interaction design - Should the results of a calculation happen automatically, or require a button push?

This is a problem I've ran into several times, and I imagine that others have ran across it too.

Basically, I have a complicated equation, and I want the user to type in the values needed to calculate the results. (For what it's worth, there are good default values for the variables in the equation.)

I've constructed a GUI that shows the user the names of the variables/fields that need to be provided, along with a control (like a numeric up/down, slider, etc.) to enter the values of the field. Below that, I have an area that shows the output of the equation.

So my question is this. Should my results area automatically change the calculated results whenever the user makes any changes to any of the input fields, or should they push a "Calculate" button to generate the results?

One other thing that probably applies is that the user is not trying to "get the right answer" out of the results. They shouldn't be using the output to then go back and modify the input to get a better answer.

Are there any benefits to requiring the button press? Is the extra click to see the results worth those benefits?

Answer

If none of the following apply, then you are probably better off without a calculation button:

Possible reasons for a calculation button:

• 
  

  Calculation has side effects to system and therefore probably shouldn't be done until user has completed input

  
  
  


• 
  

  Output interferes with input

  
  
  
  
  • 
    

    Calculation takes a noticeable amount of time (e.g. 0.5 seconds) and therefore appears after delay and may cause user to believe that the result of previous input is actually the result of last input

    
    
  
  
  • 
    

    Calculation blocks UI for more longer than half the amount of type it takes the user to type a character than (e.g. 0.1 seconds)

    
    
  
  
  
  • 
    

    Output and input appear in the same place (e.g. a simple hardware calculator)

    
    
  
  
  
  


• 
  

  Output produces with partial input may result in a confusing output (e.g. error if trying to calculate 0!)

  
  


• 
  

  You need to make sure the user has entered all inputs (hasn't forgotten to change defaults), although a calculation button alone will not be enough for this

  
  
  


• 
  

  System may miscalculate if inputs changed out of expected order (though this should really be fixed in system, not UI)