The contribution margin, sometimes used as a ratio, is the difference between a company's total sales revenue and variable costs. In other words, the contribution margin equals the amount that sales exceed variable costs. This is the sales amount that can be used to, or contributed to, pay off fixed costs.he contribution margin, sometimes used as a ratio, is the difference between a company's total sales revenue and variable costs. In other words, the contribution margin equals the amount that sales exceed variable costs. This is the sales amount that can be used to, or contributed to, pay off fixed costs.