How Is a Mortgage Payment Calculated? Formula + Example

Borrow 300,000 at 5% a year over 25 years, and the monthly payment comes out to 1,753.77. Stretch that across all 300 payments and the total interest works out to 226,131 — roughly three-quarters of the original loan, paid on top of the loan itself. That number isn't an estimate or a lender's quote. It comes straight out of one equation, and once you can see the equation, the loan stops feeling like a black box.

This guide breaks down that equation in plain language and walks through the math one step at a time. The numbers work in any currency — the formula doesn't care whether you're borrowing in dollars, pounds, euros, rupees, or pesos. To test your own figures as you read, the mortgage calculator uses the same formula and produces the same result.

What the mortgage payment formula actually is

The mortgage payment formula is a single equation that turns three inputs — the loan amount, the interest rate, and the term — into a fixed monthly payment that pays the loan off completely by the end of the term. It's the standard amortising payment formula, and the same equation governs most fixed-rate car loans, student loans, and personal loans worldwide.

Two things sit at the heart of it. First, the payment stays the same every month for the life of the loan. Second, each payment quietly does two jobs at once: it covers the interest charged on whatever balance is still outstanding, and it chips away at the principal. Early on, most of the payment is interest. Later on, most of it is principal. The total stays flat.

Why the formula is worth understanding

A mortgage is, for most households, the single biggest financial commitment they'll ever take on. The formula determines two numbers that matter more than almost anything else: how much leaves your account each month, and how much you'll have paid in interest by the time the loan is gone. Small changes to the inputs produce surprisingly large changes to both.

The OECD reports that household mortgage debt sits somewhere between 50% and 100% of disposable income across most developed economies — a range that means the monthly payment shapes household budgets for decades. Understanding how the calculation works lets you check whether a lender's quote matches the formula, model what an overpayment plan would actually achieve, and compare offers on equal footing rather than just on headline rate.

The formula also explains something that confuses a lot of first-time borrowers: two mortgages with the same advertised rate can end up costing wildly different amounts. A 25-year term and a 30-year term at identical rates look similar on a lender's website, but the total interest bills can differ by tens of thousands.

How a mortgage payment is calculated, step by step

The fixed monthly payment on an amortising mortgage comes from one equation. It looks dense at first glance, but every piece is doing a specific job.

M = P × [ r × (1 + r)^n ] / [ (1 + r)^n − 1 ]

Where:

M is the fixed monthly payment.
P is the principal — the amount borrowed at the start.
r is the monthly interest rate (the annual rate divided by 12).
n is the total number of monthly payments (years × 12).

The numerator captures how interest compounds across the life of the loan. The denominator spreads that compounded cost evenly so every payment ends up the same size. Divide one by the other and out pops a single, stable monthly figure.

One detail trips people up constantly: the rate inside the formula is the monthly rate, not the annual rate. Drop a 5% annual rate straight in without dividing by 12 and the answer comes out absurd. The same goes for the term — it has to be expressed in months. A 25-year mortgage is 300 in the formula, not 25.

The equation also assumes the loan is fully amortising, meaning the balance reaches exactly zero on the last payment. Interest-only and adjustable-rate mortgages build on the same formula but add extra steps — usually a recalculation each time the rate or repayment structure changes.

A worked example with real numbers

Take a borrower with a 300,000 loan at 5% annually, over 25 years. The numbers below work in any currency — substitute your own symbol mentally as you go.

Step 1. Convert the inputs into the units the formula expects.

P = 300,000
r = 0.05 ÷ 12 = 0.00416667 (monthly rate)
n = 25 × 12 = 300 (total monthly payments)

Step 2. Calculate (1 + r) raised to the power of n. This compound factor is the engine of the whole equation.

(1 + 0.00416667)^300 = 3.481290

Step 3. Build the numerator.

P × r × (1 + r)^n = 300,000 × 0.00416667 × 3.481290 = 4,351.61

Step 4. Build the denominator.

(1 + r)^n − 1 = 3.481290 − 1 = 2.481290

Step 5. Divide.

M = 4,351.61 ÷ 2.481290 = 1,753.77

The monthly payment is 1,753.77. Over 300 payments, that totals 526,131. Subtract the original 300,000 of principal and 226,131 is left over — that's the interest, roughly 75% of the loan amount, paid as the cost of borrowing.

Plugging the same inputs into the mortgage calculator gives the same number to the cent. The calculator also breaks each payment into its interest and principal portions for every month of the schedule.

Using the mortgage calculator

The calculator takes three primary inputs: the loan amount, the annual interest rate (as a percentage), and the term in years. It returns the monthly payment, the total amount paid, the total interest, and a year-by-year breakdown of how the balance falls over time.

Three figures are worth focusing on when the results come back. The monthly payment is the recurring cash outflow — the number that has to fit alongside everything else in the household budget. The total interest figure is the true cost of borrowing; anything over 100% of the principal means the loan costs more in interest than the amount borrowed itself. The amortisation breakdown shows how slowly principal gets paid down in the early years, which matters enormously for anyone planning to refinance or sell within the first decade.

To explore alternatives, run the mortgage calculator with different terms or overpayment amounts. Even a modest overpayment can take years off the loan and remove a real chunk of total interest.

Common scenarios

The formula stays the same — it's the inputs that shift across these situations.

Comparing a 25-year and a 30-year term

On the same 300,000 loan at 5%, stretching the term from 25 to 30 years drops the monthly payment from 1,753.77 to 1,610.46 — a saving of about 143 a month. The total interest, though, climbs from 226,131 to 279,767. That extra five years of breathing room costs around 53,600 more across the life of the loan.

A one-percentage-point rate increase

Moving from 5% to 6% on the same 300,000 loan over 25 years raises the monthly payment from 1,753.77 to 1,932.90 — an extra 179 a month. Over 25 years, that single percentage point adds roughly 53,700 to the total cost. Rate movements that look small on paper compound into substantial sums by the end of the term.

A larger deposit

Reducing the principal by putting more down at the start produces a proportional fall in the monthly payment. Borrowing 270,000 instead of 300,000 at the same 5% over 25 years cuts the monthly figure by about 175, and the total interest by about 22,600. The deposit isn't just an upfront cost — it's a permanent reduction in everything that follows.

Refinancing part-way through

When a borrower refinances a few years in, the new calculation uses the outstanding balance as the new P, the new rate as r, and the remaining term as n. Because interest dominates early payments, the outstanding balance falls slowly in the first years — something the amortisation schedule makes painfully visible once it's drawn out month by month.

Variable rates and rate resets

Adjustable-rate mortgages recalculate the payment each time the rate changes. The same formula applies, with the new rate and the remaining term plugged in fresh. A reset from 4% to 6% on a loan with 20 years still to run can lift the payment by around 15% — sometimes more, depending on how much principal is left.

Patterns commonly observed

The formula is unforgiving. Small input errors produce big output errors.

Using the annual rate instead of the monthly rate. Forgetting to divide by 12 inflates the payment dramatically — usually by something like 6-7×.
Expressing the term in years instead of months. n has to be the total number of monthly payments. A 25-year term is 300, not 25.
Ignoring fees and insurance. The formula returns the principal-and-interest payment only. Property taxes, building insurance, mortgage insurance, and product fees often add 10-20% on top of what the formula produces.
Comparing two offers on monthly payment alone. A longer term lowers the monthly figure but raises the total cost. Total interest is the fairer side-by-side comparison.
Assuming the early payments reduce the balance evenly. They don't. In the first few years, most of each payment goes to interest. Principal only overtakes interest well into the schedule — often more than halfway through.

Frequently asked questions

How is a mortgage payment calculated for a fixed-rate loan?

For a fixed-rate, fully amortising mortgage, the monthly payment comes from the formula M = P × [r × (1 + r)^n] / [(1 + r)^n − 1], where P is the principal, r is the monthly interest rate, and n is the total number of monthly payments. The annual rate has to be divided by 12 to give the monthly rate, and the term has to be converted into months. The result is a single payment that stays constant for the life of the loan, with each payment covering interest on the remaining balance and a portion of principal.

Why does the same mortgage cost so much more over 30 years than 25?

A longer term means more months of interest charged on a balance that's being paid down more slowly. On a 300,000 loan at 5%, a 30-year term produces a monthly payment about 143 lower than a 25-year term, but the total interest bill climbs by roughly 53,600 over the life of the loan. The compounding effect is the reason. Interest is charged each month on whatever principal remains, and a longer schedule leaves more principal outstanding for longer. The monthly payment looks more comfortable, but lifetime cost is materially higher.

What happens to the payment if interest rates change part-way through?

On a fixed-rate mortgage, the payment doesn't change during the fixed period regardless of what happens to market rates. On an adjustable-rate or tracker mortgage, the payment recalculates every time the rate resets. The lender plugs the new rate, the outstanding balance, and the remaining term back into the same formula, producing a new fixed payment until the next reset. A rate rise of one or two percentage points on a loan with most of its term still to run can lift the monthly payment by 15-25%, depending on how much principal remains.

Does overpaying reduce the mortgage payment or the term?

On most fixed-rate mortgages, overpaying shortens the term rather than reducing the regular payment. The monthly figure stays the same, but the loan finishes earlier and total interest drops. Some lenders allow a recast on request, which recalculates the payment using the same formula but with a lower outstanding balance, producing a smaller monthly figure. Adding 100 a month to a 300,000 loan at 5% over 25 years takes roughly 2.5 years off the schedule and saves around 26,000 in interest.

Why is the interest portion so large in the early years?

Interest each month is charged on the full outstanding balance, which is highest at the start of the loan. On a 300,000 loan at 5%, the first month's interest is roughly 1,250 out of a 1,753.77 payment — leaving only about 504 going to principal. As the balance falls, the interest portion shrinks and the principal portion grows. The crossover point, where principal becomes the larger share of each payment, typically falls between 40% and 60% of the way through the term.

How is a mortgage payment calculated when extra costs are bundled in?

The core formula returns the principal-and-interest payment only. Lenders often add escrow amounts for property taxes and insurance, plus any mortgage insurance premium, into the headline monthly figure that gets advertised. These additions don't change the formula — they sit on top of it. To compare loans on equal footing, isolate the principal-and-interest figure produced by the formula, then add the lender's escrow and insurance estimates separately. That separation shows which loan is genuinely cheaper before tax and insurance distort the picture.

Sources and methodology

The mortgage payment formula presented here matches the standard fully amortising loan equation used across academic finance and lender practice worldwide. The worked example was verified by computing the payment via the formula, summing the 300 monthly payments, and confirming the total interest against an independent amortisation schedule. The compound factor (1 + r)^n was calculated to six decimal places to keep rounding error well below one currency unit in the final monthly payment.

This article and the linked mortgage calculator use reference data from:

OECD household debt statistics — for the scale of mortgage debt across developed economies.
Bank for International Settlements credit statistics — for long-run data on residential mortgage lending.

For related calculations on affordability and total cost, try the mortgage affordability calculator and the amortisation schedule calculator, which extends the same formula to show the payment schedule month by month. For borrowers weighing mortgage capacity against existing obligations, the debt-to-income ratio calculator handles the other side of that question.

The bottom line

The mortgage payment formula is short, but the consequences are long. Three inputs — principal, rate, term — combine through one equation to produce a number that shapes household cash flow for decades. Once you can see the math, the loan stops being something a lender hands you and starts being something you can stress-test against rate moves, term extensions, and overpayment plans. Anyone weighing a new mortgage, a refinance, or even just a different deposit size benefits from running their own numbers through the formula first.