Mortgage Calculator: How to Calculate Your Monthly Payment
Updated March 2026 — amortization formulas, APR explained, real-world examples and money-saving tips
1. How a mortgage works
A mortgage is a loan used to purchase real estate, where the property itself serves as collateral. You borrow a lump sum (the principal) from a lender and repay it over time in monthly installments. Each payment consists of two parts: principal (which reduces your debt) and interest (the cost of borrowing).
In the early years, most of your payment goes toward interest. As the loan matures, the balance shifts — more goes to principal and less to interest. This is called amortization.
Understanding how mortgage math works is essential before you sign: a 0.5% difference in interest rate on a $300,000 loan over 30 years means roughly $30,000 more or less in total interest paid.
2. Fixed-rate vs adjustable-rate mortgages
The two main types of mortgage are:
| Feature | Fixed-Rate Mortgage | Adjustable-Rate (ARM) |
|---|---|---|
| Interest rate | Stays the same for the entire term | Fixed for an initial period, then adjusts periodically |
| Monthly payment | Predictable, never changes | Can increase or decrease after the fixed period |
| Initial rate | Usually higher | Usually lower (introductory rate) |
| Risk | No rate risk | Payment could rise significantly |
| Best for | Long-term homeowners who want stability | Short-term owners or when rates are expected to drop |
In Europe, fixed-rate mortgages dominate (>80% of new loans). In the US, the 30-year fixed-rate mortgage is the most popular product. ARMs (such as 5/1 or 7/1 ARMs) offer lower initial rates but carry the risk of payment increases.
3. The monthly payment formula
The standard fixed-rate mortgage payment is calculated using this formula:
Monthly payment formula:
M = P × r / (1 − (1 + r)^−n)
Where:
- M = monthly payment
- P = principal (loan amount)
- r = monthly interest rate (annual rate / 12)
- n = total number of payments (years × 12)
Example: $300,000 loan at 6.5% for 30 years.
- r = 0.065 / 12 = 0.005417
- n = 30 × 12 = 360 payments
- M = 300,000 × 0.005417 / (1 − 1.005417^−360) = $1,896/month
Total interest paid over 30 years: ($1,896 × 360) − $300,000 = $382,560. That’s more than the loan itself — which is why even small rate differences matter enormously.
4. Interest rate vs APR: what’s the difference?
When shopping for a mortgage, you’ll see two numbers:
- Interest rate: the “pure” cost of borrowing, used to calculate your monthly payment.
- APR (Annual Percentage Rate): includes the interest rate PLUS all mandatory fees — origination fees, points, mortgage insurance, closing costs. This is the number to compare when evaluating different lenders.
A mortgage with a 6.0% interest rate and 6.4% APR is more expensive than one with 6.2% interest rate and 6.3% APR, despite having a lower headline rate. Always compare APRs, not just interest rates.
In the EU, the equivalent metric is APRC (Annual Percentage Rate of Charge), which lenders must disclose by law.
5. Real-world examples with numbers
Here are three common scenarios for home buyers in 2026:
| Scenario | Loan | Rate | Term | Payment | Total interest |
|---|---|---|---|---|---|
| Starter home | $200,000 | 6.0% | 30 years | $1,199 | $231,640 |
| Family home | $400,000 | 6.5% | 30 years | $2,528 | $510,080 |
| Same home, 15-year | $400,000 | 5.8% | 15 years | $3,339 | $201,020 |
The 15-year mortgage on the same $400,000 loan saves over $309,000 in interest compared to the 30-year option. The payment is higher ($3,339 vs $2,528), but the total cost is dramatically lower. Use our free mortgage calculator to run your own numbers.
6. How to save thousands on your mortgage
- Shop around. Get quotes from at least 3–4 lenders. A 0.5% rate difference on $300,000 over 30 years saves roughly $30,000 in total interest.
- Choose a shorter term if you can afford it. Going from 30 to 15 years dramatically reduces total interest — often by 50% or more.
- Make extra payments. Even $200/month extra on a $300,000 loan at 6.5% cuts about 8 years off the loan and saves over $130,000 in interest.
- Consider refinancing. If rates drop 0.5–1% below your current rate, refinancing can save significant money. Calculate the break-even point (closing costs / monthly savings).
- Avoid PMI if possible. Private Mortgage Insurance (required with less than 20% down in the US) adds $100–300/month. A larger down payment eliminates it entirely.
7. Common mortgage mistakes
- Only looking at the monthly payment. A low payment over a longer term means far more interest paid overall. Always check total cost.
- Confusing interest rate and APR. The interest rate doesn’t include fees. Compare APRs for an apples-to-apples comparison.
- Forgetting closing costs. Budget 2–5% of the home price for closing costs (appraisal, title insurance, origination fees, etc.).
- Borrowing the maximum you qualify for. Banks approve based on debt-to-income ratios, but comfortable and maximum are very different things. Keep housing costs under 28–30% of gross income.
- Not reading the Loan Estimate. This standardized document (required in the US) shows all costs, rate, and terms. Read it carefully before closing.
8. Free online mortgage calculator
Run your mortgage numbers in seconds with our free calculator:
- Monthly payment with standard amortization
- Total interest paid over the life of the loan
- Full amortization schedule (payment by payment)
- Compare different rates, terms, and loan amounts
- Works on desktop and mobile
- No registration required — instant results
Need more financial tools? Try our compound interest calculator, loan simulator, or explore all free ANIMA tools.