imagine you have a feature class of census tracts with fields for total housing units and vacant units.

• Numerator Input: !VACANT_HU!
• Denominator Input: !TOTAL_HU!
• Desired Output: A new text field showing the percentage of vacant units.

Using the calculator with the Python 3 expression type, you would get:

round((float(!VACANT_HU!) / float(!TOTAL_HU!)) * 100, 2) if !TOTAL_HU! not in (0, None) else 0

You then format this as a string with a ‘%’ symbol. This expression safely handles cases where the total housing units might be zero.

Example 2: Calculating Percentage Change in Population

Suppose you have population data from two different years and want to calculate the percentage change.

• Numerator Input: !POP_2020! - !POP_2010! (This must be entered directly in the Field Calculator expression box)
• Denominator Input: !POP_2010!
• Desired Output: A field showing the growth or decline as a percentage.

A VBScript expression for this would look like:

IIF([POP_2010] > 0, Round((([POP_2020] - [POP_2010]) / [POP_2010]) * 100, 2), 0)

This example demonstrates how to use a more complex expression for the numerator. For more information on this you might want to look into data management best practices.

How to Use This Percentage Expression Calculator

1. Select Expression Type: Choose between Python 3 (standard for ArcGIS Pro) or VBScript (used in older ArcMap versions).
2. Enter Field Names: Type the name of your numerator (part) and denominator (total) fields. The tool automatically adds the correct syntax (!field! for Python, [field] for VBScript) but you can also type it yourself. You can also enter a static number for the denominator.
3. Set Decimal Places: Specify how many decimal places you want in the final result.
4. Generate and Copy: The tool instantly generates the expression. Click the “Copy Expression” button.
5. Paste in ArcGIS: In ArcGIS Pro or ArcMap, open the attribute table, add a new field (data type ‘Text’ or ‘String’), right-click the new field’s header, and choose “Calculate Field”. Paste the copied expression into the expression box and run the tool.

Key Factors That Affect ‘arcgis use field calculator to display percentages’

• Data Type: The most common error is performing integer division. If both numerator and denominator fields are integers, the result of the division will be truncated to zero in older Python/VBScript versions. Always cast fields to float (e.g., `float(!FIELD!)`) before dividing.
• NULL Values: If either the numerator or denominator contains a NULL value, the calculation will result in NULL. It’s good practice to include logic to handle these cases, for example by returning 0 or a specific text string like “No Data”.
• Division by Zero: A denominator of zero will cause a “division by zero” error and stop the calculation. The generated expressions on this page include checks to prevent this error.
• Field Name Syntax: Python requires field names to be enclosed in exclamation points (!FIELD_NAME!), while VBScript uses square brackets ([FIELD_NAME]). Using the wrong syntax is a frequent error.
• Output Field Type: To store a value like “25.50%”, the output field’s data type must be ‘Text’ (or ‘String’). If you calculate into a numeric field (Double, Float), you can only store the number (e.g., 25.50), not the ‘%’ symbol.
• Scripting Language Version: ArcGIS Pro uses Python 3, which handles division differently than the Python 2 used in ArcMap. In Python 3, `5 / 2 = 2.5`, whereas in Python 2, `5 / 2 = 2`. This makes casting to float even more important for cross-version compatibility.

Frequently Asked Questions (FAQ)

1. Why is my result always 0%?

This is almost always due to integer division. Ensure you convert your fields to a floating-point number before dividing. Our calculator does this automatically with `float(!FieldName!)`.

2. How do I handle NULL values in my data?

You should add a conditional check. In Python, you can check `if !FieldName! is not None`. The expressions generated by our tool include a basic check for this.

3. What is the difference between Python and VBScript field syntax?

Python uses `!FieldName!` and is case-sensitive. VBScript uses `[FieldName]` and is not case-sensitive.

4. Can I calculate the percentage from a single field?

Yes, if you have a known total. For instance, if you want to find what percentage of a total of 5000 each row’s value represents, you would use `!MyField!` as the numerator and `5000` as the denominator.

5. The tool created the expression, but ArcGIS gives me an error. Why?

Check for typos in your field names. Also, ensure the ‘Expression Type’ in the ArcGIS Calculate Field tool matches the expression you copied (Python 3 or VB).

6. How do I format the result to include more text, like “75% Complete”?

You can concatenate additional strings. In Python: `str(round(…)) + “% Complete”`. In VBScript: `Round(…) & “% Complete”`.

7. Does this work in both ArcGIS Pro and ArcMap?

Yes. ArcGIS Pro defaults to Python 3. ArcMap used VBScript as a default but also supports Python (typically version 2.x). Choose the appropriate expression type.

8. What’s the best data type for my new percentage field?

Use ‘Text’ or ‘String’ if you want to include the ‘%’ symbol. Use ‘Double’ or ‘Float’ if you only need to store the numeric value for further analysis or symbology.

Related Tools and Internal Resources

Explore these other resources to enhance your GIS skills:

• Spatial Analysis Techniques: A guide to advanced spatial analysis.
• Choropleth Mapping Guide: Learn how to best visualize your percentage data.
• Introduction to Arcade: Discover ArcGIS’s modern, cross-platform expression language.
• Python Scripting for Geoprocessing: Automate your workflows with Python.
• Data Cleaning and Preparation: Best practices for getting your data ready for analysis.
• VBScript Legacy Reference: A reference for maintaining older ArcMap scripts.