Loan Interest Calculator

How to use this calculator

The Loan Interest Calculator shows you the total interest you’ll pay over the life of a loan, not just the monthly payment. Most people focus on the monthly number and miss the bigger picture. This tool fixes that.

Standard tab is for basic loans. Enter the Loan Amount (the principal you’re borrowing), the Annual Interest Rate, and the Loan Term in years. The calculator returns your monthly payment, total amount paid, and total interest paid.

Extra Payment tab lets you add a fixed extra amount to each monthly payment. Enter the same loan details, then enter the Extra Monthly Payment. The calculator shows how many months you save and how much total interest you avoid. This is one of the most eye-opening features: even $100 extra per month on a 30-year mortgage can save tens of thousands of dollars.

Fixed vs Variable tab lets you compare a fixed-rate loan against a variable-rate estimate. Enter the fixed rate on one side, a starting variable rate and an assumed rate change per year on the other. This helps you decide whether the certainty of a fixed rate is worth the premium.

What amortization means and why it matters

Amortization is the process of paying down a loan through equal periodic payments, where each payment covers both interest and principal. Early in the loan, most of your payment goes toward interest. Later, most goes toward principal. The total payment stays the same throughout, but the split shifts.

On a 30-year mortgage, you might spend the first 10 years paying mostly interest and barely touching the principal. Understanding this is the first step to making smarter payoff decisions.

Here’s why this matters: if you’re considering selling your home in 5 years, you haven’t paid down much principal yet. The equity you’ve built is mostly from price appreciation, not from your payments. And if you’re thinking about refinancing, you need to compare the remaining loan balance and interest exposure against the new terms, not just the monthly payment difference.

Amortization also explains why extra principal payments are so powerful early in a loan. When you pay extra in month 1, every subsequent payment saves interest on that extra amount for the full remaining term. When you pay extra in month 340, you save interest only for the final 20 months.

The formulas

Monthly Payment Formula:

M = P x [r(1+r)^n] / [(1+r)^n - 1]

Where:

• M = monthly payment
• P = principal (loan amount)
• r = monthly interest rate = annual rate / 12
• n = total number of payments = term in years x 12

Total Interest Paid:

Total Interest = (M x n) - P

Remaining Balance after k payments:

Balance = P x [(1+r)^n - (1+r)^k] / [(1+r)^n - 1]

This last formula is what your lender uses to calculate your payoff amount at any given month. It’s also how extra payment calculators determine how much your balance drops when you pay ahead.

Total interest on a $200,000 mortgage at different rates

This table assumes a 30-year fixed-rate mortgage with a $200,000 loan amount and standard monthly amortization.

Interest Rate	Monthly Payment	Total Paid	Total Interest
3.0%	$843	$303,480	$103,480
4.0%	$955	$343,800	$143,800
5.0%	$1,074	$386,640	$186,640
6.0%	$1,199	$431,640	$231,640
7.0%	$1,331	$479,160	$279,160
8.0%	$1,468	$528,480	$328,480

The difference between a 3% and 7% rate on a $200,000 loan is $175,680 in total interest over 30 years. That’s not a marginal difference. That’s money you either spend on interest or keep for everything else.

Moving from 5% to 6% alone costs an extra $45,000 over the life of the loan, more than the annual income of many borrowers. This is why rate shopping matters enormously on large, long-term loans.

Real examples

Common mistakes

Mistake 1: Only comparing monthly payments, not total cost

Dealers and lenders know that consumers anchor on monthly payments. Stretching a loan from 48 to 72 months lowers the monthly number, which feels like a better deal, but you pay more interest overall and spend more years in debt. Always compare total interest paid, not just the monthly figure.

Mistake 2: Ignoring fees in rate comparisons

A loan quoted at 5.5% with $3,000 in origination fees may cost more than a loan at 5.9% with no fees, depending on how long you hold it. The APR (Annual Percentage Rate) accounts for fees in a standardized way. For mortgages especially, compare APRs rather than just the base interest rates, and factor in your expected time in the home.

Mistake 3: Assuming extra payments automatically go to principal

Some lenders apply extra payments to future scheduled payments rather than reducing the principal balance. If your extra payment doesn’t reduce principal, you don’t save the interest you expected. Call your lender or check the payment portal to confirm that overpayments are being applied to principal directly.

Mistake 4: Forgetting the effect of refinancing costs

If you refinance from 7% to 5.5% to save interest, you still pay closing costs (typically 2-3% of the loan amount). On a $400,000 mortgage, that’s $8,000-$12,000 upfront. The break-even point might be 3-5 years away. If you sell the home before that, you’ve lost money on the refinance even though the rate is lower.

How amortization front-loads interest

Most borrowers know that a 30-year mortgage at 6.5% produces a fixed monthly payment. Fewer understand that the composition of that payment shifts dramatically over time. In the early years, you’re mostly paying interest. In the later years, you’re mostly paying principal. The payment amount never changes, but what you’re actually paying for does.

Here’s how that plays out on a $300,000 mortgage at 6.5% with a monthly payment of $1,896:

The crossover point, where principal paid exceeds interest paid in a given month, doesn’t happen on this mortgage until roughly month 252 (year 21). For most of the loan’s life, you’re primarily servicing interest, not building equity.

Biweekly payments: a simple way to add a 13th payment each year

Instead of making 12 monthly payments, you make a half-payment every two weeks. Because there are 52 weeks in a year, that’s 26 half-payments, which equals 13 full payments per year instead of 12. One extra full payment annually sounds minor. The compounding effect over decades is not.

On a $300,000 mortgage at 6.5% with a $1,896 monthly payment:

Monthly schedule: 360 payments, total interest approximately $382,600. Biweekly schedule: payoff in approximately 310 payments (about 25 years 10 months). Total interest approximately $302,000.

You save roughly $80,600 in interest and pay off the mortgage about 4 years and 2 months early without any change to your actual payment amount. The only change is timing: you pay every two weeks instead of once a month. Most banks and servicers support biweekly payment schedules, though some charge a setup fee. You can also replicate the effect by simply adding 1/12 of your monthly payment as extra principal each month, which achieves the same mathematical result without needing a biweekly program.

Bottom line

Total interest paid is the number that actually tells you what a loan costs. Monthly payment is just cash flow. They’re different things, and optimizing for monthly payment while ignoring total interest is one of the more expensive financial habits people develop.

Use this calculator before you commit to any loan. Run the numbers at your actual rate, then at 0.5% higher and 0.5% lower to understand the stakes of rate negotiation. Then try the extra payment feature to see how much even modest additional payments can save you.

The math on long-term loans is unforgiving. A 30-year mortgage at a high rate means you’re spending more on interest than on the home itself. Knowing that before you sign is better than discovering it during year 15.

How amortization front-loads interest

The first payment on a long-term loan is almost entirely interest. The last payment is almost entirely principal. That’s not a quirk, it’s how amortization math works.

On a $300,000 30-year mortgage at 6.5%, the monthly payment is about $1,896. In month 1, the interest charge is $300,000 × 0.065/12 = $1,625. Principal paid: $1,896 - $1,625 = $271. The loan balance after one payment: $299,729.

In month 60 (year 5), the balance is roughly $282,000. Interest: $282,000 × 0.065/12 = $1,527. Principal: $369.

In month 180 (year 15), balance is around $240,000. Interest: $1,300. Principal: $596.

Only in the final years does the balance drop fast enough that principal payments exceed interest. This is why making extra payments in the early years of a loan saves disproportionately more interest than making the same extra payments in the later years.

Many lenders offer biweekly autopay programs for this reason. If yours doesn’t, simply make one extra principal payment per year and the effect is nearly identical.