Home Renovation Loan Calculator Widget for a Bank Website

We run a lending section on a bank-style credit organization website and need a compact calculator widget that helps visitors estimate the cost of a home renovation loan before they apply. The borrower should be able to enter the amount they want to borrow, choose a desired repayment term, and mark whether the loan will be secured by collateral, such as the apartment itself, or issued unsecured. Based on these three inputs the widget must instantly show an approximate monthly payment and the total overpayment over the whole term, using a standard annuity formula with separate rate tiers for secured and unsecured offers.

The widget will be embedded into an existing site, so it needs to be lightweight, recalculate without a page reload, and fit into our current design without pulling in heavy frameworks. Clear number formatting, input validation for minimum and maximum loan amount and term limits, plus a responsive layout for mobile visitors are all important, since most of our traffic comes from phones.

What we expect from the finished widget:
— Plain HTML, CSS and JS, no server calls required, easy to drop into an iframe or inline block
— A configurable interest rate table for secured versus unsecured terms
— Instant recalculation as the borrower moves sliders or edits fields