Entering values using the keyboard is not the only way you can edit values in a table. In some cases, you may want to perform a mathematical calculation to set a field value for a single record or even all records. You can perform simple as well as advanced calculations on all or selected records. In addition, you can calculate area, length, perimeter, and other geometric properties on fields in attribute tables. The sections below include examples of using the field calculator. Calculations are performed using Python, SQL, and Arcade.
- Python enforces indentation as part of the syntax. Use two or four spaces to define each logical level. Align the beginning and end of statement blocks, and be consistent.
- Python calculation expression fields are enclosed with exclamation points (!!).
- When naming variables, note that Python is case sensitive, so value is not the same as Value.
- After entering statements, click the Export button if you want to write them to a file. The Import button prompts you to find and select an existing calculation file.
A variety of calculations can be calculated with only a short expression.
Simple string examples
Strings are supported by a series of Python string functions, including capitalize, rstrip, and replace.
Capitalize the first character of the string in the CITY_NAME field.
Remove any space from the end of the string in the CITY_NAME field.
Replace any occurrences of "california" with "California" in the STATE_NAME field.
Characters in a string field can be accessed by indexing and slicing in Python. Indexing fetches characters at an index position; slicing fetches a group of characters. In the following table, assume that !fieldname! is a string field with a value of "abcde".
The first character
The second-to-last character
The second, third, and fourth characters
Python also supports string formatting using the format() method.
Combine FieldA and FieldB, separated by a colon.
Simple math examples
Python provides tools for processing numbers. Python also supports a number of numeric and mathematical functions, including math, cmath, decimal, random, itertools, functools, and operator.
x + y
x plus y
1.5 + 2.5
x - y
x minus y
3.3 - 2.2
x * y
x times y
2.0 * 2.2
x / y
x divided by y
4.0 / 1.25
x // y
x divided by y (floor division)
4.0 // 1.25
x % y
x modulo y
8 % 3
negative expression of x
x = 5
x is unchanged
x = 5
x ** y
x raised to the power of y
2 ** 3
!Rank! * 2
Calculate volume of a sphere given a radius field.
4.0 / 3.0 * math.pi * !Radius! ** 3
In ArcGIS Pro, Python 3 is used, and in ArcGIS Desktop, Python 2 is used. Python 2 uses integer math, meaning that dividing two integer values always produces an integer value (3 / 2 = 1). In Python 3, dividing two integer values produces a float value (3 / 2 = 1.5).
Python built-in functions
Python includes a number of built-in functions, including max, min, round, and sum.
Calculate the maximum value for each record from a list of fields.
max([!field1!, !field2!, !field3!])
Calculate the sum for each record from a list of fields.
sum([!field1!, !field2!, !field3!])
Use code blocks
With Python expressions and the Code Block parameter, you can do the following:
- Use any Python function in the expression.
- Access geoprocessing functions and objects.
- Access properties of feature geometry.
- Access the new random value operator.
- Reclassify values using if-then-else logic.
|Expression type||Code block|
Supports Python functionality. The code block is expressed using Python functions (def). Geometry properties are expressed using geoprocessing objects, such as Point objects, where appropriate.
Supports Arcade functionality.
Supports SQL expressions.
SQL expressions were implemented to better support calculations using feature services and enterprise geodatabases, particularly regarding performance. Instead of performing calculations one feature or row at a time, a single request is set to the feature service or database.
In ArcGIS Desktop, the Calculate Field tool supports VB, PYTHON, and PYTHON_9.3 expression types. The VB expression type, which is supported in some products, is not supported on 64-bit products, including ArcGIS Pro.
PYTHON and PYTHON_9.3 keywords are still supported in ArcGIS Pro for backward compatibility but are not listed as choices. Python scripts that use these keywords will continue to work.
The only difference between the Python 3 expression type and the legacy PYTHON_9.3 keyword is that Python 3 returns the values in date fields as Python datetime objects.
The Python 3 expression type is not related to the version of Python installed with ArcGIS Pro. It is just the third Python-related keyword historically (after PYTHON and PYTHON_9.3).
Python functions are defined using the def keyword followed by the name of the function and the function’s input arguments. A Python function can be written to accept any number of input arguments (including none). A value is returned from the function using a return statement. The function name is your choice (don't use spaces or leading numbers).
If a value is not explicitly returned from a function with a return statement, the function returns None.
Python enforces indentation as part of the syntax. Use four spaces to define each logical level. Align the beginning and end of statement blocks, and be consistent.
When using the math samples below, assume an expression type of Python 3.
Round a field's value to two decimal places.
Expression: round(!area!, 2)
Use the math module to help convert meters to feet. The conversion is raised to the power of 2 and multiplied by the area.
Expression: MetersToFeet((float(!shape.area!))) Code Block: import math def MetersToFeet(area): return math.pow(3.2808, 2) * area
Calculate fields using logic with Python
Logical patterns can be included in a code block using if, else, and elif statements.
Classify based on field values.
Expression: Reclass(!WELL_YIELD!) Code Block: def Reclass(WellYield): if (WellYield >= 0 and WellYield <= 10): return 1 elif (WellYield > 10 and WellYield <= 20): return 2 elif (WellYield > 20 and WellYield <= 30): return 3 elif (WellYield > 30): return 4
In addition to the following code samples, see the Geometry unit conversions section below for more information about converting geometry units.
Calculate the area of a feature.
Calculate the maximum x-coordinate of a feature.
Calculate the vertex count of a feature.
Expression: getVertexCount(!shape!) Code Block: def getVertexCount(feat): partnum = 0 # Count the number of points in the current multipart feature partcount = feat.partCount pntcount = 0 # Enter while loop for each part in the feature (if a singlepart # feature, this will occur only once) while partnum < partcount: part = feat.getPart(partnum) pnt = part.next() # Enter while loop for each vertex while pnt: pntcount += 1 pnt = part.next() # If pnt is null, either the part is finished or there # is an interior ring if not pnt: pnt = part.next() partnum += 1 return pntcount
For a point feature class, shift the x-coordinate of each point by 100.
Expression: shiftXCoordinate(!SHAPE!) Code Block: def shiftXCoordinate(shape): shiftValue = 100 point = shape.getPart(0) point.X += shiftValue return point
Geometry unit conversions
Area and length properties of the geometry field can be modified with unit types expressed with an @ sign.
If the data is stored in a geographic coordinate system and a linear unit (for example, feet) is supplied, the length calculation is converted using a geodesic algorithm.
Converting the areal units on data in a geographic coordinate system produces questionable results since decimal degrees are not consistent across the globe.
Calculate a feature's length in yards.
Calculate a feature's area in acres.
Geodesic area and length can also be calculated using geodesicArea and geodesicLength properties with @ followed by a unit of measure keyword.
Calculate a feature's geodesic length in yards.
Calculate a feature's geodesic area in acres.
Calculate the current date.
Calculate the current date and time.
Calculate the date to be December 31, 2000.
Expression: datetime.datetime(2000, 12, 31)
Calculate the number of days between the current date and the value in a field.
Expression: (datetime.datetime.now() - !field1!).days
Calculate a date by adding 100 days to the date value in a field.
Expression: !field1! + datetime.timedelta(days=100)
Calculate a string representing the date using the ctime method in the datetime module. The example creates a string in the format: 'Mon Feb 22 10:15:00 2021'.
Calculate the day of the week (for example, Sunday) for a date value in a field.
Calculate a formatted string from a date field using the datetime module's strftime method and an explicit format string. The example will create a string in the format: '02/22/2021, 10:15:00'.
Expression: !field1!.strftime("%m/%d/%Y, %H:%M:%S")
String calculations can be completed using a variety of Python coding patterns.
Return the three rightmost characters.
Replace any cases of an uppercase P with a lowercase p.
Concatenate two fields with a space separator.
Expression: !SUB_REGION! + " " + !STATE_ABBR!
Convert to proper case
The following examples show different ways to convert words so that each word has the first character capitalized and the rest of the letters in lowercase.
Expression: ' '.join([i.capitalize() for i in !STATE_NAME!.split(' ')])
The Python re module provides regular expression matching operations that you can use to perform complex pattern matching and replacement rules for strings.
Replace St or St. starting a new word at the end of the string with the word Street.
Expression: update_street(!ADDRESS!) Code Block: import re def update_street(street_name): return re.sub(r"""\b(St|St.)\Z""", 'Street', street_name)
Accumulative and sequential calculations
Accumulative and sequential calculations can be performed using global variables.
Calculate a sequential ID or number based on an interval.
Expression: autoIncrement(10, 5) Code Block: rec = 0 def autoIncrement(start=1, interval=1): global rec if rec == 0: rec = start else: rec += interval return rec
Calculate the accumulative value of a numeric field.
Expression: accumulate(!FieldA!) Code Block: total = 0 def accumulate(increment): global total if total: total += increment else: total = increment return total
Calculate the percentage increase of a numeric field.
Expression: percentIncrease(float(!FieldA!)) Code Block: lastValue = 0 def percentIncrease(newValue): global lastValue if lastValue: percentage = ((newValue - lastValue) / lastValue) * 100 else: percentage = 0 lastValue = newValue return percentage
Random values can be calculated using the random module.
Use the numpy site package to calculate random float values between 0.0 and 1.0.
Expression: getRandomValue() Code Block: import numpy def getRandomValue(): return numpy.random.random()
Use the random module to calculate random integers between 0 and 10.
Expression: random.randint(0, 10) Code Block: import random
Calculate null values
In a Python expression, null values can be calculated using a Python None.
The following calculation only works if the field is nullable.
Use a Python None to calculate null values.