Mortgage Scenario Comparison (Pro) United Kingdom

What the mortgage scenario calculator does

The calculator puts up to three rate and term combinations for the same loan side by side, showing the monthly payment, total interest and total repaid for each and which is cheapest overall. You see all three outcomes together instead of running separate calculations.

The deal with the lowest monthly payment is not necessarily the cheapest over its life, and the lowest headline rate does not always win once the term is counted. This Pro tool shows those trade-offs so the decision rests on total cost.

How the comparison works

Each scenario is a standard repayment mortgage: the same loan repaid at that scenario’s rate over its term with a level monthly payment that clears the balance by the end. The total repaid is the monthly payment times the number of months, and the total interest is the total repaid minus the loan. The tool works out all three for each scenario, then ranks them on total repaid and shows the saving of the cheapest against the most expensive.

Why monthly payment and total cost diverge

A lower monthly payment often means a higher total cost. A longer term spreads the repayment over more months, so each payment is smaller, but the debt is outstanding for longer and interest builds up over more years.

On a £250,000 loan at 5% over 25 years, the monthly payment is £1,461.48. Over 30 years at the same rate it falls to £1,342.05, noticeably easier on the budget. But the 25-year mortgage repays about £438,443 in total and the 30-year one about £483,139, so the lower payment costs about £44,700 more over the life of the loan.

Rate versus term

A third scenario in that example lowers the rate instead: 4.5% over 25 years. The monthly payment is £1,389.58, between the other two, but the total repaid is about £416,874, the lowest of the three, saving about £66,265 against the 30-year option.

So rate and term must be weighed together. A modest cut in the rate over a sensible term can beat both a longer term at the same rate and a shorter term at a higher rate, and ranking the scenarios on total repaid shows which.

What the tool leaves out

The calculator uses no tax figures and adds no fees, which keeps the comparison like for like but leaves real-world costs outside the model. For genuine offers, account separately for arrangement and product fees, cashback, early repayment charges and the rate that applies when an introductory period ends. A deal that looks cheapest here can lose its place once a large product fee is included.

Assumptions and sensible use

The model assumes one fixed rate for the whole term of each scenario and a capital and interest repayment structure. Real mortgages often move to a higher standard variable rate after an initial fixed or discounted period, so a long projection at a low introductory rate can be optimistic. Where an offer has an introductory rate, run a scenario at the likely follow-on rate too.

Enter your loan amount, set up to three combinations worth weighing, and compare the monthly payment, total interest and total repaid before using the cheapest-overall result to shortlist. For an actual mortgage decision, confirm the figures and wider costs with a qualified mortgage adviser.

This is general information only and not financial advice.

About This Mortgage Scenario Calculator

Use this mortgage scenario calculator to get an instant estimate based on the figures you enter. Adjust the inputs above to match your situation, or browse all our calculators to plan your next step.