Contrition margin indicates why operating income changes as the number of units sold changes. The contrition margin when Emma sell 5 packages is $400 (1,000 in total revenues $600 in total variable costs); the contrition margin when Emma sells 40 packages is $3,200 ($8,000 in total revenues minus $4,800 in total variable costs). When calculating the contribution margin, be sure to subtract all variable costs. For example, if Emma incurred some variable selling costs because she paid a commission to salespeople for each package they sold at the fair, variable costs would include the cost of each package plus the sales commission paid on it.