How to Calculate Your Monthly Mortgage Payment (With Formula and Examples)
Buying a home is the biggest financial decision most people will ever make, and yet surprisingly few buyers actually understand how their monthly mortgage payment is calculated. They plug numbers into an online calculator, get a result, and move on. But understanding the math behind that number gives you real power — the power to compare loan offers intelligently, negotiate better terms, and plan for the true cost of homeownership.
In this guide, we'll break down the exact formula lenders use to calculate your monthly payment, walk through three real-world examples step by step, and cover the additional costs that turn your "monthly payment" into the actual check you write each month. You can follow along with our Mortgage Calculator to verify the numbers.
The Mortgage Payment Formula
Every fixed-rate mortgage payment is calculated using this formula:
M = P × [r(1 + r)^n] / [(1 + r)^n − 1]
Where:
- M = your monthly payment (principal + interest only)
- P = the principal loan amount (what you borrow, not the home price)
- r = your monthly interest rate (annual rate ÷ 12)
- n = total number of payments (loan term in years × 12)
The key detail most people miss: r is the monthly rate, not the annual rate. A 6% annual rate means r = 0.06 / 12 = 0.005 per month. Get this wrong and every calculation downstream is off.
Example 1: $250,000 at 6% for 30 Years
Let's say you're buying a $312,500 home, putting 20% down ($62,500), and borrowing $250,000 at a 6% fixed rate for 30 years.
- P = $250,000
- r = 0.06 / 12 = 0.005
- n = 30 × 12 = 360
Plugging into the formula:
M = 250,000 × [0.005 × (1.005)^360] / [(1.005)^360 − 1]
(1.005)^360 = 6.02258
M = 250,000 × [0.005 × 6.02258] / [6.02258 − 1]
M = 250,000 × 0.030113 / 5.02258
M = 250,000 × 0.005996
M = $1,498.88
Your principal and interest payment is $1,498.88 per month. Over 30 years, you'll pay a total of $539,595 — meaning $289,595 goes to interest alone. That's more than the original loan amount.
Example 2: $400,000 at 7% for 30 Years
A more expensive home with a higher rate — common in the current market:
- P = $400,000
- r = 0.07 / 12 = 0.005833
- n = 360
M = 400,000 × [0.005833 × (1.005833)^360] / [(1.005833)^360 − 1]
(1.005833)^360 = 8.11652
M = 400,000 × [0.005833 × 8.11652] / [8.11652 − 1]
M = 400,000 × 0.04735 / 7.11652
M = 400,000 × 0.006653
M = $2,661.21
At $2,661.21 per month, you'll pay $957,636 over the life of the loan — $557,636 in interest. That one percentage point increase from 6% to 7% on $400K adds roughly $265 per month compared to what a 6% rate would cost ($2,398/mo).
Example 3: $350,000 at 5.5% for 15 Years
Shorter terms mean higher monthly payments but dramatically less interest paid:
- P = $350,000
- r = 0.055 / 12 = 0.004583
- n = 15 × 12 = 180
M = 350,000 × [0.004583 × (1.004583)^180] / [(1.004583)^180 − 1]
(1.004583)^180 = 2.28175
M = 350,000 × [0.004583 × 2.28175] / [2.28175 − 1]
M = 350,000 × 0.010455 / 1.28175
M = 350,000 × 0.008158
M = $2,855.35
The monthly payment is $2,855.35 — higher than Example 1 despite a lower rate, because you're paying it off in half the time. But the total paid is only $513,963, meaning you pay $163,963 in interest. Compare that to $289,595 in interest from Example 1 on a smaller loan. The 15-year term saves you a staggering amount.
Monthly Payment Comparison Table: $300,000 Loan
To see how dramatically interest rates affect your payment, here's a table showing the monthly principal and interest payment for a $300,000 loan at various rates and terms:
| Interest Rate | 15-Year Monthly | 30-Year Monthly | 30-Year Total Interest |
|---|---|---|---|
| 5.0% | $2,372 | $1,610 | $279,767 |
| 5.5% | $2,451 | $1,703 | $313,212 |
| 6.0% | $2,532 | $1,799 | $347,515 |
| 6.5% | $2,613 | $1,896 | $382,633 |
| 7.0% | $2,696 | $1,996 | $418,527 |
| 7.5% | $2,781 | $2,098 | $455,157 |
| 8.0% | $2,867 | $2,201 | $492,480 |
Notice the pattern: going from 5% to 8% on a 30-year loan increases your monthly payment by $591, but the total interest paid nearly doubles from $279,767 to $492,480. This is why even a quarter-point rate reduction is worth negotiating for.
Understanding Amortization: Where Your Money Goes
Here's something that surprises most first-time buyers: in the early years of a 30-year mortgage, most of your payment goes to interest, not principal. On a $300,000 loan at 6%, your first payment of $1,799 breaks down as roughly $1,500 in interest and only $299 toward the principal balance. By month 180 (halfway through), you're paying about $993 in interest and $806 in principal. The split doesn't favor principal until roughly year 19.
This is why selling a home after just a few years can feel financially painful — you've barely made a dent in the loan balance despite making years of payments. It's also why extra payments early in the loan have an outsized impact. An extra $200 per month in year one saves far more in total interest than an extra $200 per month in year 25, because you're eliminating principal that would have accumulated interest for decades.
What the Formula Doesn't Include: PITI and Beyond
The formula above gives you the principal and interest (P&I) portion of your payment. Your actual monthly obligation — what lenders call PITI — includes:
- Principal & Interest: the number from the formula
- Property Taxes: typically 1%–2% of the home's assessed value per year, divided by 12. On a $350,000 home at 1.2%, that's $350/month.
- Homeowners Insurance: usually $100–$300/month depending on location and coverage
- PMI (Private Mortgage Insurance): required if your down payment is less than 20%, typically 0.5%–1% of the loan amount per year. On a $300,000 loan, that's $125–$250/month.
- HOA Fees: if applicable, often $200–$500/month for condos and planned communities
In practice, someone with a $1,799 P&I payment on a $300,000 loan might actually pay $2,500–$2,800/month when all costs are included. Always calculate the full PITI when budgeting.
PMI: The Cost of a Small Down Payment
If you put less than 20% down on a conventional loan, you'll pay Private Mortgage Insurance. PMI protects the lender (not you) in case of default. The rate depends on your credit score and loan-to-value ratio:
| Down Payment | LTV Ratio | Typical PMI Rate | Monthly Cost ($300K loan) |
|---|---|---|---|
| 3% | 97% | 0.85%–1.15% | $213–$288 |
| 5% | 95% | 0.70%–1.00% | $175–$250 |
| 10% | 90% | 0.40%–0.70% | $100–$175 |
| 15% | 85% | 0.30%–0.50% | $75–$125 |
| 20%+ | ≤80% | None | $0 |
The good news: PMI isn't permanent. Once you reach 20% equity (through payments or appreciation), you can request its removal. By law, your lender must automatically cancel PMI when you reach 22% equity based on the original purchase price.
The Power of Extra Payments
Making extra payments toward your principal can save you tens of thousands of dollars and years off your loan. Let's look at the impact on our Example 1 ($250,000 at 6%, 30 years, $1,498.88/month):
| Extra Monthly Payment | Payoff Time | Interest Saved |
|---|---|---|
| $0 (minimum only) | 30 years | $0 |
| +$100 | 25 years, 3 months | $52,039 |
| +$250 | 21 years, 6 months | $104,682 |
| +$500 | 17 years, 10 months | $157,235 |
An extra $250/month saves you over $104,000 in interest and gets you mortgage-free 8.5 years early. Use our Loan Payoff Calculator to model your own extra payment scenarios.
Fixed-Rate vs. Adjustable-Rate Mortgages (ARM)
Everything above assumes a fixed-rate mortgage, where the interest rate stays the same for the entire loan. Adjustable-rate mortgages (ARMs) work differently:
A 5/1 ARM has a fixed rate for the first 5 years, then adjusts annually based on a benchmark rate (like the Secured Overnight Financing Rate, or SOFR) plus a margin. A typical ARM might start at 5.5% for 5 years, then adjust between 3.5% and 10.5% depending on rate caps.
ARMs make sense when you're confident you'll sell or refinance within the fixed period. They carry real risk if rates spike and you can't refinance. For most buyers planning to stay long-term, fixed-rate mortgages offer predictability that's worth the slightly higher initial rate.
When an ARM might make sense:
- You plan to sell the home within 5–7 years
- You expect interest rates to decrease (and plan to refinance)
- The initial rate savings are substantial (1%+ lower than fixed)
- You have financial flexibility to handle payment increases
Tips for Getting a Lower Monthly Payment
- Improve your credit score before applying. A score of 760+ typically gets the best rates. Going from 680 to 760 can save 0.5%–1% on your rate.
- Shop multiple lenders. Rates can vary by 0.5% or more between lenders on the same day. Get at least three quotes.
- Consider buying points. One discount point (1% of the loan amount) typically lowers your rate by 0.25%. On a $300,000 loan, paying $3,000 upfront to go from 6.5% to 6.25% saves $51/month — a break-even point of about 5 years.
- Make a larger down payment. Beyond avoiding PMI at 20%, a larger down payment means a smaller loan and lower monthly payment.
- Choose a longer term if cash flow is tight. A 30-year term has lower payments than 15-year, even if you plan to make extra payments when possible.
Frequently Asked Questions
How much house can I afford on a $75,000 salary?
A common guideline is that your total housing payment (PITI) should not exceed 28% of your gross monthly income. On a $75,000 salary, that's $6,250/month gross, so a maximum of $1,750/month for housing. After taxes, insurance, and PMI, that typically supports a home price of roughly $280,000–$320,000 with 10% down at current rates. However, your actual affordability depends on your other debts, down payment, credit score, and local tax rates.
Is it better to get a 15-year or 30-year mortgage?
A 15-year mortgage has higher monthly payments but saves you a massive amount in interest. If you can comfortably afford the 15-year payment while still saving for retirement and emergencies, it's the better financial move. If the 15-year payment would strain your budget, take the 30-year and make extra payments when you can — you get the lower required payment as a safety net while still paying down the loan faster.
Does making biweekly payments actually save money?
Yes. Paying half your monthly payment every two weeks results in 26 half-payments per year — equivalent to 13 full monthly payments instead of 12. That one extra payment per year can shave about 4–5 years off a 30-year mortgage and save tens of thousands in interest. However, make sure your lender applies biweekly payments correctly; some hold the first half-payment until the second arrives, negating the benefit.
What credit score do I need to get the best mortgage rate?
Most lenders offer their best rates to borrowers with a FICO score of 760 or above. You can still get a mortgage with a score as low as 580 (FHA loans) or 620 (conventional loans), but you'll pay significantly higher rates. A borrower at 660 might pay 0.5%–1.5% more than someone at 760, which on a $300,000 loan translates to $100–$300 more per month.
Should I pay off my mortgage early or invest the extra money?
This depends on your mortgage rate versus expected investment returns. If your mortgage is at 3%, and you expect 8%–10% average stock market returns, investing the extra money mathematically wins. At 7%+ mortgage rates, the guaranteed "return" of paying off your mortgage is very competitive with market returns and carries zero risk. Many people split the difference: make some extra payments for peace of mind while also investing. Use our Compound Interest Calculator to compare scenarios.