When I decided to add a boat loan calculator to my site, my first instinct was to reach for an API or at least a server-side calculation endpoint. Felt like the "proper" way to handle financial math.
Then I stopped and asked myself: why?
Every number the user needs — monthly payment, total interest, full amortization schedule, affordability limit — can be derived from a handful of inputs. There's no sensitive data involved, no database query, nothing that actually needs a round trip to a server.
So I built the whole thing in vanilla JS on a WordPress custom page template. PHP serves the page, JS does all the math. No AJAX calls, no REST endpoints, no plugins handling the logic.
The core of it is the standard amortization formula:
M = P[r(1+r)^n] / [(1+r)^n - 1]
Where P is the principal, r is the monthly interest rate, and n is the number of payments. Everything else — total interest, amortization breakdown, reverse affordability calculation — is just derivatives of this single formula running in the browser.
The result is a calculator that loads instantly, works offline, and puts zero load on the server. You can see it live at boat loan calculator.
The only thing WordPress does here is render the template. The actual product is a static HTML shell with a JS brain.
If you're building any kind of calculator tool — loan, mortgage, ROI, whatever — seriously consider whether you need a backend at all. Chances are you don't.
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