What This Calculator Does

Keep the existing equal-installment, equal-principal and interest-only monthly modes, and add monthly extra principal or a one-time principal payment in a selected period. Compare the actual schedule with the same loan without extras: payoff month, months saved, interest saved and adjusted final payment.

When to Use It

Compare cash-flow burden and early principal repayment for a fixed-rate loan. For taxes, insurance, PMI and HOA use the mortgage budget calculator. Principal here is the amount borrowed, rather than a home’s purchase price.

Inputs Explained

Principal is 1000–1 trillion, nominal annual rate 0–30%, and term 1–30 years with years×12 a whole number. Zero interest works. Extra principal is 0–1 trillion; its selected one-time period is a whole number from 1 through the original month count. Enter a valid first-payment month in years 0001–9999; the final month must remain in range. Loan-type choices label scenarios rather than impose lender eligibility or policy.

How the Calculation Works

Monthly rate r = annual percentage/1200 and n = years×12. Equal installments use M = P·r/[1−(1+r)^−n], or P/n at zero rate. Equal principal uses P/n scheduled principal plus interest on the opening balance. Interest-only pays opening-balance interest until the final original period. Each month accrues interest first, pays scheduled principal, then applies monthly extra and the selected one-time extra. Principal cannot exceed the remaining balance. The smaller balance changes next month’s interest, not interest already accrued this month.

Example

Borrow 1200 at 0% for one year and add 100 monthly. The scheduled payment is 100 and each actual payment is 200, finishing in six months instead of twelve. A first payment in 2026-11 gives payoff in 2027-04 versus 2027-10. Interest savings are zero because the baseline is interest-free. With 150 extra monthly, four payments of 250 leave an adjusted final payment of 200.