How to Calculate Your Monthly Mortgage Payment (And What the Bank Doesn't Tell You)
Every mortgage payment you make is split between two things — and for most of the loan's life, the bank is getting the lion's share. In the first year of a 30-year mortgage at 6.75%, nearly 80% of each payment goes to interest. The principal barely moves. This isn't hidden — it's how amortization works — but very few first-time buyers understand it before signing.
Understanding the maths behind your mortgage payment takes about five minutes and changes how you think about your loan forever. Here's the complete guide: how to calculate any payment from scratch, what actually happens to your money each month, and exactly how much a single percentage point in rate difference costs you over 30 years.
The Mortgage Payment Formula
Every mortgage payment is calculated using the amortization formula, which ensures the loan is paid off to exactly zero at the end of the term — regardless of how much of each payment goes to interest versus principal along the way.
Where:
- M = monthly payment
- P = principal (loan amount)
- r = monthly interest rate (annual rate ÷ 12)
- n = total number of payments (years × 12)
This formula computes a fixed payment that, made every month at rate r for n months, exactly repays the original loan with interest — no more, no less.
Key Definition
Amortization is the process of spreading loan repayment across equal payments over a fixed term. Each payment covers the interest owed for that month first, then any remainder reduces the principal. Because the principal shrinks slowly at first, early payments are mostly interest — this is why your balance barely moves in the first few years of a 30-year mortgage.
Worked Example: A $350,000 Home
Let's run the numbers on a realistic purchase:
- Home price: $350,000
- Down payment: 20% = $70,000
- Loan amount (P): $280,000
- Interest rate: 6.75% annually
- Term: 30 years (360 monthly payments)
Monthly rate: r = 6.75% ÷ 12 = 0.5625% = 0.005625
That $1,816 covers principal and interest only. Your real all-in housing payment will also include:
- Property taxes (typically 1–2% of home value per year, paid monthly via escrow)
- Homeowner's insurance (~$150–$300/month depending on location)
- PMI (private mortgage insurance, required when down payment is under 20%)
Your total monthly housing cost is often $300–$700 higher than the P&I figure.
Where does that $1,816 go in month one?
| Amount | |
|---|---|
| Interest (£280,000 × 0.5625%) | $1,575 |
| Principal | $241 |
| Total payment | $1,816 |
You pay $1,575 in interest and only $241 toward the loan balance. By month 360 (the final payment), those figures almost completely reverse. The Mortgage Calculator shows the full amortization schedule — every month's split for the entire loan life.
How Interest Rates Change Your Payment
A single percentage point in rate has a far larger effect on your total cost than most buyers realise. Here's what a $300,000 loan costs at different rates on a 30-year term:
| Interest Rate | Monthly Payment | Total Interest Paid | vs. 5% Rate |
|---|---|---|---|
| 5.00% | $1,610 | $279,767 | — |
| 6.00% | $1,799 | $347,515 | +$67,748 |
| 7.00% | $1,996 | $418,527 | +$138,760 |
| 8.00% | $2,201 | $492,376 | +$212,609 |
Moving from 5% to 7% costs an extra $386 per month — and $138,760 more in total interest over 30 years. This is why homebuyers who get quotes from just one or two lenders often leave tens of thousands of dollars on the table.
⚠️ Rate vs APR
The interest rate and the APR (Annual Percentage Rate) are different numbers. The APR folds in lender fees and closing costs spread across the loan term, so it's always higher than the stated rate. When comparing lenders, compare APRs — not interest rates alone. A lender advertising a lower rate but charging high origination fees may cost more overall than a competitor with a slightly higher rate and minimal fees.
In the UK, most mortgages are fixed for 2–5 years then revert to a variable rate — the 30-year fixed common in the US doesn't exist. In Australia, variable-rate mortgages tied to the RBA cash rate are most common, with fixed options available for shorter periods. The amortization formula is identical regardless of country; only the term structure differs. Use the Loan Calculator to model any scenario in your preferred currency.
15-Year vs 30-Year: Which Wins?
Using the same $280,000 loan at 6.75%:
| Term | Monthly Payment | Total Interest | Interest Saved |
|---|---|---|---|
| 30-year | $1,816 | $373,760 | — |
| 15-year | $2,481 | $166,580 | $207,180 |
The 15-year mortgage costs $665 more per month but saves over $207,000 in interest. If you can comfortably afford the higher payment, the 15-year wins — financially, every time.
The critical word is "comfortably." Choosing a 15-year mortgage and then struggling to make payments creates financial stress and eliminates your emergency cushion. The 30-year mortgage with deliberate extra payments gives you a lower required minimum (your safety net) while letting you pay extra whenever your budget allows. Which leads to the most powerful lever most homeowners never use.
💡 Pro Tip: Run the Rent vs Buy Comparison First
Before committing to any mortgage, compare the real total cost of buying versus renting over 5–10 years — factoring in appreciation, maintenance (typically 1–2% of home value per year), and the opportunity cost of tying up a large down payment. The Rent vs Buy Calculator runs this side-by-side analysis with your actual numbers.
The Power of Extra Payments
On a 30-year $280,000 mortgage at 6.75%, making just one extra monthly payment per year — $1,816 as a voluntary 13th payment — cuts approximately 4.5 years off the loan term and saves roughly $58,000 in interest.
Extra payments work because every dollar goes directly to principal. Less principal outstanding means less interest charged next month, which means slightly more of your regular payment reduces principal next month too. It's compounding working in your favour.
Three practical approaches:
- Round up: Pay $1,900 instead of $1,816 — an extra $84/month that costs little but makes a meaningful long-term difference
- Annual lump sum: Apply a year-end bonus or tax refund as a principal payment once per year
- Bi-weekly payments: Pay half your monthly amount every two weeks — you make 26 half-payments per year, naturally equivalent to 13 full monthly payments
To understand why compounding makes early extra payments so powerful, the Compound Interest Calculator is worth exploring — it illustrates exactly how interest snowballs on any balance over time.
Frequently Asked Questions
What's the difference between a fixed-rate and variable-rate mortgage?
A fixed-rate mortgage locks in your interest rate for the entire loan term — your payment is identical in month 1 and month 360. A variable-rate (or adjustable-rate) mortgage has an interest rate tied to a market benchmark index that rises and falls. Variable rates are typically lower to start, but carry risk if rates increase significantly. In the US, the 30-year fixed is the most popular product. In the UK and Australia, 2-to-5-year fixed terms followed by reversion to a variable rate are the norm.
How much house can I actually afford?
A commonly used rule is the 28/36 guideline: your housing payment (P&I plus taxes and insurance) should not exceed 28% of gross monthly income, and your total debt payments (housing plus car, student loans, and credit cards) should stay below 36%. A household earning $8,000/month gross can safely target a housing payment up to $2,240. The Mortgage Calculator lets you reverse-engineer the maximum loan for any target monthly payment.
Does making extra payments lower my required monthly payment?
No — unless you specifically request a loan recast, which some lenders offer for a small fee. Extra payments reduce your principal balance and shorten the loan term, but your contractual monthly payment stays the same until the loan is paid off. The benefit is faster payoff and less total interest, not a lower minimum. This is actually useful: you keep the lower minimum as a financial safety net while building equity faster in good months.
Should I pay discount points to lower my interest rate?
Each discount point costs 1% of the loan amount and typically reduces your rate by 0.25%. To decide whether it's worth it, find your break-even point: divide the upfront cost of the points by the monthly savings. If one point on a $280,000 loan costs $2,800 and saves you $46/month, your break-even is about 61 months (just over 5 years). If you plan to stay in the home longer than the break-even period, buying points makes financial sense. If you expect to sell or refinance sooner, skip the points.
What happens to my mortgage if I refinance?
Refinancing replaces your existing mortgage with a new one — typically at a lower rate or for a different term. The new loan pays off the old balance, and you start a fresh amortization schedule. The key cost of refinancing is closing costs (typically 2–5% of the loan amount), which reset your break-even clock. A refinance makes sense when the interest savings over your expected remaining time in the home outweigh the closing costs. The Loan Calculator can compare your current loan terms with potential refinance scenarios.
Calculate Your Mortgage Before You Sign
Understanding your mortgage payment before you commit — not after — puts you in a far stronger position at the negotiating table and gives you a realistic picture of your monthly budget. Use the Mortgage Calculator to model any loan scenario, compare 15-year vs 30-year terms, and see your full amortization schedule in seconds.
