There is no min/max function present in Revit. You can limit a parameter to a certain range though with an IF or nested IF statement. Below is the Revit help file which explains it pretty well. Post again if you get stuck.
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You can use conditional statements in formulas to define actions in a family that depend on the state of other parameters. With conditional statements, the software enters values for a parameter based on whether a specified condition is satisfied. Conditional statements are useful in certain circumstances; however, they make families more complex and should be used only when necessary.
For most type parameters, conditional statements are unnecessary, because the type parameter itself is like a conditional statement: If this is the type, then set this parameter to a specified value. Instance parameters are a more productive place to use conditional statements, particularly when they are used to set a parameter that does not vary continuously.
Syntax for Conditional Statements
A conditional statement uses this structure: IF (<condition>, <result-if-true>, <result-if-false>
This means that the values entered for the parameter depend on whether the condition is satisfied (true) or not satisfied (false). If the condition is true, the software returns the true value. If the condition is false, it returns the false value.
Conditional statements can contain numeric values, numeric parameter names, and Yes/No parameters. You can use the following comparisons in a condition: <, >, =. You can also use Boolean operators with a conditional statement: AND, OR, NOT. Currently, <= and >= are not implemented. To express such a comparison, you can use a logical NOT. For example, a<=b can be entered as NOT(a>b).
The following are sample formulas that use conditional statements.
Simple IF: =IF (Length < 3000mm, 200mm, 300mm)
IF with a text parameter: =IF (Length > 35', “String1”, “String2&rdquo
IF with logical AND: =IF ( AND (x = 1 , y = 2), 8 , 3 )
IF with logical OR: =IF ( OR ( A = 1 , B = 3 ) , 8 , 3 )
Embedded IF statements: =IF ( Length < 35' , 2' 6" , IF ( Length < 45' , 3' , IF ( Length < 55' , 5' , 8' ) ) )
IF with Yes/No condition: =Length > 40 (Note that both the condition and the results are implied.)
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