A mortgage is the single largest financial commitment most households ever make. The monthly payment depends on three variables: how much you borrow, the interest rate, and how long you take to repay. This calculator computes your exact monthly payment and total interest, then shows how additional monthly contributions compress your payoff timeline.
How it works
The mortgage payment formula distributes your loan amount plus compounded interest evenly across each month of the term. Early payments go mostly to interest; later payments go mostly to principal. This is why making extra payments early has an outsized impact — every extra dollar applied to principal avoids years of compound interest on that dollar.
Worked example
On a $300,000 loan at 6.5% for 30 years, your monthly payment is $1,896.20. Over 30 years you pay $682,633 total — $382,633 of that is interest, more than the loan itself. Adding just $200 extra per month shortens the loan by about 6 years and saves roughly $95,000 in interest.
What affects your mortgage payment
Four factors dominate: loan amount (principal), annual interest rate, loan term, and any extra payments. A 1% difference in rate on a $300,000 30-year mortgage changes your payment by roughly $180/month and your lifetime interest by over $70,000. Shorter terms (15 years instead of 30) raise the monthly payment but drastically reduce total interest — often by half.
How extra payments compound savings
Each extra dollar of principal removes not just itself from the loan, but also all future interest that would have accumulated on it. On a 30-year loan at 6.5%, an extra $100/month reduces the payoff time by about 4 years and saves roughly $55,000. Unlike refinancing, extra payments have no closing costs and can be stopped or adjusted any time.
Taxes, insurance, and PMI
This calculator shows principal and interest only. Real monthly housing costs also include property taxes (typically 0.5–2.5% of home value annually), homeowner's insurance, and private mortgage insurance (PMI) if your down payment was under 20%. Add these separately to plan your true monthly housing budget.
Fixed vs. adjustable rate
This calculator assumes a fixed-rate mortgage — the rate stays constant for the entire term. Adjustable-rate mortgages (ARMs) start lower but can reset higher after an initial period (commonly 5, 7, or 10 years). For ARMs, your real lifetime cost depends on rate movements that cannot be known in advance.
Amortization schedules and payment breakdown
An amortization schedule maps every payment across your loan term, showing exactly how much goes to principal versus interest each month. Early in the loan, interest dominates: on a $300,000 mortgage at 6.5%, your first payment allocates $1,625 to interest and only $271 to principal. By month 360, that flips — nearly the entire payment reduces principal. This front-loading is why paying extra early matters enormously. An extra $200 in month 12 saves more interest than an extra $200 in month 300. Most lenders provide amortization schedules; use them to track your actual payoff progress and verify that extra payments are applied to principal, not held as escrow.
Down payment strategy and its long-term impact
Your down payment percentage shapes your entire loan profile. A 20% down payment ($60,000 on a $300,000 home) avoids PMI entirely and reduces your loan to $240,000, lowering monthly payments and total interest substantially. Putting down 10% triggers PMI, typically costing 0.5–1.5% of the loan annually until you reach 20% equity — often thousands of dollars over several years. Conversely, a larger down payment (30–40%) dramatically compresses your payoff timeline and interest burden. The trade-off: larger down payments reduce liquidity and opportunity cost. If you have $60,000, deploying it all into a down payment may forgo higher returns in investments or emergency reserves. Model both scenarios to find your optimal balance between lower borrowing costs and financial flexibility.
Prepayment strategies and penalty considerations
Making extra principal payments is one of the most effective ways to reduce lifetime interest, but execution matters. Bi-weekly payments (half your monthly amount every two weeks) result in 26 half-payments yearly — equivalent to 13 full payments instead of 12 — saving substantial interest without strain. Lump-sum payments (tax refunds, bonuses) applied directly to principal offer immediate high-impact relief. Before committing to aggressive prepayment, verify your loan has no prepayment penalty. Some mortgages, especially older loans or those with special features, impose fees for early payoff. A penalty can negate 2–3 years of extra payment savings. Review your loan documents or contact your lender directly. Once confirmed penalty-free, prepayment becomes a powerful, low-risk wealth-building tool that requires no refinancing, no new paperwork, and complete flexibility to adjust or pause.