Mortgage Interest Calculator
Use this mortgage interest calculator to estimate interest costs in a few different ways. You can check the monthly interest on an interest-only mortgage, the total interest over a full repayment term, or the interest over a shorter period (such as the first 5 years).
This calculator helps you estimate interest costs using three different views.
- Monthly interest (interest-only): the interest-only payment is roughly loan amount times interest rate, divided by 12.
- Total interest (repayment, full term): uses a standard repayment mortgage model (capital and interest) over the full term.
- Interest over a period: estimates how much interest you would pay in the first part of a repayment mortgage (for example, the first 5 years).
It does not include fees, insurance, taxes, or rate changes over time.
This is a guide only. Your actual costs can differ depending on your deal, your lender, and whether rates change.
If you need advice for your situation, consider speaking to a qualified mortgage adviser.
This page helps you estimate mortgage interest in a few common ways: (1) monthly interest on an interest-only balance, (2) total interest over a full repayment term, and (3) interest paid over a shorter period (for example the first 5 years).
Monthly interest (interest-only) is a simple estimate: loan amount × annual rate ÷ 12. It assumes the balance does not reduce.
Total interest (repayment) uses an amortisation schedule: each monthly payment covers interest due for the month and repays some of the principal. Early payments are typically interest-heavy; later payments are principal-heavy.
Interest over a period (e.g. 5 years) is estimated by running that same amortisation model for a limited number of months and summing the interest portion.
These outputs are estimates only. Real lenders can calculate interest daily, apply different rounding, and your rate can change over time.
Last verified: 04/03/2026. Primary source for repayment-model methodology: Money and Pensions Service mortgage repayment calculator (algorithmic transparency record).
There are no official “mortgage interest rates” tables for consumer mortgages. Your interest rate depends on your lender, product, credit profile, loan-to-value (LTV), and the market.
For this calculator, the key “rate input” is the annual interest rate you enter. The model assumes it stays constant for the period you’re estimating (unless you change it and re-run).
Tip: if you are modelling a fixed-rate deal, use the fixed rate for the fixed period. If you are modelling a tracker/SVR, treat the output as a snapshot rather than a forecast.
Worked examples (illustrative):
Example 1 — Monthly interest-only cost
- Balance: £200,000
- Rate: 5.00% per year
- Monthly interest ≈ £200,000 × 0.05 ÷ 12 = £833.33
Example 2 — Repayment mortgage: total interest over full term
- Loan: £200,000
- Rate: 5.00% per year (fixed for illustration)
- Term: 25 years
- Estimated monthly payment: £1,169.18
- Estimated total interest over 25 years: £150,754.02
Example 3 — Interest paid in the first 5 years (repayment)
- Using the same £200,000 / 5% / 25y example, estimated interest paid in the first 60 months is £47,311.18.
- Estimated balance after 5 years: £177,160.38.
Note: changing the rate, term, and starting balance will change these figures significantly.
- Rate changes: tracker/SVR products can change; this tool assumes the rate you enter stays constant for the modelled period.
- Daily interest: many lenders calculate interest daily, so exact figures can differ from monthly approximations.
- Fees: arrangement fees, broker fees, and insurance are not included.
- Overpayments and payment holidays: these change the interest path; use the overpayment calculator for overpayment scenarios.
- Offset mortgages: interest may be charged on a net balance (not modelled here).
- Figures are estimates. Your actual rate and payments can change over time.
- “Total interest” uses a standard repayment mortgage model (capital and interest).
- “Monthly interest” assumes an interest-only mortgage where the balance does not reduce.
- It does not include lender fees, broker fees, valuation fees, legal costs, insurance, or taxes.
Monthly interest (interest-only) is roughly loan amount × interest rate ÷ 12.
Total interest (repayment) and interest over a period use an amortisation schedule for a repayment mortgage.
- Estimate full repayments and totals using the main mortgage calculator.
- Model overpayments if you want to reduce interest and term.
- Read: How mortgage interest is calculated.
- Glossary: APRC and SVR.