How a mortgage payment is worked out
A fixed-rate mortgage payment is calculated with a single formula, sometimes called the amortisation formula. It takes three things — how much you borrowed, your interest rate, and how long you have to repay — and returns one figure: the identical amount you pay every month until the loan is gone.
The reason every payment is the same size is the whole point of amortisation. Rather than paying a huge amount of interest early and a tiny amount later, the schedule is engineered so each monthly payment is level. Behind that flat payment, though, the split between interest and principal shifts month by month — and understanding that shift is where most of the useful decisions live.
The one trap in the formula is the rate. It uses the monthly rate, not the annual one you were quoted. If your rate is 5.5%, then r is 0.055 ÷ 12 ≈ 0.004583. And n is the number of monthly payments, so a 30-year term is 30 × 12 = 360, not 30. Getting either of those wrong is the single most common reason a hand-calculated figure comes out wildly off.
Worked example
Say you borrow $250,000 at 5.5% over 30 years.
First convert the inputs: r = 0.055 ÷ 12 = 0.0045833, and n = 30 × 12 = 360.
Plug them in and the formula returns a monthly payment of about $1,419.47. Over the full 360 payments that's roughly $511,010 paid in total — meaning about $261,010 of interest, slightly more than the amount you originally borrowed.
That last line surprises people, and it's the most important thing a mortgage calculator can show you: on a long term at a normal rate, the interest can exceed the loan itself. Seeing the total interest — not just the comfortable monthly figure — is what turns a calculator from a curiosity into a decision-making tool.
Why your early payments are almost all interest
Interest is charged on whatever you still owe. At the start of a mortgage you owe the full amount, so the interest slice of your payment is at its largest. Whatever is left over after interest goes toward the principal — and early on, that leftover is small.
As the balance drops, the interest charged each month drops with it, so more of your fixed payment starts eating into the principal. The effect snowballs toward the end. Here's what the first and last few months of that $250,000 example look like:
| Month | Interest | Principal | Balance |
|---|---|---|---|
| 1 | $1,145.83 | $273.64 | $249,726 |
| 2 | $1,144.58 | $274.89 | $249,451 |
| 60 | $1,057.28 | $362.19 | $230,323 |
| 180 | $740.72 | $678.75 | $160,879 |
| 359 | $12.94 | $1,406.53 | $1,413 |
| 360 | $6.48 | $1,412.99 | $0 |
In month one, over $1,145 of your $1,419 payment is pure interest and only $274 reduces the debt. By the final month it has completely reversed. This is why paying a mortgage for five years can feel like the balance has barely moved — because, proportionally, it hasn't.
How overpaying changes the maths
Every extra dollar you pay beyond the required amount goes straight to the principal. It skips the interest queue entirely. And because it permanently lowers the balance, it also lowers every future interest charge for the rest of the loan — so a single overpayment keeps paying you back, month after month.
Using the calculator above, add an extra amount in the "extra each month" field and you'll see two things change: the total interest falls, and the loan finishes early. On our $250,000 example, an extra $150 a month pays the mortgage off around six years sooner and saves roughly $61,500 in interest — from overpayments totalling only about $43,000. That gap between what you put in and what you save is the compounding at work.
Worth checking first: some mortgages charge early-repayment fees or cap how much you can overpay each year. The maths always favours overpaying, but confirm your specific loan allows it penalty-free before you commit to a plan.
What this calculator does — and doesn't — include
The figure here is principal and interest only. That's the part determined purely by the loan, so it's the part a calculator can compute exactly. Your real monthly housing cost usually includes several things that vary too much by location and property to bake in:
- Property tax — set by your local authority and reassessed periodically.
- Home insurance — required by most lenders, priced by the property and provider.
- Association or service charges — common for flats, condos, and managed developments.
- Mortgage insurance — often required when your deposit is below a threshold, and removable later.
A useful habit: take the principal-and-interest figure from this tool, then add your best estimate of those extras on top to get a realistic monthly budget. Lenders bundle them into one payment, but they're separate costs — and only the loan portion is fixed by the formula.
Common mistakes when calculating a mortgage
Using the annual rate as the monthly rate. The formula needs the annual rate divided by 12. Skip that step and your payment comes out roughly twelve times too high.
Using years instead of months for the term. A 30-year loan is 360 payments. Entering 30 tells the formula the loan is repaid in thirty months.
Judging affordability on the monthly payment alone. A longer term lowers the monthly figure but raises the total interest, sometimes dramatically. Always look at both numbers together.
Forgetting taxes and insurance. The true cost of ownership is meaningfully higher than principal and interest. Budget for the extras from the start.