How Mortgage Payments Are Calculated

Most people know their monthly mortgage payment but have no idea how it is calculated. Understanding the maths reveals why paying a little extra early on can save you a surprising amount, and why the bank earns so much from your loan.

The Two Components of Every Payment

Every mortgage payment splits into two parts: interest (what the bank earns) and principal (what reduces your debt). In the early years, the vast majority of each payment is interest. Only later does the balance shift toward principal.

On a £200,000 mortgage at 4% over 25 years, your first monthly payment of around £1,056 breaks down roughly as: £667 interest, £389 principal. After 20 years, the same payment is: £133 interest, £923 principal. The payment never changes, but what it does changes enormously.

The Formula

This is called the amortization formula. It calculates the fixed payment that will exactly pay off the loan over the term, with interest applied to the outstanding balance each month.

Why Early Payments Are Mostly Interest

Each month, interest is calculated on the remaining balance. When you have just borrowed £200,000, the interest charge is large. As the balance reduces, so does the interest portion. This is why extra payments in the early years are so powerful — they reduce the balance that future interest is calculated on.

The True Cost of a Mortgage

On that £200,000 loan at 4% over 25 years, total payments come to around £316,800. You borrow £200,000 and pay back £316,800 — the bank earns £116,800 in interest alone. This is not a hidden fee; it is simply the cost of borrowing a large sum over a long period.

How to Pay It Off Faster

• Overpay monthly — even £50/month extra can cut years off the term
• Lump sum payments — bonuses or windfalls applied to the principal have a compounding benefit on all future interest
• Shorter term — a 20-year mortgage costs more per month but far less overall than a 30-year one
• Remortgage — securing a lower rate when your fixed period ends is often the single biggest saving available

Fixed vs Variable Rate

A fixed rate locks your interest rate for a set period (typically 2 or 5 years). Your payments are predictable. A variable rate moves with the market — cheaper when rates are low, more expensive when they rise. Neither is universally better; it depends on where rates are and your tolerance for uncertainty.