For informational purposes only. Not financial, investment, or tax advice. Results are estimates based on the inputs provided. Consult a qualified financial professional before making financial decisions.
Calculator tool
How this calculator works
Use the explanation to understand the formula, assumptions, and practical limits behind the calculator result.
The Amortization Payment Formula
Every standard loan — personal, auto, mortgage, business — uses the same amortized payment formula:
- = principal (loan amount)
- = periodic interest rate = annual rate ÷ 12 (for monthly payments)
- = total number of payments = term in years × 12
- Total interest paid = PMT × −
- Total cost = PMT ×
Each month, interest is charged only on the remaining outstanding balance. As principal falls, the interest portion of each payment shrinks and the principal portion grows — this is reducing-balance amortization.
Worked Example — $20,000 Loan at 6% for 5 Years
Monthly rate:
Payments:
- Total paid: $386.66 × 60 = $23,199.60
- Total interest: 20,000 = $3,199.60
Rate Has More Impact Than Term
| Rate | 3-Year PMT | 3-Yr Interest | 5-Year PMT | 5-Yr Interest |
|---|---|---|---|---|
| 4% | $590.62 | $1,262.32 | $368.33 | $2,099.80 |
| 6% | $608.44 | $1,903.84 | $386.66 | $3,199.60 |
| 8% | $626.57 | $2,556.52 | $405.53 | $4,331.80 |
| 12% | $664.29 | $3,914.44 | $444.89 | $6,693.40 |
Reducing the rate from 8% to 6% on a $20,000 5-year loan saves $1,132 — more than most borrowers save by shortening the term by one year.
Rate beats term: On this table, jumping from 4% to 12% at 5 years costs $4,593 more in total interest than changing rate alone. Negotiating a lower rate before signing delivers more savings than any term adjustment.
The Extra Payment Effect
Paying one extra monthly payment per year on a 270 in interest. On a 30-year mortgage at 7%, one extra annual payment saves 4–5 years and tens of thousands. Every extra dollar applied to principal today eliminates all future interest that would have accrued on that dollar.
Flat-Rate vs Reducing-Balance: A Critical Difference
This calculator uses reducing-balance — the standard for mortgages, auto loans, and most regulated personal loans.
Flat-rate loans (offered by some consumer lenders and car dealers) charge interest on the full original principal for the entire term, regardless of how much you have repaid. A 7.5% flat rate is not 7.5% APR:
| Stated Rate | Method | Effective APR |
|---|---|---|
| 7.5% | Reducing-balance | 7.5% |
| 7.5% | Flat-rate | ~13–14% |
| 10% | Flat-rate | ~18–19% |
Flat-rate loans look cheaper because the quoted percentage is lower — but you pay interest on principal you have already repaid. Always ask lenders which method applies, and compare using APR, not the stated rate.
Before accepting any loan offer: Ask "Is this flat-rate or reducing-balance?" A quoted 10% flat rate is approximately 18–19% APR on a reducing-balance basis — nearly double. The stated rate alone is not enough information to compare offers.
Frequently asked questions
What is the total cost difference between a 3-year and 5-year loan at the same rate?
On a $20,000 loan at 6%: a 3-year loan costs 3,200 — the longer term adds 608 to 221 per month.
The right choice depends on your cash flow. If you can comfortably afford the 3-year payment, the lower total cost is compelling. If the higher payment would strain your budget, the 5-year retains flexibility — you can always make extra payments to accelerate payoff without obligation.
What is the difference between a flat-rate and a reducing-balance loan?
A reducing-balance loan charges interest only on the outstanding principal — so as you repay, the monthly interest amount decreases. This calculator uses reducing-balance.
A flat-rate loan charges interest on the original full amount for the entire term. If a lender quotes 7.5% flat on a 5-year loan, the effective reducing-balance equivalent is roughly 13–14% APR — nearly double. Flat rates look lower but are significantly more expensive. Always verify the method and compare APR figures across offers.
How does my credit profile affect the interest rate I pay?
Your credit profile can strongly affect the rate, but the exact bands vary by country, lender, product, collateral, income, and debt obligations.
Use the calculator to compare quoted offers instead of relying on generic score bands. On a $20,000 5-year loan, the difference between 8% and 18% can add several thousand dollars in extra interest, so improving your profile or comparing lenders before applying can matter.
When comparing offers, ask for the APR or total cost of credit, not only the monthly payment. Fees can make a lower stated interest rate more expensive.
Does this calculator work for mortgages, auto loans, and personal loans?
Yes — the PMT formula is identical for any fixed-rate, reducing-balance amortized loan. The differences between loan types lie in rate, term, and what additional costs are layered on top:
- Mortgage: longer terms (15–30 years), lower rates, plus property taxes, insurance, and possibly PMI
- Auto loan: medium terms (3–7 years), mid-range rates, plus insurance and registration
- Personal loan: shorter terms (1–7 years), higher rates, sometimes origination fees
This calculator gives the P&I component. Factor in product-specific costs before comparing total loan affordability.
What is the difference between a secured and an unsecured loan?
A secured loan requires collateral — an asset the lender can seize if you default. Mortgages use the home; auto loans use the vehicle. The lender's reduced risk translates to lower interest rates.
An unsecured loan (most personal loans, credit cards) has no collateral — the lender relies entirely on your creditworthiness, so rates are higher. Default on either type damages credit and can lead to collections. Converting unsecured debt to secured debt (e.g., using a home equity loan to pay credit cards) increases catastrophic risk even if the rate is lower.
Should I pay off a loan early?
Early payoff can reduce total interest because the balance falls sooner. The savings are largest when the rate is high, the remaining term is long, and extra payments are applied directly to principal.
Before paying early, check for prepayment penalties, unpaid fees, and whether the lender applies extra payments to principal automatically. Also compare the payoff with other priorities such as emergency savings, higher-interest debt, and required monthly cash flow.
This calculator can show the interest effect, but it does not decide whether early payoff is the best financial move for your full situation.
