Finance Calculator

Consumers comparing home improvement finance options had no way to see the real cost of different loan products side by side. Retailers needed a calculator they could use with customers that showed FCA-compliant representative examples, total cost of credit, and payment schedules. The existing tools were either too basic (simple monthly payment calculators that ignored APR compounding) or locked into specific lender platforms with no flexibility.

I deployed it as a standalone subdomain (calculator.barneygoodman.co.uk) rather than an embedded component, so it can be white-labelled and shared independently of any particular site. The PostMessage API handles dynamic iframe height resizing when embedded elsewhere, which means no fixed height constraints and no scrollbar issues regardless of the host page.

APR calculation uses the Newton-Raphson iterative method for FCA-compliant accuracy. Simplified approximation formulae break at edge cases (deferred payment periods, balloon payments, very short terms), but Newton-Raphson converges to the correct answer regardless of the loan structure. I chose client-side only with no backend because all calculation logic is deterministic and there is no data persistence requirement. Zero hosting cost, instant response, works offline.

Users input a loan amount, select a term, and choose an interest rate or product. The calculator runs Newton-Raphson iterations to compute the precise APR, then generates the full payment schedule, total cost of credit, and FCA-compliant representative example. All calculations happen in the browser. The interface adapts to any container width via the PostMessage resize protocol, making it embeddable on third-party sites without layout issues.

Retailers can embed a compliant calculator on their own websites. Consumers get transparent loan comparisons with accurate total cost figures. The tool handles the edge cases (deferred payment periods, balloon payments, variable rates) that simpler calculators get wrong, which matters for FCA compliance.