How the calculators work

Standard (amortizing) loan calculator

The required monthly payment on a fixed-rate loan uses the standard amortization formula, where P is the balance, r is the monthly interest rate (annual rate ÷ 12), and n is the number of months:

payment = P × r × (1 + r)ⁿ / ((1 + r)ⁿ − 1)

At 0% interest, this simplifies to a flat split: payment = P / n.

From there, the loan is simulated month by month: each month's interest is the current balance times the monthly rate; the rest of the payment (plus any extra payment that month) reduces the principal. This is what lets extra payments shorten the payoff and reduce total interest — every extra dollar goes straight to principal, so every future month's interest is calculated on a smaller balance.

Deferment works by accruing interest on the original balance for the deferment period without any payment; if capitalization is enabled, that accrued interest is added to the balance before the standard amortization schedule begins.

Income-driven / income-contingent repayment calculator

Each year, the payment is calculated directly from income:

payment = max(0, income − threshold) × rate%

Interest (or indexation) accrues on the balance every month at the given annual rate, independent of the payment. If a year's payment is less than that year's accrued interest, the difference is added to the balance (negative amortization); otherwise the difference reduces it. Income is assumed to grow at a constant annual rate for projection purposes. If the balance reaches zero before the forgiveness term ends, the loan is paid off; otherwise, any remaining balance at the end of the term is shown as forgiven.