How loan payments are calculated
Most installment loans — personal loans, auto loans, mortgages, student loans — use the standard amortization formula. Each monthly payment is split between interest (calculated on the remaining balance) and principal (the portion that actually pays down the loan). Early in the loan, most of your payment goes to interest; later, most goes to principal.
The amortization formula
M = P × [r(1+r)^n] / [(1+r)^n - 1] where M is the monthly payment, P is the principal, r is the monthly interest rate (annual rate ÷ 12), and n is the number of months.
How term length affects total cost
Longer terms mean smaller monthly payments but dramatically more interest paid over the life of the loan. A $25,000 loan at 7.5% APR costs $5,059 in interest over 60 months, but $8,557 in interest over 84 months — an extra $3,498 for the same principal.
Making one extra payment per year — applied directly to principal — on a 60-month loan can shave 6–9 months off the term and save hundreds in interest.
Frequently asked questions
What is amortization?
Amortization is the process of paying off a loan in equal installments. Each payment covers interest (calculated on the remaining balance) plus principal. Early payments are mostly interest; late payments are mostly principal. The amortization schedule shows this split for every payment.
Is a longer loan term always better because of lower payments?
No. Longer terms mean lower monthly payments but dramatically more total interest. A $25,000 loan at 7.5% over 60 months costs $5,059 in interest; the same loan over 84 months costs $8,557 — $3,498 more for the same principal.
What is the difference between APR and interest rate?
Interest rate is the cost of borrowing the principal. APR (Annual Percentage Rate) includes the interest rate PLUS upfront fees (origination fees, points, broker fees), expressed as an annualized rate. APR is always ≥ interest rate. Always compare loans using APR.
Can I pay off a loan early?
Usually yes, but check for prepayment penalties. The Equal Credit Opportunity Act limits prepayment penalties on most consumer loans. Mortgages typically have no prepayment penalty; some personal loans and auto loans do.
What is a 'simple interest' loan?
Interest is calculated daily on the outstanding principal. Most auto loans and personal loans use simple interest. Making payments early reduces principal faster, lowering total interest. Compound interest loans (rare for consumers) charge interest on accrued interest.
What credit score do I need for a personal loan?
Most lenders require 660+ for a personal loan at a reasonable rate. Scores 740+ qualify for the best rates. Below 660, expect rates above 20% — consider alternatives like secured loans, credit union loans, or 0% intro APR credit cards.
How does loan-to-value (LTV) affect my rate?
LTV is the loan amount divided by the asset value. Lower LTV (more down payment) typically qualifies for lower rates because the lender has more cushion if they need to foreclose. Mortgages with LTV above 80% require PMI.
Glossary of key terms
- Principal
- The original amount borrowed, before interest. Each payment reduces principal.
- Amortization Schedule
- A table showing how each payment is split between principal and interest over the life of the loan.
- APR
- Annual Percentage Rate — interest rate plus fees, expressed as an annual rate. The true cost of borrowing.
- Prepayment Penalty
- A fee charged for paying off a loan early. Banned on most consumer mortgages; still common on some auto and personal loans.
- Loan-to-Value (LTV)
- Loan amount divided by asset value. Lower LTV = lower risk for lender = better terms for borrower.
Common mistakes to avoid
- Comparing loans by interest rate alone — fees can make a 'low rate' loan more expensive
- Extending loan term to lower payments — you'll pay thousands more in interest
- Not understanding the amortization curve — early payments barely reduce principal
- Ignoring prepayment penalties before signing
- Using payday loans or title loans — effective APRs often exceed 300%
Pro tips
- Round up your monthly payment to the nearest $50 — the extra goes to principal and shaves months off the term.
- Make biweekly payments instead of monthly — 26 half-payments equal 13 full payments per year, with no extra effort.
- Refinance when rates drop by 1%+ — the closing costs are usually recouped within 2-3 years.
- Always get pre-approved before shopping for a car or home — gives you negotiating power and prevents dealer markups.
- Avoid 'rule of 78s' loans (front-loaded interest) — these make early payoff nearly impossible. Banned in most states but still legal in some.
Results are estimates for educational purposes only and not financial advice. Consult a licensed professional for advice specific to your situation.