Using formulas to calculate field values

See Formulas in Action

Create a board from the Field Formula Template.
Then open Amazing Fields settings to see the field details.

Setting Up Formulas

Amazing Fields allows you to calculate the value of fields from other fields on the same Trello card.  Formulas are currently supported for Text, Number, and Checkbox field types.  Formulas are a supporter only feature so the person setting it up needs to be a supporter.  Once setup, the formula can be used by as many users and boards as you would like.

The image below shows an example formula.  

To setup a calculation:

  • Open the Amazing Fields setting panel
  • Select a field you want to compute using a formula
  • Select "Calculate field value from formula" to enable the formula
  • Start typing in the formula you would like
  • Add references to card fields by clicking on "Insert Field"

Using Formulas

Formulas should not contain cycles. If a field formula references other fields in a way that references the original field, the evaluation order is undefined and may lead to inconsistencies.

Once a field is set to be calculated, you can not edit it directly.  Amazing Fields will automatically update the calculation anytime you make a change to any other fields on the card.  This includes updates the user makes directly and updates made by things like linked custom fields.

If the formula fails when evaluated then Amazing Fields will popup an alert message with the details of the failure. The most common error is that the value being calculated doesn't make sense for the field type in use. Make sure to filter values and handle edge cases in your formulas.

Supported Functions

Amazing Fields supports over 100+ of the most common functions used in spreadsheets like Excel and Google sheets.  You can see a full list in the Supported Formula Functions reference.  If you see any additional ones you would like added, please let me know.

Default Field Values

Fields may be empty either when you create a new card or if you clear the field.  For example a numeric field like "cost" has no value on new cards.  This can cause problems for field calculations because the formulas expect all fields they use to have values.  As a result, all empty fields used in a formula will be calculated based upon a default value.  This does not actually set the field to the default value, it only adjusts the calculations behind the scenes.

The default values for each field type are:

  • Number: 0.0
  • Text: blank value
  • Checkbox: false
  • Date: 1/1/1970 00:00:00
  • Dropdown: 
    • single select: '' (ie. empty string)
    • multi-select: []  (ie. empty array)

Clearing Field Values

There are some cases where you need to compute a value that "clears" the field to having no value.  To do this you should return the constant value NULL. This will tell the powerup to clear the field value.  See this example where the user has a checkbox that determines if the total price should be calculated and if it is not checked the value is cleared.

Common Formula Issues

  • Comparisons using equals (=) are not working

When comparing two values you must use double equals ( ==)instead of single equals (=).  See the list below for the full list of comparison operations.

  • Using random values (RAND(), RANDBETWEEN()) updates too often

As described above, Amazing Fields updates all field formulas anytime the user edits any field on the card and when any custom field or other data causes a card to update.  When using with formulas involving random numbers this can cause some surprises because the value will keep changing.  Because of this I don't recommend using random values to determine a field output.  

Formula Details

Formulas are evaluated using the amazing Jexl library.  The libary allows formulas that are very similar to Excel and Google Sheets but also has a few differences and extra supported operations.  For full technical details, see the Jexl page.

Unary Operators

Operation Symbol
Negate !

Example:  !CompletedField

Binary Operators

Operation Symbol
Add, Concat +
Subtract -
Multiply *
Divide /
Divide and floor //
Modulus %
Power of ^
Logical AND &&
Logical OR ||

Example:  (4+5)*6

Comparisons

Comparison Symbol
Equal ==
Not equal !=
Greater than >
Greater than or equal >=
Less than <
Less than or equal <=
Element in array or string in

The  in operator can be used to check for a substring: "Cad" in "Ron Cadillac", and it can be used to check for an array element: "coarse" in ['fine', 'medium', 'coarse']

Example:  NameField == "John Snow"

Ternary operator

Conditional expressions check to see if the first segment evaluates to a truthy value. If so, the consequent segment is evaluated. Otherwise, the alternate is. If the consequent section is missing, the test result itself will be used instead.

Expression Result
"" ? "Full" : "Empty" Empty
"foo" in "foobar" ? "Yes" : "No" Yes
ProgressField > 70.0 ? "Good" : "Bad" Good

Value Types

Type Examples
Booleans truefalse
Strings "Hello "user"", 'Hey there!'
Numerics 6, -7.2, 5, -3.14159
Arrays ['hello', 'world!']
Note: Boolean values for true and false are true and false not TRUE and FALSE. Attempts to use the uppercase values will fail and not resolve correctly.

Groups

Parentheses work just how you'd expect them to:
Expression Result
(83 + 1) / 2 42
1 < 3 && (4 > 2 || 2 > 4) true

Constant Values

The following values may be referenced in a formula.

Identifier Description
NULL Represents an empty value. Clears the field to contain nothing.
CURRENT_VALUE The current value of the field. Useful to determine if the field has been set yet or not.
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