How do you calculate how much your roommate has to pay when he earns more than you do?

To calculate how much your roommate should pay when he earns more, you can use a proportional method based on income. First, determine the total income of both roommates and the total rent or shared expenses. Then, calculate each person's percentage of the total income and use that percentage to allocate the expenses accordingly. This way, your roommate pays a larger share of the expenses relative to his higher income.