There are a few ways to calculate ACA subsidies, but the most common way is to use your household income, family size, and the cost of your premium.
Income: The subsidies are available to those with incomes between 100-400% of the federal poverty level. For a family of four, that is $9700 to $388000 per year.
Family size: The subsidies are available to families of any size, as long as at least one person in the family is qualified for ACA.
Premium cost: The subsidies are available to help offset the cost of your monthly premium. The amount of subsidy you receive is based on the cost of the second-lowest silver plan in your area.