Monthly payment, total interest, and the real cost of your home loan.
A mortgage is almost certainly the largest financial commitment you will ever make, so understanding the numbers before you sign anything is essential. A mortgage calculator takes four inputs — home price, down payment, interest rate, and loan term — and tells you exactly what your monthly payment will be, how much total interest you will pay over the life of the loan, and what the home truly costs after financing. The difference between a well-informed mortgage decision and a poorly-informed one can be worth tens of thousands of dollars.
Most first-time buyers focus almost entirely on the monthly payment, which is understandable — it has to fit in your budget today. But the monthly payment is only part of the picture. On a $400,000 home with a 30-year loan at 7%, you will pay over $558,000 in interest alone before you own the home outright. That means the total cost of the home is nearly $960,000 — more than double the purchase price. Knowing that number before you buy changes how you think about the decision.
Lenders use the standard amortization formula to calculate your monthly payment: M = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1], where P is the loan principal, r is the monthly interest rate (annual rate divided by 12), and n is the total number of payments (years × 12). This formula ensures that every payment covers both the interest accrued that month and a portion of the principal, so the loan is fully paid off at the end of the term.
In the early years of a mortgage, the vast majority of each payment goes toward interest, not principal. On a $320,000 loan at 7%, your first monthly payment of roughly $2,129 consists of about $1,867 in interest and only $263 in principal reduction. By year 20, that ratio has flipped — the same payment now applies about $1,100 to principal. This front-loading of interest is why paying extra early in your loan has such a dramatic effect on total cost.
A 30-year loan gives you a payment of about $2,329/month but costs $488,000 in total interest. A 15-year loan raises your payment to $3,146/month — $817 more — but cuts total interest to $216,000. The 15-year option saves you $272,000 in interest. Whether that trade-off is worth it depends on your cash flow and what else you would do with that $817/month.
Most lenders cap your total monthly debt payments — including the new mortgage — at 43% of gross monthly income. If you earn $8,000/month, your total debt payments cannot exceed $3,440. If you already have $800 in car and student loan payments, your maximum mortgage payment is $2,640.
A credit score above 760 typically qualifies you for the best available rates. The difference between a 620 score and a 760 score on a $350,000 loan can be 1.5–2 percentage points, which translates to $300–$400/month in additional payment and over $100,000 in extra interest over 30 years.
A larger down payment reduces your loan size and eliminates private mortgage insurance (PMI) if you reach 20%. PMI typically costs 0.5–1.5% of the loan annually — on a $350,000 loan, that is $145–$437/month added to your payment until you have 20% equity.
30-year mortgages have lower monthly payments but significantly higher total interest. 15-year mortgages cost more each month but build equity faster and save dramatically on interest. Some lenders also offer 10- and 20-year terms.
These vary wildly by location — from under 0.5% annually in Hawaii to over 2% in New Jersey and Illinois. On a $400,000 home, that is a difference of $2,000–$8,000 per year added to your true housing cost.
Typically 0.5–1% of home value per year. On a $400,000 home, budget $167–$333/month. Lenders require this, and it is usually escrowed into your monthly payment.
A commonly cited rule is to budget 1% of home value annually for maintenance. On a $400,000 home, that is $333/month that does not appear in your mortgage payment but must come from somewhere.
The 30-year mortgage is by far the most popular choice in the US because of its lower monthly payment, which gives borrowers more flexibility. However, the lower payment comes at an enormous cost in interest. The 15-year mortgage costs more each month but can save hundreds of thousands of dollars over the life of the loan and builds equity at roughly twice the speed.
The right choice depends on your situation. If your income is stable and you have strong cash flow, the 15-year option is often the better financial decision — the forced savings of higher payments plus dramatically lower interest can accelerate wealth building. If you are early in your career, have other high-priority goals (retirement contributions, emergency fund), or expect income variability, the 30-year with intentional extra payments gives you flexibility without locking you into a higher minimum payment. The key insight: a 30-year mortgage does not prevent you from paying it off early. You can make 15-year-equivalent payments voluntarily and retain the option to revert to the minimum if needed.