See your monthly P&I payment, total interest cost, and exactly how many years an extra $200 a month cuts off your loan.
The lender quotes you a tidy monthly payment and stops there. What they do not put on the same page is the part that should make you sit up: on a 30-year loan at today's rates, you will hand over more in interest than you borrowed in the first place. This calculator puts the whole picture in front of you — principal versus interest in your very first payment, total interest across the life of the loan, and a side-by-side look at what five different extra-payment amounts do to both.
The numbers here are grounded in your actual inputs — Home Price, Down Payment, Interest Rate (APR), Loan Term in years, and an optional Extra Monthly Payment. The tool computes monthly P&I, builds an amortization schedule, and charts how your loan balance shrinks with and without extra payments so you can see the payoff date move in real time.
Why the total interest number should be your first stop
On a $350,000 home with a $70,000 down payment at 6.5% for 30 years, your loan is $280,000. The monthly principal and interest payment comes out to roughly $1,770. That feels manageable. What is less comfortable is that you will pay approximately $357,000 in interest over the life of that loan — more than the original loan amount. The calculator surfaces that number upfront, not buried in the fine print.
That interest-to-loan ratio is the single most useful thing to know before you decide on a term. A 15-year mortgage at the same rate carries a higher payment — around $2,440 per month — but total interest drops to roughly $159,000. That is a $198,000 difference. Seeing both scenarios before you sign is the entire point of running the numbers here.
What the first month's split actually looks like
The tool shows you first-month interest versus first-month principal in the key metrics row. On a $280,000 loan at 6.5%, interest in month one is roughly $1,517 — about 86% of your total payment. You reduce the actual loan balance by only $253. That ratio is not a rounding error; it is how amortization works. Interest front-loads, and principal repayment accelerates only in the back half of the loan.
This matters when you are weighing whether to refinance early, sell in seven years, or pay extra now. The breakeven grid in the tool models exactly when extra payments start to meaningfully dent principal versus just keeping pace with interest. Most buyers who look at that grid start making extra payments earlier than they planned.
The extra payment lever and what it actually saves
The Extra Monthly Payment field is what the calculator calls 'the magic lever.' On that $280,000 loan at 6.5%, adding just $200 per month extra cuts the loan from 30 years to about 24.5 years and saves roughly $67,000 in interest. Adding $500 per month drops it to around 21 years and saves $112,000. The bar chart in the tool compares all five extra-payment scenarios side by side so you can see the savings and time reduction at a glance.
What the chart makes visible is that the earlier you start extra payments, the bigger the compounding benefit. An extra $200 added in year one of a 30-year mortgage does more work than $200 added in year fifteen. If you have any room in your monthly budget, even a small consistent extra payment changes the trajectory of the loan significantly.
Using the amortization schedule for real decisions
The tool generates a full amortization table showing balance, principal paid, and interest paid for each year of the loan. If you are planning to sell in seven years, scroll to year seven and read the remaining balance — that is what you will owe at the sale, before you see any equity from appreciation. For a $280,000 loan at 6.5%, you still owe roughly $246,000 after seven years. Whether that leaves you above or below what you paid matters for the next purchase.
The balance line chart with and without extra payments is the clearest way to see this. The red line shows the slow early decline of the standard payment schedule; the teal line with extra payments breaks away from it visibly within the first few years. Seeing the gap widen is more persuasive than any table.
Rate changes have an outsized effect — here is why
Move the Interest Rate input up or down by half a point and watch total interest change. Going from 6.5% to 7.0% on a $280,000 loan adds roughly $29,000 in total interest over 30 years. Going from 6.5% to 6.0% saves roughly $27,000. Half a percentage point is not a rounding error on a 30-year loan.
The same logic applies to loan term. Choosing a 20-year term instead of 30 at 6.5% raises the monthly payment by about $400 but cuts total interest nearly in half. Whether that monthly delta fits your cash flow is a different question — but the total cost difference is real and the tool shows it instantly when you toggle the Loan Term field.
How to use it
- Enter the Home Price and Down Payment in dollars — the tool computes the loan amount automatically.
- Set Interest Rate (APR) to the rate your lender quoted, and choose Loan Term in years (10, 15, 20, or 30).
- Optionally enter an Extra Monthly Payment to see how early payoff scenarios compare in the bar charts.
- Read the four key metrics: monthly payment, first-month interest split, total interest, and years saved.
- Open the Amortization tab to review year-by-year balance and interest paid, and use it to model a planned sale date.
Who it's for
- First-time buyer comparing 15 vs 30-year term — Inputs the same $300K loan at 6.75% and sees that 15 years costs $740 more per month but saves $192,000 in interest, then decides whether the cash flow difference fits.
- Existing homeowner weighing refinance timing — Enters current balance, remaining term, and new rate to see whether the interest savings on a refi justify the closing costs they will pay upfront.
- Buyer deciding whether to put 10% or 20% down — Runs both down payment amounts to compare monthly payment differences and the total interest impact before choosing a down payment strategy.
- Investor modeling debt service on a rental property — Uses the monthly P&I output to subtract from projected rent and see whether net cash flow is positive at current rates before making an offer.
Key terms
- Amortization
- The process of spreading a loan into fixed payments over time, each covering a portion of interest and principal. Early payments are interest-heavy; later ones shift toward principal.
- Principal
- The original loan balance, separate from interest. When you make extra payments, they reduce principal directly and lower future interest charges.
- APR (Annual Percentage Rate)
- The yearly cost of the loan expressed as a percentage, including the base interest rate. Use this figure, not the nominal rate, for accurate payment calculations.
- Loan-to-Value (LTV)
- The loan amount divided by the home's appraised value. Below 80% LTV typically eliminates PMI requirements; this calculator's down payment field drives that ratio.
- Payoff date
- The month and year when the loan balance reaches zero. Extra monthly payments move this date earlier and reduce total interest paid between now and then.
Frequently asked questions
Does this calculator include property taxes and insurance?
No — it calculates principal and interest only. Property taxes, homeowner's insurance, and PMI are separate line items that vary by location and lender. Add your estimated monthly escrow amount on top of the P&I figure this tool returns to get your total payment.
How accurate is the amortization schedule?
The schedule uses standard fixed-rate amortization math: each month's interest is the remaining balance multiplied by the monthly rate, and principal is the difference between the payment and interest. It matches what a lender produces assuming no prepayment penalties and a fixed rate for the full term.
What does 'Extra Monthly Payment' actually do to the loan?
It applies directly to principal reduction each month, which lowers the balance used to calculate interest the following month. That compounding effect means even a modest extra payment reduces both total interest and payoff date by more than a simple multiplication would suggest.
Should I include my down payment in the home price field?
Enter the full purchase price in Home Price and your down payment separately. The tool subtracts one from the other to compute the loan amount. If you enter only the loan amount in Home Price and zero for down payment, the math still works — you just lose the down payment percentage display.
What APR rate should I use if I have not locked a rate yet?
Use the rate from your pre-approval letter, or check current average rates for your loan type and credit profile. Even rough estimates are more useful than the default — model a quarter-point higher and lower to see how sensitive your payment is to rate movement before you lock. Run the numbers free here, save the scenario, and bring the amortization table to your lender conversation.