We build financial calculators (FIRE number, compound interest, mortgage, net salary) for Vextor Capital (https://vextorcapital.com/tools) that run entirely client-side, in five languages, with no signup and no backend call per calculation.
A few decisions that mattered more than expected:
Locale-aware number formatting is the actual hard part. "1.234,56" vs "1,234.56" breaks silently if you parse input with the wrong assumption. We normalize on input and format on output separately, never trusting a single regex to do both directions.
Precision, not just rounding. Compound interest over 30 years amplifies floating-point drift if you're not careful with how you accumulate intermediate values. We keep a higher-precision running total internally and only round for display.
Language, currency, and locale are three separate settings. An Italian user might want output in USD. We decoupled UI language from number formatting from currency symbol instead of bundling them into one locale dropdown.
No backend means no server-side validation net. All bounds-checking (negative rates, impossible time horizons) happens client-side, with sane defaults so the widget never silently returns nonsense.
Nothing here is novel computer science, it's mostly about not cutting corners on the boring parts. Happy to go deeper on any of these if useful.
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