Add up all expected retirement income sources
Estimate Social Security benefits
Estimate pension income
Estimate annuity income
Estimate withdrawals from retirement accounts
Estimate investment income
Estimate rental or business income
Subtract expected taxes
Subtract expected healthcare costs
Subtract expected housing costs
Subtract expected debt payments
Subtract expected living expenses
Adjust for inflation
Compare total income to total expenses
Divide annual income by 12 to get monthly income
Recalculate regularly as assumptions change