Mortgage Calculator: How We Calculate
Mortgage Calculator: How We Calculate
Tax year coverage: N/A — the mortgage calculator uses user-provided interest rates, not tax year constants.
Estimates only — not tax, legal, or financial advice. Full disclaimer below.
What This Calculator Does
This calculator models: UK mortgage monthly payments and total cost across fixed, variable, tracker, and discounted rate types, for both repayment and interest-only mortgages. It does not model: Affordability assessment, early repayment charges, overpayments, offset accounts, or tax relief on mortgage interest.
The calculator takes a loan amount, interest rate, mortgage term, and rate type, and returns the monthly payment, total interest paid, and total repayment over the full term. It supports two-period products (fixed-variable, discounted) where the rate changes partway through the term.
To use the calculator directly, go to the Mortgage Calculator. For a step-by-step guide, see the Mortgage Calculator Guide.
How We Calculate
Step 1 — Determine the loan amount and product fee
The loan amount is derived from the property price and deposit (or LTV ratio, depending on the input mode). The product fee is added to the loan amount for payment calculation purposes — it is repaid over the mortgage term, not paid upfront.
Total Principal = Loan Amount + Product Fee
Users who pay the product fee upfront should enter £0 as the fee amount.
Step 2 — Calculate the monthly interest rate
All rates are annualised internally. The monthly rate is:
Monthly Rate = Annual Rate / 12
Interest is calculated monthly on the outstanding balance. This is a standard simplification — many UK lenders calculate interest daily (ACT/365), which can produce small differences (see Assumptions and Limitations).
Step 3 — Calculate the monthly payment
Repayment (capital and interest)
The standard amortisation formula is used:
Monthly Payment = Principal × (monthlyRate / (1 − (1 + monthlyRate)^(−term × 12)))
This produces a fixed monthly payment that covers both interest and capital repayment, with the balance fully repaid at the end of the term.
Interest-only
Monthly Payment = Principal × Annual Rate / 12
The monthly payment covers interest only. The full principal is repaid at the end of the term as a lump sum.
Step 4 — Handle two-period rate types
For mortgages where the rate changes partway through the term:
Fixed-Variable
- The fixed rate applies for the fixed term (e.g., 2 or 5 years)
- The outstanding balance at the end of the fixed period is calculated
- A new amortisation is computed over the remaining term using the variable rate (repayment mortgages)
- For interest-only, the payment simply switches to the variable rate on the same principal
Discounted
- The discounted rate applies for the discounted term
- The outstanding balance at the end of the discounted period is calculated
- A new amortisation is computed over the remaining term using the variable rate (repayment mortgages)
- For interest-only, the payment switches to the variable rate on the same principal
Tracker
Tracker Rate = Base Rate + Lender Margin
The tracker rate applies for the full term. The base rate entered by the user is held constant — future Bank of England rate changes are not forecast.
Step 5 — Calculate total cost
Total Repayment = Total Interest Paid + Loan Amount + Product Fee
For repayment mortgages, total interest is the sum of all monthly interest payments over the term. For interest-only mortgages, total interest is the monthly payment multiplied by the number of months.
The calculator also produces a year-by-year breakdown showing cumulative repayment and interest paid at each year-end.
These examples can be reproduced using the calculator with the same inputs.
Worked Examples
Example 1 — Repayment mortgage, fixed rate (MTG-METH-001)
Inputs:
- Property price: £250,000
- Deposit: £62,500 (25% LTV)
- Loan amount: £187,500
- Product fee: £999 (added to loan)
- Mortgage type: Repayment
- Interest rate type: Fixed
- Fixed rate: 5.00% annual
- Term: 25 years
Calculation:
- Total principal = £187,500 + £999 = £188,499
- Monthly rate = 5.00% / 12 = 0.4167%
- Monthly payment = £188,499 × (0.004167 / (1 − (1.004167)^−300)) = £1,101.95
- Total interest paid = £142,084.91
- Total repayment = £142,084.91 + £187,500 + £999 = £330,583.91
Result: Monthly payment £1,101.95, total cost £330,583.91 over 25 years.
Audit test: MTG-METH-001
Example 2 — Interest-only mortgage, fixed rate (MTG-METH-002)
Inputs:
- Property price: £250,000
- Deposit: £62,500 (25% LTV)
- Loan amount: £187,500
- Product fee: £999 (added to loan)
- Mortgage type: Interest-only
- Interest rate type: Fixed
- Fixed rate: 4.50% annual
- Term: 20 years
Calculation:
- Total principal = £187,500 + £999 = £188,499
- Monthly payment = £188,499 × 4.50% / 12 = £706.87
- Total interest paid = £706.87 × 240 months = £169,649.10
- Total repayment = £169,649.10 + £187,500 + £999 = £358,148.10
Result: Monthly payment £706.87, total cost £358,148.10 over 20 years. The principal (£188,499) is repaid as a lump sum at the end of the term.
Audit test: MTG-METH-002
Tax Constants and Rates
The mortgage calculator does not use tax year constants. All interest rates are user-provided — the calculator does not set or assume any particular rate. Rate validation ensures inputs are between 0% and 10%.
Assumptions and Limitations
Assumptions
Interest rates remain constant within each rate period. Impact: results differ from actual payments if rates change during the variable period. When this matters: all variable, tracker, and discounted scenarios. Tracker mortgages diverge as soon as the base rate changes.
No overpayments or underpayments. Impact: results overstate total interest for users who make regular overpayments. When this matters: investors using rental surplus to overpay, which can significantly reduce term and total interest.
Product fee is added to the loan (not configurable). Impact: monthly payments are higher than if the fee were paid upfront. When this matters: when the product fee is significant (£999+). Users who pay upfront should enter £0.
Monthly interest calculation (not daily). Impact: small differences from lender illustrations using daily interest (ACT/365). When this matters: rarely — the difference is typically a few pence per month. May matter when comparing against a lender's exact quote.
Payments occur monthly at equal intervals. Impact: users with weekly or fortnightly schedules will have different results. When this matters: investors using non-standard payment schedules.
Limitations
- Affordability assessment (income multiples, stress testing)
- Early repayment charges (ERCs) and exit fees
- Overpayment modelling
- Offset account effects
- Lender fees and insurance products (MIG, higher lending charges)
- Tax relief on mortgage interest (Section 24 — handled in the Rental Yield Calculator)
- Flexible mortgages with payment holidays
- Current account mortgages
- Islamic (Sharia-compliant) mortgages
When to Get Professional Advice
This calculator is a starting point for comparing mortgage products. A mortgage adviser can factor in lender-specific criteria, affordability assessment, early repayment charges, and product features that a general-purpose calculator cannot model. Always consult a qualified adviser before making mortgage decisions.
FAQ
Does this calculator account for tax relief on mortgage interest? No. Tax relief under Section 24 is handled in the Rental Yield Calculator, which applies the mortgage interest tax credit for individual ownership and deducts financing costs for company ownership. The mortgage calculator shows gross mortgage costs only.
Why does my lender's quote differ slightly from the calculator? Most UK lenders calculate interest daily (ACT/365 convention) rather than monthly. The calculator uses monthly interest calculation, which is a standard simplification. The difference is typically a few pence per month but can accumulate over long terms.
Can I model a mortgage where the rate changes after a fixed period? Yes. Select "Fixed-Variable" or "Discounted" as the rate type. The calculator computes the outstanding balance at the end of the initial period and recalculates the payment using the new rate for the remaining term.
Does the calculator include the product fee in the monthly payment? Yes. The product fee is added to the loan amount and repaid over the term. If you pay the fee upfront, enter £0 as the fee amount — the calculator will exclude it from all calculations.
What is the maximum mortgage term? The calculator supports terms from 1 to 25 years. Buy-to-let mortgages typically have terms of 15–25 years.
Disclaimer
This calculator provides estimates based on the methodology described above. It is not financial advice. Mortgage rates and terms depend on lender criteria, your financial circumstances, and product availability. Always verify with a qualified mortgage adviser. PropMatch.uk accepts no liability for decisions based on calculator outputs.
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