Loan Calculator — Monthly Payments, Interest & Amortization
A loan calculator takes three inputs — principal, interest rate, and term — and tells you exactly what you will pay each month, how much of that is interest, and what the loan costs in total. Understanding these numbers before you sign is the difference between an affordable commitment and a decade of regret.
The Monthly Payment Formula Explained
Every fixed-rate loan uses the same amortization formula to calculate the monthly payment:
M = P × [r(1 + r)ⁿ] ÷ [(1 + r)ⁿ − 1]
Where:
- M — monthly payment
- P — principal (the amount borrowed)
- r — monthly interest rate (annual rate ÷ 12)
- n — total number of monthly payments (years × 12)
For example, a $200,000 mortgage at 6% for 30 years: r = 0.06 ÷ 12 = 0.005, n = 360. The formula produces a monthly payment of $1,199. Over 30 years that is $431,676 paid in total — $231,676 in interest on a $200,000 loan. The loan calculator does this arithmetic instantly so you can adjust any variable and see the result in real time.
How Amortization Works
Amortization is the process of spreading loan repayment across equal monthly payments. Although each payment is the same dollar amount, the split between principal and interest shifts dramatically over the loan term.
In the early months, most of each payment covers interest — because interest is calculated on the outstanding balance, which is still close to the full principal. As you pay down the balance, less interest accrues each month, so more of each fixed payment chips away at the principal. By the final year of a 30-year mortgage, a payment that was 80% interest at the start is now almost entirely principal.
This is why making even small extra payments early in a loan's life has an outsized effect on total interest — every extra dollar reduces the balance that future interest is calculated against.
How to Use the Loan Calculator
- Enter the loan amount (principal). This is the amount you are borrowing — not the purchase price. For a mortgage, subtract your down payment. For a refinance, use your current outstanding balance.
- Enter the annual interest rate. Use the rate on your loan offer, not the APR (which includes fees). For comparison shopping, run the calculator with each lender's quoted rate to see the total interest difference over the full term.
- Set the loan term. Common terms are 5 years for personal and car loans, and 15 or 30 years for mortgages. Shorter terms mean higher monthly payments but significantly less total interest.
- Read the summary. The calculator shows your monthly payment, total amount paid, and total interest. These three numbers give you an immediate read on affordability and cost. Cross-check the monthly payment against your take-home pay using the salary calculator — a common rule of thumb is that housing costs should not exceed 28–30% of gross monthly income.
- Review the amortization schedule. The full schedule shows the principal and interest split for every payment over the life of the loan. Scroll through to see how quickly (or slowly) your equity builds, and how a few extra payments per year would compress the payoff date.
Monthly Payment and Total Interest by Loan Scenario
The table below shows calculated monthly payments and total interest for common loan amounts at 5% and 7% annual interest rates. Use the loan calculator to model your specific figures.
| Principal | Rate | Term | Monthly Payment | Total Interest |
|---|---|---|---|---|
| $10,000 | 5% | 5 years | $189 | $1,323 |
| $10,000 | 7% | 5 years | $198 | $1,881 |
| $20,000 | 5% | 5 years | $377 | $2,645 |
| $20,000 | 7% | 5 years | $396 | $3,761 |
| $50,000 | 5% | 5 years | $944 | $6,613 |
| $50,000 | 7% | 5 years | $990 | $9,403 |
| $200,000 | 5% | 30 years | $1,074 | $186,511 |
| $200,000 | 7% | 30 years | $1,331 | $279,016 |
Note how a 2 percentage point rate difference on a 30-year $200,000 mortgage adds $92,505 in total interest — more than 46% of the original loan amount. Rate shopping is not a minor detail.
Tips for Reducing Total Interest Paid
Make a larger down payment
A larger down payment reduces the principal, which reduces both the monthly payment and total interest. On a home purchase, putting down 20% instead of 10% on a $300,000 property reduces the loan from $270,000 to $240,000 — saving roughly $27,000 in total interest on a 7%, 30-year mortgage, plus eliminating the requirement for private mortgage insurance (PMI).
Choose a shorter term
A 15-year mortgage on the same $200,000 at 7% produces a monthly payment of $1,798 — $467 more per month than the 30-year equivalent — but saves over $150,000 in total interest and builds equity twice as fast. If the higher payment is manageable on your budget, the shorter term is almost always the better financial outcome. Use the percentage calculator to work out what percentage of your take-home pay the higher payment would represent.
Make extra principal payments
On an amortizing loan, any payment above the required monthly amount goes directly to principal. Adding $100/month to a $200,000 30-year mortgage at 7% cuts roughly 4.5 years off the loan term and saves over $50,000 in interest. Most lenders allow extra payments with no penalty — check your loan agreement and specify that extra payments should be applied to principal, not prepaid interest.
Refinance when rates drop meaningfully
Refinancing replaces your existing loan with a new one at a lower rate. The break-even point — when your monthly savings offset the closing costs — typically falls between 18 and 36 months. If you plan to stay in the property beyond that and rates have dropped by at least 0.75–1 percentage point, refinancing usually makes financial sense. Run the numbers with the loan calculator using your current balance and remaining term as inputs.
Shop multiple lenders before committing
Rate differences between lenders on the same loan type routinely exceed 0.5 percentage points. On a $300,000 30-year mortgage, that gap is worth over $30,000 in total interest. Mortgage pre-approval applications within a 45-day window are typically treated as a single credit inquiry under FICO scoring models, so shopping aggressively carries minimal credit score impact.
Common Questions
What is the difference between a mortgage and a personal loan?
Both use the same amortization math. The key differences are collateral, term, and rate. A mortgage is secured against the property — if you default, the lender can foreclose. This collateral allows lenders to offer much lower rates and longer terms (10–30 years). Personal loans are unsecured, so lenders charge higher rates to compensate for the risk, and terms are typically 1–7 years. The loan calculator works for both — just enter the correct principal, rate, and term for the loan type.
What is the difference between interest rate and APR?
The interest rate is the cost of borrowing the principal expressed as an annual percentage. The APR (Annual Percentage Rate) includes the interest rate plus fees — origination fees, mortgage points, broker fees, and some closing costs — expressed as a single annualised figure. APR is always equal to or higher than the interest rate and is a better basis for comparing loan offers from different lenders. For calculating monthly payments, use the interest rate, not the APR.
What is a fixed rate vs a variable rate?
A fixed-rate loan locks in the same interest rate for the entire term. Your monthly payment never changes, which makes budgeting straightforward. A variable-rate (or adjustable-rate) loan starts with a fixed period — often 5, 7, or 10 years — and then adjusts periodically based on a benchmark rate like the Secured Overnight Financing Rate (SOFR). Variable rates are typically lower at the start but carry the risk of rising payments. The loan calculator models fixed-rate loans; for variable rates, calculate the payment at the introductory rate and again at a stress-tested higher rate to understand your exposure.
How does the amortization schedule help with planning?
The schedule shows your outstanding balance at any point in time, which matters for several decisions. If you are considering selling a property, your balance tells you the minimum sale price needed to pay off the loan. If you are evaluating a refinance, your current balance is the new principal. If you want to pay off the loan early, the schedule shows exactly how much you still owe on any given date. It also illustrates the equity-building rate — early in the schedule, equity grows slowly; later, it accelerates.
Can I use this calculator for car loans?
Yes. Enter the financed amount (purchase price minus down payment and trade-in), the dealer or lender's quoted rate, and the term (typically 36, 48, 60, or 72 months). The calculator will produce the monthly payment and total interest, which you can compare against the dealer's quoted payment to verify the numbers are correct before signing.
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Enter any loan amount, interest rate, and term to get your monthly payment, total interest, and a full amortization schedule — free, no signup, runs in your browser.
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