The monthly mortgage payment formula is:
M = P × [r(1+r)^n] / [(1+r)^n − 1]
Where:
Example: $300,000 loan at 6.5% for 30 years:
r = 0.065 / 12 = 0.005417, n = 360
M = 300,000 × [0.005417 × (1.005417)^360] / [(1.005417)^360 − 1] = $1,896/month
Over the full 30 years, total payments = $1,896 × 360 = $682,560. You paid $382,560 in interest on top of the $300,000 principal — more than the home's original price again in interest.
The choice between a 15-year and 30-year mortgage is one of the most significant financial decisions homeowners make:
| 15-Year Mortgage | 30-Year Mortgage | |
|---|---|---|
| Loan Amount | $350,000 | $350,000 |
| Interest Rate | 6.0% | 6.5% |
| Monthly Payment | $2,955 | $2,212 |
| Total Payments | $531,900 | $796,320 |
| Total Interest Paid | $181,900 | $446,320 |
| Interest Savings | $264,420 saved with 15-year | |
The 15-year mortgage saves $264,420 in interest but requires $743/month more. The 30-year's lower payment provides flexibility — you can always make extra payments to pay off faster without being locked into the higher obligation. Many financial advisors suggest taking the 30-year but paying it as if it were a 20-year mortgage.
Several rules of thumb help determine how much mortgage you can responsibly take on:
Lenders also assess your debt-to-income ratio (DTI). Most conventional lenders require a DTI below 43%, with the best rates going to borrowers under 36%.
Down payment impact: A 20% down payment avoids Private Mortgage Insurance (PMI), which costs 0.5–1.5% of the loan annually. On a $400,000 home, that is $166–$500/month in extra cost until you reach 20% equity.
In the early years of a mortgage, the vast majority of each payment goes toward interest, not principal. This is called amortization:
| Payment # | Payment Amount | Interest Portion | Principal Portion | Balance Remaining |
|---|---|---|---|---|
| 1 | $1,896 | $1,625 | $271 | $299,729 |
| 12 | $1,896 | $1,611 | $285 | $297,979 |
| 60 (yr 5) | $1,896 | $1,549 | $347 | $285,982 |
| 180 (yr 15) | $1,896 | $1,280 | $616 | $235,888 |
| 300 (yr 25) | $1,896 | $758 | $1,138 | $139,128 |
| 360 (yr 30) | $1,896 | $10 | $1,886 | $0 |
Based on $300,000 at 6.5% for 30 years. Notice that after 5 years of payments, you have only paid down ~$14,000 of principal. This is why making extra principal payments early is so impactful.
Mortgage rates vary based on both market conditions and personal financial factors. Here is what affects your rate:
| Credit Score Range | Approximate Rate Adjustment | Monthly Payment Difference* |
|---|---|---|
| 760–850 | Best available rate (e.g., 6.5%) | — |
| 700–759 | +0.25% | +$50/month |
| 680–699 | +0.5% | +$101/month |
| 660–679 | +0.75% | +$153/month |
| 640–659 | +1.5% | +$311/month |
| 620–639 | +2.25% | +$474/month |
*Based on a $300,000 30-year mortgage. A poor credit score could cost $474 more per month — $170,640 extra over the life of the loan.
Even small additional payments can save tens of thousands in interest and years off your mortgage:
Before making extra payments, ensure your mortgage has no prepayment penalty and consider whether the money might earn more in tax-advantaged investments (401k, IRA) if your mortgage rate is low.
"Before taking on a mortgage, consider your total monthly housing costs — principal, interest, taxes, and insurance. Most financial experts recommend keeping housing costs below 28% of gross monthly income to maintain financial stability."
A common guideline is the 28/36 rule: spend no more than 28% of gross monthly income on housing costs, and no more than 36% on total debt payments. On a $75,000/year salary, that means a maximum housing payment of about $1,750/month.
A 15-year mortgage has higher monthly payments but saves dramatically on interest — often $150,000–$300,000 over the loan life. A 30-year mortgage provides lower, more flexible payments. Many experts suggest a 30-year mortgage with aggressive extra payments, giving you flexibility without being locked in.
The biggest factors are: credit score (760+ gets best rates), down payment size (20%+ is ideal), loan term (15-year rates are lower), loan type (conventional vs FHA vs VA), and current Federal Reserve monetary policy which influences all rates.
Private Mortgage Insurance (PMI) is required on conventional loans when your down payment is less than 20%. It costs 0.5–1.5% of the loan per year ($1,500–$4,500 annually on a $300,000 loan). Avoid it by putting 20%+ down, or request cancellation once your equity reaches 20%.
The interest rate is the cost of borrowing the principal. APR (Annual Percentage Rate) includes the interest rate plus other loan costs (origination fees, points, mortgage insurance). APR is always higher than the interest rate and gives a more accurate picture of total loan cost. Use APR to compare different loan offers.
In the early years of a mortgage, most of each payment is interest. Over time, more goes to principal. For example, on a $300,000 mortgage at 6.5%, your first payment is about 86% interest and 14% principal. By year 25, it flips to roughly 40% interest and 60% principal.
A larger down payment reduces PMI, lowers your loan amount, and may improve your rate. Extra mortgage payments reduce interest costs and build equity faster. The best choice depends on your current mortgage rate vs. expected investment returns — if your mortgage rate is 7% and investments return 10%, extra investments may win.
Conventional loans generally require a minimum of 620, but rates are best at 760+. FHA loans allow scores as low as 580 with a 3.5% down payment, or 500 with a 10% down payment. VA and USDA loans have no official minimum but lenders typically require 620+.