What is the difference between APR and the interest rate?

The interest rate is the annual cost of borrowing the principal expressed as a percentage — it drives the amortization formula but excludes fees. APR (Annual Percentage Rate) is a broader measure required by the Truth in Lending Act that rolls in most lender fees and expresses them as an annualised rate. For example, a 6.0% rate with $3,000 in fees on a $200K 30-year loan results in roughly a 6.12% APR. Comparing APRs (not rates) across lenders gives you the true cost comparison — but even APR excludes some costs like appraisal and title fees, so factor those in separately.

How do extra payments reduce the total interest I pay?

Extra payments reduce the principal directly, which shrinks the balance that future interest accrues on. On a 6% mortgage, every $1,000 extra payment reduces your principal by $1,000 AND saves $60 in interest the first year alone. That $60 saved remains in your balance, accelerating the next period's principal reduction — a compounding effect. Over 25-30 years, a one-time $1,000 extra payment in the first year saves approximately $3,500-$5,700 in total interest (depending on rate), because you avoid paying interest on that $1,000 for the remainder of the loan.

Should I choose a 15-year or 30-year mortgage?

The 15-year mortgage offers a lower interest rate (typically 0.5-1% lower than 30-year) and dramatically less total interest. However, the higher monthly payment reduces your financial flexibility. A good middle ground: take the 30-year for the lower required payment, but make extra payments as if it were a 15-year when your budget allows. This gives you the safety of the lower minimum payment with the interest savings of the accelerated schedule — but be sure your loan has no prepayment penalty.

What is amortization and how does it differ from simple interest?

Amortization is the process of paying off debt through regular payments where each payment covers interest accrued plus a portion of principal, with the principal portion growing over time. Simple interest loans charge interest only on the remaining principal each period (no compounding on unpaid interest). Most mortgages, auto loans, and student loans are amortized. Credit cards, by contrast, typically compound interest daily on the unpaid balance, making them far more expensive for long-term borrowing.

How does my credit score affect my loan rate?

Credit scores are the primary determinant of mortgage and auto loan rates. As of 2024-2025, the spread between excellent (760+) and fair (620-639) credit on a 30-year fixed mortgage can exceed 1.5 percentage points. On a $300K loan, this difference costs roughly $95,000 in additional interest over 30 years. Before applying for a loan, check your credit report, correct errors, pay down revolving balances to lower your utilisation ratio, and avoid opening new credit for 3-6 months to maximise your score.

Calculators2026-05-10

Loan Calculator: Amortization, APR vs Interest Rate, and How Extra Payments Save Thousands

A loan is deceptively simple: you borrow a principal amount, agree to pay it back over time, and the lender charges interest for the privilege. But the relationship between loan amount, interest rate, and term length produces results that defy intuition — a $300,000 mortgage at 6% over 30 years costs $647,515 in total, meaning you pay more in interest ($347,515) than the house itself. A difference of just one percentage point in interest rate changes the monthly payment by $190 and the total cost by $68,000 on that same mortgage. Our free loan calculator uses the standard amortization formula to compute your exact monthly payment, generates a full principal-vs-interest breakdown for every payment in the loan's life, and lets you model the effect of extra payments — showing you exactly how many years and how many thousands of dollars one extra payment per year shaves off your loan.

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Loan & Mortgage Calculator

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Step-by-Step Guide

Enter Your Loan Details

Input the loan amount (principal), annual interest rate (as a percentage, e.g., 6.5 for 6.5%), and loan term in years. The calculator supports both fixed-rate amortised loans (standard for mortgages and auto loans) and simple-interest loans. For adjustable-rate loans, you can model multiple rate periods by adding rate adjustment dates. All currency fields accept any numeric format — the calculator is currency-agnostic and displays whatever decimal separator you prefer.

Review Your Monthly Payment and Amortization Schedule

Your monthly payment is displayed instantly along with: total payments (monthly × term in months), total interest paid, and total cost (principal + interest). Below this, the full amortization schedule shows every payment period — payment number, starting balance, payment amount, interest portion, principal portion, and ending balance. A colour-coded stacked bar chart makes the interest-vs-principal ratio visible at a glance for each year of the loan.

Model Extra Payment Scenarios

Add extra payments — one-time lump sums, recurring monthly extras, or annual additional payments — and the calculator recomputes the entire amortization schedule. The results show: new payoff date (how many years earlier), total interest saved (in currency and as a percentage), and a side-by-side comparison of the original vs. accelerated schedule. A single extra payment of $1,000 on a 30-year $300K mortgage can save over $3,500 in interest — the calculator shows you exactly why.

Tips & Best Practices

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The amortization formula: M = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1], where M = monthly payment, P = principal, r = monthly interest rate (annual rate ÷ 12), n = total number of payments (years × 12). For a $300K loan at 6% for 30 years: r = 0.005, n = 360, M = 300,000 × [0.005(1.005)³⁶⁰] / [(1.005)³⁶⁰ − 1] = $1,798.65. This formula guarantees the balance reaches exactly $0 after the final payment.

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APR (Annual Percentage Rate) is NOT the same as the interest rate. APR includes the interest rate plus lender fees (origination fee, points, closing costs) expressed as an annualised percentage. A loan advertised at 6.0% interest with $5,000 in fees on a $300K loan might have an APR of 6.15%. By law in the US, lenders must disclose both — compare APRs, not interest rates, when shopping for loans.

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The first 5 years of a 30-year mortgage are almost entirely interest. On a $300K 6% loan, your first payment splits to $1,500 interest and only $298.65 principal. After 5 years (60 payments), you've paid $88,259 in interest but reduced your balance by only $19,726 — you still owe $280,274. This front-loaded interest structure is why moving or refinancing within the first few years erases much of the financial benefit of homeownership.

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A 15-year mortgage vs. 30-year: on $300K at 6%, the 30-year payment is $1,799 (total interest = $347,515), while the 15-year payment is $2,531 (total interest = $155,582). The 15-year loan costs $732 more per month but saves $191,933 in interest — and you own the home in half the time. If your budget allows the higher payment, the total cost savings are enormous.

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Extra payment strategy: making one extra monthly payment per year (bi-weekly payments achieve the same effect naturally) on a 30-year mortgage pays it off in approximately 24 years instead of 30, saving tens of thousands in interest. On the $300K 6% loan, one extra $1,799 payment annually saves $64,288 in interest and knocks 5 years off the loan term. There is almost no easier way to save this much money.

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Debt-to-income (DTI) ratio = total monthly debt payments divided by gross monthly income. Most mortgage lenders cap DTI at 43% (the qualified mortgage limit), with many preferring 36% or below. Our calculator displays the estimated DTI for your loan based on the income you enter, helping you gauge approval likelihood before formally applying.

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Balloon payments: some commercial and private loans structure a low monthly payment for 5-7 years, then require the entire remaining principal as a single balloon payment. A $100K loan at 5% amortised over 30 years but due in 7 years: monthly payment = $537, but the balloon after 7 years = $87,843. If you cannot refinance when the balloon comes due, you risk losing the asset. Always model balloon scenarios before signing.

Frequently Asked Questions

The formula M = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1] derives from the present value of an annuity — the sum of all discounted future payments must equal the principal. Each payment covers the interest accrued since the last payment (r × remaining balance), with the remainder reducing principal. Because the balance shrinks with each payment, the interest portion decreases and the principal portion increases over time, even though the total payment stays constant. This is why the amortization schedule is non-linear: early payments are mostly interest, late payments are mostly principal.

A loan calculator is the single most valuable financial tool you can use before signing a loan agreement. In 30 seconds, it reveals the true cost of borrowing — the total interest, the amortization curve, and the staggering savings from even modest extra payments. Run the numbers before you commit; your future self will thank you.

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