Mortgages Explained: Amortisation, LTV, Fixed vs Variable Rates, and the Power of Overpayments
The EMI calculator gives a monthly payment — but total interest, LTV impact on rate, fixed vs tracker mortgages, and the overpayment calculation are what determine whether a mortgage is actually affordable over 25 years. Here's how mortgage amortisation works with real numbers.
By sadiqbd · June 10, 2026
A mortgage is the most significant financial commitment most people ever make — and the EMI is the starting point, not the endpoint, of understanding it
The EMI calculator gives you a monthly payment number. What it doesn't automatically show you is the total interest cost over the life of the loan, how an interest rate change affects your repayments, what happens when you overpay, or how a tracker mortgage differs from a fixed mortgage in practice.
How mortgage amortisation works
A mortgage (or any amortising loan) is structured so that each payment covers that month's interest plus a portion of the principal. In the early years, interest dominates; later, principal repayment dominates.
The calculation for each period:
Interest portion = Outstanding balance × (Annual rate ÷ 12) Principal portion = Monthly payment − Interest portion New balance = Outstanding balance − Principal portion
Example — £200,000 mortgage at 4% over 25 years:
Monthly payment: £1,055
| Month | Balance | Interest | Principal | Balance after |
|---|---|---|---|---|
| 1 | £200,000 | £667 | £388 | £199,612 |
| 12 | £195,618 | £652 | £403 | £195,215 |
| 60 | £178,432 | £595 | £460 | £177,972 |
| 120 | £153,862 | £513 | £542 | £153,320 |
| 180 | £122,916 | £410 | £645 | £122,271 |
| 240 | £83,500 | £278 | £777 | £82,723 |
| 300 | £32,700 | £109 | £946 | £31,754 |
Total interest paid over 25 years: approximately £116,500 — 58% of the original loan amount.
Fixed vs. variable mortgage rates
Fixed-rate mortgage: the interest rate is locked for a set period (typically 2, 5, or 10 years in the UK; 15 or 30 years in the US). Monthly payments are predictable regardless of how market rates move.
Advantages: certainty; protection against rate rises. Disadvantages: typically higher initial rate than variable; early repayment charges during the fixed period.
Variable rate / tracker mortgage: the interest rate moves with a reference rate (Bank of England base rate, or LIBOR/SONIA for UK products). Monthly payments change when the reference rate changes.
Advantages: lower initial rate when the base rate is low; no ERCs (early repayment charges) on many tracker products. Disadvantages: payment uncertainty; exposure to rate rises.
Standard Variable Rate (SVR): the lender's own variable rate, which they can change at will. Most mortgages revert to SVR when the initial deal period ends — SVRs are typically significantly higher than fixed or tracker rates. Remortgaging before reverting to SVR is standard financial advice.
Loan-to-Value (LTV) and its impact on rate
LTV = Loan amount ÷ Property value × 100
A £180,000 mortgage on a £200,000 property = 90% LTV.
LTV directly affects the interest rate offered:
| LTV | Typical rate band (approximate) |
|---|---|
| 60% or under | Best available rates |
| 75% | Good rates, most products available |
| 85% | Higher rates, fewer lenders |
| 90% | Significantly higher rates |
| 95% | Highest rates, limited lenders |
A borrower with a 40% deposit (60% LTV) may pay 0.5–1% less interest than one with a 10% deposit (90% LTV). On a £200,000 mortgage over 25 years, a 0.5% difference in rate changes the monthly payment by approximately £55 and the total interest by approximately £16,500.
The overpayment calculation
UK lenders typically allow overpayments of up to 10% of the outstanding balance per year without early repayment charges on fixed-rate products (always check your specific terms).
Impact of overpaying £200/month on a £200,000 mortgage at 4% over 25 years:
Normal repayment: 25 years, total interest £116,500 With £200/month overpayment: approximately 19.5 years, total interest approximately £86,000
Interest saving: approximately £30,500 and loan cleared 5.5 years early.
The mathematics: overpaying reduces the outstanding balance, which reduces the next month's interest charge, which means more of the regular payment goes to principal. The effect compounds — each overpayment produces a cascade of slightly reduced interest charges in every subsequent month.
Remortgaging: when and why
When a fixed-rate deal ends (typically after 2 or 5 years), the mortgage reverts to the lender's SVR — often 2–3% higher than the fixed rate. For a £180,000 outstanding balance, a 2% rate increase costs approximately £300/month extra.
The remortgage timeline:
- 6 months before deal end: start shopping for new deals
- 3–6 months before: apply and lock in a new rate (many products allow you to lock a rate in advance)
- Deal end: switch to new product without reverting to SVR
Remortgaging vs. product transfer:
- Product transfer: staying with the same lender, switching to a new deal. Often quicker, no legal fees.
- Remortgage: moving to a new lender. May offer better rates; involves a solicitor and valuation fee (though many remortgages offer free legal/valuation).
How to use the EMI Calculator on sadiqbd.com
- Enter principal, rate, and tenure
- Calculate monthly EMI — your base repayment figure
- Change the rate — see how a 1% rate rise affects your payment (important for variable rate mortgages)
- Change the tenure — see how a shorter term reduces total interest despite higher monthly payments
- Calculate the total interest — monthly payment × total months − principal = total interest cost
Frequently Asked Questions
Is it better to take a longer mortgage term and overpay, or take a shorter term from the start? A longer term with deliberate overpaying provides flexibility — if finances are tight in a given month, you can revert to the minimum payment. A shorter term commits you to the higher payment regardless of circumstances. For people with stable income, the shorter term is fine; for variable income or uncertain financial situations, the longer term with optional overpayments is generally advisable.
What is stamp duty / SDLT and how does it affect the deposit needed? Stamp Duty Land Tax (UK) is a tax on property purchases above certain thresholds. It must be paid from the buyer's own funds — it cannot be mortgaged. First-time buyers receive relief on SDLT up to £425,000 (as of 2024, though rates change). This tax cost must be factored into the total deposit and purchase cost calculation.
Is the EMI Calculator free? Yes — completely free, no sign-up required.
The monthly payment is the visible number. The total interest cost, the rate sensitivity, and the impact of overpayments are the numbers that determine whether a mortgage is genuinely manageable over the long term.
Try the EMI Calculator free at sadiqbd.com — calculate monthly loan repayments, compare rates and terms, and see the total cost of borrowing instantly.
