How Are Mortgage Payments Calculated?
A mortgage payment can stay almost identical every month even though what happens inside the payment changes continuously.
With a standard fixed-rate amortising mortgage, each payment first covers the interest charged on the outstanding balance. What remains reduces the principal. As that balance becomes smaller, the interest portion usually falls and the principal portion rises.
The formula determines the payment. The amortisation schedule shows what the payment actually does over time.
Updated August 2026
The short answer
A standard mortgage payment is mainly determined by three numbers:
- the amount borrowed
- the interest rate
- the number of payments
The payment is calculated so that, if the scheduled payments are made and the interest rate does not change, the mortgage balance reaches zero at the end of the term.
A larger loan or higher interest rate generally increases the payment. A longer term usually lowers the payment but gives interest more time to accumulate.
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The three numbers behind the payment
Amount borrowed
This is the mortgage principal after the down payment. A ₹50,00,000 home with a ₹10,00,000 down payment leaves a ₹40,00,000 mortgage before any financed fees.
Interest rate
The rate determines how much interest accrues against the outstanding balance. The exact conversion from an annual rate to a payment-period rate can differ by country and mortgage convention.
Number of payments
A 20-year mortgage with monthly payments normally contains 240 scheduled payments; a 30-year mortgage contains 360. More payments spread the principal over a longer period, but usually increase total lifetime interest.
A mortgage payment worked through with real numbers
A ₹50,00,000 home with a ₹10,00,000 down payment, borrowing at 8.5% over 20 years with monthly payments. Taxes and insurance are left out here so the mathematical mortgage payment stays clear.
Mortgage principal
₹40,00,000
Monthly principal & interest
₹34,712.93
Total interest
₹43,31,103
Total repayment
₹83,31,103
The schedule runs for 240 payments, and interest makes up 52.0% of everything repaid.
Illustrative example — 8.5% is not a current market rate.
What happens inside the first payment?
| Opening balance | Interest | Principal | Closing balance | |
|---|---|---|---|---|
| Payment of ₹34,712.93 | ₹40,00,000 | ₹28,333.33 | ₹6,379.60 | ₹39,93,620 |
At the beginning of the mortgage, interest is being calculated against almost the full ₹40,00,000 balance. That makes the interest share comparatively large — about ₹28,333 of the ₹34,713 payment.
Once the principal portion of the first payment reduces the balance, the next period begins with a slightly smaller amount outstanding. That means the next interest charge is also slightly smaller.
The payment stays similar. The split does not.
| Point in mortgage | Payment | Interest | Principal | Remaining balance |
|---|---|---|---|---|
| Payment 1 | ₹34,712.93 | ₹28,333.33 | ₹6,379.60 | ₹39,93,620 |
| End of year 1 | ₹34,712.93 | ₹27,818.27 | ₹6,894.66 | ₹39,20,391 |
| End of year 5 | ₹34,712.93 | ₹25,037.90 | ₹9,675.03 | ₹35,25,087 |
| End of year 10 | ₹34,712.93 | ₹19,936.25 | ₹14,776.68 | ₹27,99,753 |
| Final payment | ₹34,712.93 | ₹244.15 | ₹34,468.78 | ₹0 |
The scheduled payment may barely move, but the amount reducing the balance grows as the mortgage progresses.
The mortgage payment formula
Payment = P × r × (1 + r)n / ((1 + r)n − 1)
- P = mortgage principal
- r = effective interest rate per payment period
- n = total scheduled payments
The formula finds one periodic payment large enough to pay the interest due each period and reduce the remaining balance to zero by the end of the mortgage.
How the quoted annual rate becomes a payment-period rate depends on the mortgage convention in use — dividing by twelve is not universally correct. Canadian mortgages, for example, conventionally compound semi-annually. FinanceCalcWorks applies the convention supported for the selected country and shows the relevant assumptions alongside the calculator.
What if the interest rate is 0%?
Without interest or fees the payment is simply the principal divided by the number of payments. A ₹12,00,000 mortgage over 120 monthly payments gives ₹10,000.00 per month, and total repayment equals the amount borrowed. The difference between that and a normal mortgage is exactly what interest adds.
What happens when the mortgage rate changes?
The principal is identical in all three rows below — ₹40,00,000 over 20 years. Only the cost of borrowing changed.
| Rate | Monthly payment | Total interest | Total repayment |
|---|---|---|---|
| 7.5% | ₹32,223.73 | ₹37,33,695 | ₹77,33,695 |
| 8.5% | ₹34,712.93 | ₹43,31,103 | ₹83,31,103 |
| 9.5% | ₹37,285.25 | ₹49,48,459 | ₹89,48,459 |
Because the mortgage runs for many years, one percentage point changes the monthly payment by about ₹2,572 and lifetime interest by roughly ₹6,17,356.
Illustrative rate comparison — not current mortgage rates.
A lower monthly payment can cost more overall
| Term | Monthly payment | Total interest | Total repayment |
|---|---|---|---|
| 15 years | ₹39,389.58 | ₹30,90,125 | ₹70,90,125 |
| 20 years | ₹34,712.93 | ₹43,31,103 | ₹83,31,103 |
| 30 years | ₹30,756.54 | ₹70,72,354 | ₹1,10,72,354 |
The 30-year mortgage spreads repayment across more months, so the required monthly payment is lower — about ₹3,956 less than the 20-year version. But the balance remains outstanding for considerably longer, and total interest rises by ₹27,41,251.
Monthly affordability and lifetime cost are different questions.
How the down payment changes the mortgage
The down payment does not enter the amortisation formula as a separate variable. It changes the principal. Same ₹50,00,000 home, same rate and term:
| Down payment | Mortgage | Monthly payment | Total interest |
|---|---|---|---|
| ₹5,00,000 | ₹45,00,000 | ₹39,052.05 | ₹48,72,491 |
| ₹10,00,000 | ₹40,00,000 | ₹34,712.93 | ₹43,31,103 |
| ₹15,00,000 | ₹35,00,000 | ₹30,373.81 | ₹37,89,715 |
Your mortgage payment may not be your full monthly housing cost
Principal and interest describe the mortgage itself. Depending on the property, lender and country, the household may also pay property tax, home insurance, mortgage insurance, service or association charges and other recurring property costs.
FinanceCalcWorks keeps the principal-and-interest payment separate from optional ownership costs so it is clear what the mortgage itself costs and what the wider ownership estimate includes. The Mortgage Calculator has fields for those costs when you want the fuller picture.
What does amortisation mean?
Amortisation is the process of gradually reducing the mortgage balance through scheduled payments. For each payment: interest is calculated, the payment covers that interest, the remainder reduces principal, and the next period starts with a smaller balance. Repeat until the balance reaches zero.
View a full amortisation schedule →
What happens if you pay more than required?
Adding ₹5,000 to every payment on the same mortgage changes both the payoff date and the lifetime cost:
| Scenario | Payments made | Total interest |
|---|---|---|
| Scheduled payments only | 240 | ₹43,31,103 |
| Plus ₹5,000 per payment | 178 | ₹30,32,149 |
The extra amount finishes the mortgage 62 payments early and avoids about ₹12,98,954 of interest, because each early principal reduction lowers the balance that future interest is calculated on.
A fixed payment does not mean you pay the same interest every month
On a normal fixed-rate amortising mortgage, the rate may stay fixed and the scheduled payment may stay fixed. The interest amount inside the payment usually does not. Interest depends on the outstanding balance, so as the balance declines the interest portion falls and the principal portion rises — visible in the table above, where the interest share moves from ₹28,333 to ₹244.15.
Five mortgage-payment mistakes worth avoiding
- Comparing mortgages only by monthly payment — a lower payment may simply come from a longer term.
- Assuming the interest rate applies to the original balance every year. Amortising interest is calculated against the outstanding balance.
- Treating principal and interest as the whole housing budget. Taxes, insurance and service charges may sit outside the mortgage payment.
- Assuming every country calculates periodic mortgage interest identically. Conventions differ.
- Assuming an extra payment automatically has the same effect everywhere. Lender rules, penalties and how payments are applied can differ.
Is a mortgage payment the same as an EMI?
In India, the term EMI — equated monthly instalment — is commonly used for a level monthly loan payment. For a standard fixed-rate amortising home loan the concept is broadly similar: the scheduled payment contains both interest and principal.
Terminology and interest conventions still differ between countries and products, which is why FinanceCalcWorks adapts terminology and calculation assumptions where supported rather than treating every mortgage as identical.
Calculate your own mortgage
Change the home price, down payment, rate or term and see the monthly payment, total interest, balance over time and amortisation schedule update together.
Also relevant: Affordability Calculator · Down Payment Calculator · Overpayment Calculator
Questions about mortgage payments
Why is so much of my mortgage payment interest at the beginning?
Interest is calculated against the outstanding mortgage balance. That balance is highest at the start, so the interest charge is also highest. As principal is repaid, less interest is charged and more of the payment goes toward principal.
Does a longer mortgage term lower the payment?
Usually yes. The principal is spread across more payments. The trade-off is that the mortgage stays outstanding longer, which can substantially increase total interest.
Does a larger down payment reduce my monthly mortgage payment?
Usually, because a larger down payment reduces the amount that needs to be borrowed. Holding the rate and term constant, a smaller principal produces a smaller scheduled payment.
Why is my lender's mortgage payment different from this calculator?
A lender may use different compounding conventions, payment timing, fees, insurance requirements, taxes, rounding rules or product-specific terms. FinanceCalcWorks provides estimates based on the assumptions displayed by the calculator.
Do extra mortgage payments reduce interest?
When an extra payment is applied to principal, it reduces the outstanding balance sooner. Future interest can then be calculated against a smaller balance. Loan terms may include restrictions or charges, so actual results depend on the mortgage agreement.
Are property taxes and insurance part of the mortgage formula?
No. The standard principal-and-interest mortgage formula does not include them. They can still be important parts of the total monthly housing cost.
How these examples were calculated
The examples on this page use the same calculation logic as the FinanceCalcWorks Mortgage Calculator. Calculations use full precision internally and values shown here are rounded for readability.
Mortgage conventions vary by country and product. The calculator shows the assumptions used for the selected scenario, including how the annual rate is converted to a payment-period rate.
See the calculation methodology for how formulas are documented and tested.
Examples are for general information and planning, not financial, tax or legal advice. Actual terms, costs and rules may differ.