Note: the Computed Field Module can be used instead of this custom code (in Drupal 5.x and 6.x)

Imagine you have two existing number fields, called field_product_price and field_postage_price. You want to create a computed field field_total_cost which adds these two fields. Create a new computed field with the name 'Total Cost', and in your computed field's configuration set the following:

• Computed Code:
  
  $node_field[0]['value'] = $node->field_product_price[0]['value'] + $node->field_postage_price[0]['value'];
  • If you are using the E-commerce module the price value is available as $node->price
  • If you are using the UBERCART module the public price value is available as $node->sell_price
• Check 'Display this field'
• Display Format:
  
  $display = '' . $node_field_item['value'];
• Check 'Store using the database settings below'
• Data Type: float
• Data Length: 10,2
• Default Value: 0.00
• Check 'Not NULL'
• Check 'Sortable'

While the example above uses a price and postage which both can be handled using ecommerce modules or Ubercart, there are other examples when you would want to compute two fields and the above instructions/example provide guidance.