To calculate damage during combat use the two cards below to setup your Attacker and Blocker. A card's Power value goes in the left box and Toughness value in the right. Select any abilities that are in play such as First Strike, Deathtouch, etc, and when you're ready click the Battle button!
The Result section will show who won the battle. Lifelink if returned will be in a green box. The other boxes will light up according to how the card was set.