Estimate the monthly payment and total cost of any fixed-rate loan, then see how each payment splits between interest and principal and what the quoted APR leaves out.
A fixed-rate loan is amortized: every payment is the same size, but its composition shifts over time. Early payments are mostly interest, because the outstanding balance is large. As the balance shrinks, more of each payment attacks the principal, until the final payment is almost entirely principal. The lender computes your payment once, at origination, so that the very last payment brings the balance to exactly zero. Everything about the deal, the rate you shopped for, the term you picked, is frozen into that one number.
Variables in plain terms. M is the monthly payment. P is the loan amount, the principal. i is the monthly interest rate, the annual rate divided by 12, so 6.5 percent becomes 0.0054167. n is the total number of payments, years times 12. The formula balances two forces: the compounding growth of the unpaid balance at rate i against the steady drain of the payment. The payment is the single number where those forces cancel at month n.
Run the defaults, a 300,000 dollar loan at 6.5 percent for 30 years. That is i of 0.0054167 and n of 360 payments. The formula returns a monthly payment of 1,896.20 dollars. Multiply out the full term and you pay 682,633 dollars in total, of which 382,633 dollars is interest. Interest exceeds the original loan itself. On a 15-year version at 6.0 percent, the payment jumps to 2,531.57 dollars but total interest falls to 155,683 dollars, less than half. That gap is the entire argument for taking term length seriously.
The payment scales almost linearly with loan size, which makes shopping by monthly payment deceptively easy. The interest column is where the real differences live.
| Loan scenario | Monthly payment | Total interest |
|---|---|---|
| $200,000 at 6.5% for 30 years | $1,264.14 | $255,088.98 |
| $300,000 at 6.5% for 30 years (default) | $1,896.20 | $382,633.47 |
| $400,000 at 6.5% for 30 years | $2,528.27 | $510,177.95 |
| $300,000 at 7.0% for 30 years | $1,995.91 | $418,526.69 |
| $300,000 at 6.0% for 30 years | $1,798.65 | $347,514.57 |
| $300,000 at 6.5% for 15 years | $2,613.32 | $170,397.98 |
Read the rate rows closely. Moving from 6.5 to 7.0 percent costs 99.71 dollars per month, but 35,893 dollars over the term. Moving from 6.5 down to 6.0 saves 35,119 dollars of interest for a payment drop of 97.55 dollars. Half a point sounds like a rounding error at the kitchen table; over 360 payments it is the price of a car. This is why the rate deserves more negotiation effort than almost any other part of a home purchase.
Two numbers matter more than the payment itself: when your payment starts reducing the loan faster than it accrues interest, and when you actually own half the house. On the default loan both arrive much later than most borrowers expect.
| Point in the loan | Interest | Principal | Balance left |
|---|---|---|---|
| Month 1 | $1,625.00 | $271.20 | $299,728.80 |
| Month 60 (year 5) | $1,523.20 | $373.01 | $280,832.93 |
| Month 120 (year 10) | $1,380.41 | $515.80 | $254,328.38 |
| Month 180 (year 15) | $1,182.95 | $713.25 | $217,677.42 |
| Month 233 (year 19.4) | $946.50 | $949.70 | $173,789.36 |
| Month 257 (year 21.4) | $815.04 | $1,081.17 | $149,387.24 |
| Month 300 (year 25) | $532.33 | $1,363.87 | $96,912.49 |
| Month 360 (final) | $10.22 | $1,885.99 | $0.00 |
The first payment is 85.7 percent interest: 1,625 dollars of the 1,896.20 goes to the lender and 271 dollars to the loan. Principal does not overtake interest until month 233, which is 19.4 years into a 30-year term. The balance does not fall below half the original amount until month 257, when 71 percent of the term is already spent.
This is the arithmetic behind the advice to pay extra early. An extra 200 dollars in month 1 prevents 30 years of interest on that 200. The same 200 dollars in month 300 prevents five years of it. Same money, same loan, wildly different effect, which is why the payoff calculator asks when the extra payments start.
The rate you negotiate is the note rate, and it determines the payment. The APR is a different number: it folds the lender's fees into the same calculation, so you can compare offers with different cost structures. Take the default loan and add one discount point (3,000 dollars) plus 4,000 dollars of closing costs. The payment is unchanged at 1,896.20 because fees are paid at closing rather than financed, but you only net 293,000 dollars, and the APR rises to 6.728 percent.
That 0.228 point spread is the honest full-term cost, and it assumes you keep the loan for all 30 years. Most people do not. Recalculate the same 7,000 dollars of fees against an earlier exit and the effective annual cost climbs sharply, because the fee is spread over fewer years of interest savings.
| You sell or refinance after | Effective annual cost of the 7,000 dollars |
|---|---|
| 3 years | 7.384% |
| 5 years | 7.071% |
| 7 years | 6.939% |
| 10 years | 6.843% |
| 30 years (the quoted APR) | 6.728% |
Two rules follow. First, an APR is only comparable between offers with the same term, because the same fee is diluted over twice as many payments on a 30-year loan. Running the same 7,000 dollars against a 15-year loan at 6.0 percent produces a 6.371 percent APR, a spread of 0.371 rather than 0.228, so a shorter loan makes every upfront fee more expensive in APR terms. Second, if you expect to move or refinance within a few years, price the fees against that horizon instead of the quoted APR. A low rate with heavy points is a good deal only if you stay long enough to earn the points back.
Biweekly payment plans are sold as a shortcut to paying off a 30-year mortgage in about 15 years. The mechanism is real, the shortcut is not. Paying half the monthly payment every two weeks produces 26 half-payments a year, which is 13 full payments instead of 12. The extra payment goes to principal, and the loan retires early.
On the default loan, a true biweekly schedule finishes in 628 half-payments, about 24.2 years, and cuts total interest from 382,633 dollars to 294,512, a saving of 88,122 dollars. Paying an extra one-twelfth of the payment with each monthly bill reaches the same place: 290 months and 295,377 dollars of interest. The two are within 900 dollars of each other because they are the same idea counted differently.
So the honest summary is five to six years early, not fifteen. If a company offers to set up a biweekly program for a fee, you are paying for a calendar reminder: add one-twelfth to your monthly principal instead, keep the money, and skip the enrollment paperwork. The one genuine advantage of the biweekly structure is that the extra payment happens automatically, which matters more for some people than the fee does.
The figure this calculator produces is principal and interest only, the part the amortization formula controls. A real mortgage payment usually arrives with property tax and homeowners insurance escrowed on top, plus HOA fees where they apply, and those parts rise over time even though the loan piece never moves. Budget on the escrowed number from your Loan Estimate, not the calculator output alone. For auto loans, add registration and insurance; for personal loans, the quoted figure is usually complete.
The payment this calculator prices is only the headline. The amortization calculator splits every payment into interest and principal and shows the year the split flips. If the payment fits, the mortgage payoff calculator shows what extra principal does to the term. And for revolving debt, where the payment floats instead of amortizing, the credit card minimum payment calculator shows why the same math never finishes the job.
No, this shows principal and interest only. For a full housing cost picture, add property tax, homeowners insurance, and any HOA fees.
This calculator assumes a fixed rate. For adjustable-rate mortgages, model the worst case using the maximum rate allowed by your loan terms.
Not necessarily. If your rate is low and you can reliably invest extra cash at a higher return, the longer term can win. But that requires discipline; the shorter term forces the saving automatically.
Because interest accrues on the outstanding balance, and on day one the balance is the entire loan. On the default example the first payment includes about 1,625 dollars of interest and only 271 dollars of principal. The ratio shifts every month as the balance falls, but principal does not exceed interest until month 233, nearly two decades in. The year-by-year table above shows the crossover.
Mathematically, paying off a 6.5 percent loan early is a guaranteed 6.5 percent return, which beats most safe investments after tax. It loses only if your rate is low and your alternative return reliably exceeds it. The practical case for extra payments is strongest on high-rate debt and weakest on old mortgages written at 3 percent.
For a fixed-rate, fully amortizing loan with no fees rolled in, it matches lender figures to the penny, and the page's math is verified against an independent model before publishing. Real quotes differ by closing costs and escrow, which is why the APR on your Loan Estimate will read slightly above your contract rate.
All calculation logic on this page is implemented from the primary documents below. Each one is listed with the figure it supplies, so any rate or threshold in the result can be traced back to its published source.
Same published formulas, same sourcing standard, one page each.