Mortgage Calculator
Estimate your monthly mortgage payment including the total interest over the life of the loan.
Mortgage Amortization
Formula
M = P[r(1+r)^n] / [(1+r)^n – 1]
Example
A $400,000 home with $80,000 down at 6.5% for 30 years = ~$2,023/month.
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Understanding the Mortgage
On a 30-year mortgage, the total interest paid usually exceeds the original loan amount itself. Knock 0.5% off the rate and you save tens of thousands. Add 5 years to the term and you give back even more in interest than you saved on the monthly payment.
How it actually works
Estimate your monthly mortgage payment including the total interest over the life of the loan.
The formula is straightforward arithmetic once the inputs are correct; the value of the calculator is in handling the algebraic manipulation reliably and removing transcription errors. Plug in your specific inputs above and the result appears as you type, so you can immediately see how each variable affects the answer.
What the numbers really say
$400,000 at 6.5% over 30 years: $2,528/month, $510,178 total interest. Same loan at 6.0%: $2,398/month, $463,353 total interest - a $46,825 lifetime savings for half a percent. Same loan at 6.5% but 15 years: $3,485/month, $227,338 total interest - a $282,840 lifetime savings for the steeper monthly payment. The differences compound far past intuition.
The deeper context most users miss
Beyond the mortgage payment itself, homeownership carries hidden costs most calculators do not capture: maintenance and repairs typically run 1-3% of home value annually, property taxes have risen faster than general inflation in most US markets, and major capital expenses (roof, HVAC, water heater) hit in unpredictable bursts. A useful frame: take the calculator's monthly P&I number and multiply by 1.4-1.5 for a realistic true monthly housing cost. Renters who run this comparison sometimes find that owning at current rates is meaningfully more expensive than equivalent rentals, with the offset being equity buildup and inflation hedging on the asset itself.
What people get wrong
- Treating pre-approval as affordability. Banks qualify you on debt-to-income ratios that do not consider lifestyle, savings goals, or financial slack. Borrow 70-85% of the pre-approval and you have a stable mortgage. Borrow the maximum and a layoff or rate reset becomes a crisis.
- Skipping the 15-year vs 30-year comparison. Most buyers default to 30-year because the monthly is lower. The 15-year option saves enormous total interest if cash flow allows.
- Forgetting PITI extras. Monthly P&I is only part of the housing cost. Property tax (1-2% of home value/yr), homeowners insurance, PMI if under 20% down, HOA, and maintenance reserves typically add 30-50% to the principal-and-interest figure.
- Not running the refinance break-even. When rates drop, the savings only matter if you stay in the home long enough to recoup closing costs. Run a break-even calculation; if you might move within the break-even window, the savings disappear.
When this calculator helps most
The mortgage calculator is most useful when you are making a real decision - comparing options, sizing a commitment, sanity-checking a quote, or planning ahead. The output is precise to your inputs; the inputs themselves are the place to slow down. Spend extra time on the assumptions you are making about rate, term, timing, or context-specific variables - those swing the answer far more than the formula's arithmetic does. A 5% change in the input often produces a 10-20% change in the output, which means small input errors compound into large output errors.
Where the math comes from
Standard amortization formulas as taught in financial mathematics courses (Brealey-Myers, Principles of Corporate Finance). Federal Reserve Economic Data (FRED) tracks historical mortgage rates. The CFPB's Loan Estimate disclosure regulates the format banks must use to present the same numbers.
Questions and answers
How much house can I afford?
The conservative rule is the 28/36 rule - housing costs (PITI) under 28% of gross monthly income, total debt under 36%. In high-cost areas this is hard to meet, so many buyers go closer to 30/40. Banks may approve more, but the math gets uncomfortable when income drops or expenses rise.
Is a 20% down payment necessary?
Not legally - many programs allow 3-5% down - but under 20% means PMI (private mortgage insurance), which is roughly 0.5-1.5% of loan amount per year. PMI drops off automatically at 78% loan-to-value. If you can put down 20%, the math typically favors doing so.
Should I pay points to lower my rate?
Points are upfront fees (1 point = 1% of loan amount) that reduce the rate. Math works if you stay in the home past the break-even point - typically 5-7 years. Run the calculation: paid points / monthly savings = months to break even.
What is the difference between fixed and adjustable rate?
Fixed-rate locks the rate for the life of the loan. Adjustable-rate (ARM) starts lower for a fixed period (5, 7, or 10 years) then adjusts annually based on a benchmark. ARMs can be cheaper if you definitely sell or refinance before adjustment, riskier if you stay.
Does the loan amount include closing costs?
Only if you roll them in. Closing costs (typically 2-5% of purchase price) are paid at closing - either out of pocket or financed. Financing them increases your loan amount, monthly payment, and lifetime interest.
How does property tax fit into the calculation?
Property tax (typically 1-2% of home value annually) is paid separately. Many lenders escrow it into the monthly payment, collecting 1/12 each month and paying the bill when due. The calculator's P&I figure does not include this.
Sources & References
Authoritative references consulted in building this calculator and educational content. These are primary sources — check directly for the most current figures.
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