Mortgage Amortisation Explained: Where Does Your Payment Go?
Most of your early mortgage payments go straight to interest, not the house. On a £200,000 mortgage at 5% over 25 years, your first payment of £1,162 includes £833 in interest and just £329 towards your actual loan balance. That's amortisation — and understanding it can change how you approach your mortgage strategy.
Amortisation sounds like financial jargon, but it's simple: it's the schedule that shows how much of each monthly payment chips away at the principal (what you actually owe) versus interest (what the lender takes). This guide explains the mechanics, shows you real breakdowns, and answers the questions homeowners ask most often.
What Is Amortisation and Why It Matters
Amortisation is the process of repaying a loan through equal monthly instalments over a fixed period. Each payment covers both interest (the lender's cost of lending) and principal (reduction in what you owe). The split between the two changes every month — at the start, mostly interest; by the end, mostly principal.
Here's the formula lenders use to calculate your payment:
Payment = P × (r(1+r)^n) / ((1+r)^n - 1)
Where P is the principal (loan amount), r is the monthly interest rate (annual rate ÷ 12), and n is the total number of payments.
Don't worry if that looks scary. Your mortgage calculator does this automatically. The point is: the lender calculates a single monthly payment that, if paid consistently for your mortgage term, will pay off the entire loan plus interest.
Breaking Down Your Monthly Payment: Principal vs Interest
Let's work through a real scenario. You've taken a £200,000 mortgage at 5% fixed for 25 years.
Your monthly payment is £1,162.
Month 1:
- Interest due: £200,000 × (5% ÷ 12) = £833
- Principal reduction: £1,162 − £833 = £329
- Balance after payment: £199,671
Month 2:
- Interest due: £199,671 × (5% ÷ 12) = £832
- Principal reduction: £1,162 − £832 = £330
- Balance after payment: £199,341
See the pattern? As your balance shrinks, the interest portion shrinks with it, and the principal portion grows. But in the early years, interest dominates your payment.
Month 150 (halfway through, year 12.5):
- Interest due: £679
- Principal reduction: £483
- Balance: £101,239
Month 300 (final payment, year 25):
- Interest due: £5
- Principal reduction: £1,157
- Balance: £0
Over the whole 25 years, you'll have paid £348,600 total (300 payments of £1,162) — that's £148,600 in interest alone. Compounding does the heavy lifting, in the wrong direction.
This is why a longer mortgage term (like a 40-year mortgage) drops your monthly payment but almost doubles your total interest. A shorter term (15 years instead of 25) hurts monthly cash flow but saves you huge money in total interest.
Your Amortisation Schedule: How to Read It
Your lender will send you an amortisation schedule — a table showing every payment, the interest and principal split, and your remaining balance. Learning to read your mortgage statement is vital, especially if you're considering early repayment or overpayments.
Most schedules look like this:
| Payment # | Payment | Interest | Principal | Balance |
|---|---|---|---|---|
| 1 | £1,162 | £833 | £329 | £199,671 |
| 2 | £1,162 | £832 | £330 | £199,341 |
| ... | ... | ... | ... | ... |
| 150 | £1,162 | £679 | £483 | £101,239 |
| 300 | £1,162 | £5 | £1,157 | £0 |
If you overpay (say, an extra £100/month), here's what happens: that £100 comes straight off the principal, not the interest. It doesn't change next month's payment — instead, it shortens your mortgage term and saves you a pile of interest later. Paying off your mortgage faster or investing instead is a common dilemma; the numbers let you decide.
How LTV and Interest Rates Affect Your Amortisation
Your LTV — the ratio of loan to property value — is the single biggest factor in the interest rate you're offered. A 60% LTV (40% deposit) typically gets rates 0.5–1% lower than a 90% LTV.
On that same £200,000 mortgage over 25 years:
- At 4% (good LTV): £1,037/month, £111,200 total interest
- At 5% (typical LTV): £1,162/month, £148,600 total interest
- At 6% (higher LTV): £1,299/month, £189,700 total interest
The difference isn't just in monthly payment — it's in total interest. A 1% rate difference costs you £37,500 over 25 years. This is why saving a bigger deposit is worth the wait, and why checking your mortgage affordability upfront (including realistic rate assumptions) matters.
If your mortgage is variable (tracker or SVR), your amortisation is less certain because your payment can change. The Bank of England base rate influences what lenders charge, so your payment rises and falls with it. Fixed-rate mortgages lock in your amortisation schedule for the entire term, which is why they're easier to plan around.
Early Repayment and Early Repayment Charges
If you overpay or pay off early, your lender might charge an Early Repayment Charge (ERC). These are typically 1–5% of the outstanding balance — on a £200,000 mortgage, that's £2,000–£10,000.
Why? Because you've broken your agreement and the lender loses expected interest income. The ERC is meant to offset that loss. However, paying off via overpayments (adding money to regular payments) is usually charge-free; it's only lump-sum settlements or moving to a new lender that trigger ERCs. Always check your mortgage offer. MoneyHelper (gov.uk's guidance service) has more on early repayment and what to consider before paying off early.
The amortisation schedule shows you exactly when you'll have paid off each chunk of principal. If you're planning to move or remortgage in 5 years, you'll know how much principal you'll have reduced and whether an ERC is worth it for the interest you'd save.
Common Misconceptions About Amortisation
"My payment goes mostly to the house." False in the early years. On a 25-year mortgage, you've paid 50% of the principal by year 12.5, but you've already paid 78% of the total interest. The back half of your mortgage is where principal finally dominates. This is why understanding mortgage fees and hidden costs upfront is critical — the headline rate doesn't tell the whole story.
"A longer mortgage term means I pay more interest." True, but it's not always wrong. A 40-year mortgage at 5% costs more total interest than a 25-year at 5%. But a 40-year mortgage at a lower rate (because you're paying less monthly and look less risky to the lender) might actually cost less total interest than a 25-year at a higher rate. The math depends on your actual offer.
"Overpaying £100/month saves me £100 of interest." Not quite. Overpayment saves you interest on the interest (the compounding part). On that £200,000 mortgage, £100/month extra would save you roughly £28,000 in total interest and shorten your mortgage by about 4 years. Useful, not trivial.
"Interest-only mortgages are cheaper." Cheaper monthly, yes. But you owe the whole principal at the end. If you're betting rates will fall or your income will rise, that might make sense. For most people, repayment mortgages fit better because you're actually paying down what you owe.
Frequently Asked Questions
Q: How much of my first payment goes to interest? A: On a typical £200,000 mortgage at 5% over 25 years, about 72% of your first payment (£833 out of £1,162) is interest. That ratio shifts every month as your balance drops. By payment 150, it's 58% interest. By the last payment, it's less than 1%.
Q: Why do I owe so much interest if I'm paying every month? A: Because interest is calculated on the outstanding balance. Early on, that balance is huge. Each payment chips away at it slowly, so the interest stays high. It's not unfair — it's just how compound interest works in reverse. You pay interest on what you still owe, not on what you've paid.
Q: If I overpay £50/month, how much faster is my mortgage done? A: It depends on your term and rate, but on a £200,000 mortgage at 5% over 25 years, £50/month extra reduces the term by roughly 2–2.5 years and saves you about £15,000 in interest. Use our mortgage calculator with overpayment figures to see your exact timeline.
Q: Can I change my amortisation schedule? A: Your lender sets the repayment schedule based on your loan amount, rate, and term. You can't change it without refinancing or remortgaging. However, you can overpay or pay lump sums to accelerate your principal reduction — that's allowed (usually charge-free during your rate term) and changes how fast your balance drops.
Q: Is a 40-year mortgage worth it? A: Not usually, unless you genuinely can't afford the monthly payment on a 25-year term and rates are falling. A 40-year mortgage costs significantly more in total interest and leaves you with a mortgage into your 70s. Check your actual numbers: can you stretch to 30 years instead? We've written more on whether a 40-year mortgage makes sense.
Q: What happens to my amortisation if I remortgage? A: You're starting a new amortisation schedule. If you remortgage 5 years into a 25-year mortgage, you have 20 years left. If you remortgage to another 25-year term, you're back to the steep interest-heavy curve at the start. If you go shorter (15 years), payments rise but total interest falls. Always compare apples-to-apples: your remaining term vs the new term you're choosing.
Q: How do I know if my lender's amortisation calculation is correct? A: Use the formula or our mortgage calculator to verify. Plug in your loan amount, interest rate, and term. Your monthly payment should match your mortgage offer. If it doesn't, contact your lender.
Next Steps
Amortisation is powerful knowledge. Now that you understand how your payment splits between interest and principal, you can make smarter decisions about your mortgage:
- Use our mortgage calculator to model different scenarios: higher deposit (lower LTV), shorter term, different rates.
- Check your mortgage affordability — stress-test your budget for rate rises.
- If you already have a mortgage, pull your latest statement and find the amortisation schedule. Note how the principal portion is growing month by month. If you're 5+ years in, you're starting to see real progress.
- Decide whether to pay off or invest — amortisation shows you the true cost of your mortgage over time, so you can weigh it against other financial goals.