Create a template overload of this file:
buddypress\bp-templates\bp-legacy\buddypress\members\single\profile\profile-loop.php
Look at bp_the_profile_field_name() and bp_the_profile_field_value() – they display the value.
You’ll need to write a conditional using
bp_get_the_profile_field_name and bp_get_the_profile_field_value() so that you can use the values without displaying them. 
For example: 
if( bp_get_the_profile_field_name() == 'current_weight' )
  $current_weight = bp_get_the_profile_field_value(); 
if( bp_get_the_profile_field_name() == 'starting_weight' )
  $starting_weight = bp_get_the_profile_field_value(); 
$weight_lost = $starting_weight - $current_weight; 
echo '<br/>weight lost so far: ' . $weight_lost;
		
	 
	
	
	
 
		
			
	
	
		
		 @shanebp Thank you so very much for the examples.  I wish I knew more of what I was doing to implement it.  Was kind of hoping their could have been some way to calculate this on the profile page through a simple additional field-such as a display only field.  I wish I was smart like you and this would be a no brainer and it would be done already….
Will