From Draftsperson.net
Applies To:
 Revit Structure 2008,2009. May apply to other versions
Tip:
When entering distances or other parameters interactively, you can use a handy calculator built into in Revit. Prefix a formula with '=' to envoke the inbuilt calculator.
Example: Copy a wall three times a given distance: In the Copy command enter pick the first point and instead of picking a second point simply enter =3*600. Results are the wall copies 1800 from first point. Revit calls this “entering a formula”. There are other formulas you can input:
Other examples:
=1000+500 
result: 1500 
=1000/2 
result: 500 
=1000500 
result: 500 
=(100*2)+300 
result: 500 
Formula abbreviations for arithmetic operations & trigonometric functions:
Addition: 
+ 
Subtraction: 
 
Multiplication 
* 
Division 
/ 
Exponentiation 
/ 
Division 
^ {example: x^y is x raised to the power of y} 
Logarithm 
log 
Square root 
sqrt {example: sqrt(16) } 
Sine 
sin 
Cosine 
cos 
Tangent 
tan 
Arcsine 
asin 
Arccosine 
acos 
Arctangent 
atan 
e raised to an x power 
exp 
Absolute Value 
abs 
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