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7/4/2026
How I Built a Free Financial Calculator for 5 Countries (and what I learned about personal finance along the way)

How I Built a Free Financial Calculator for 5 Countries (and what I learned about personal finance along the way)

Short summary

A developer built 20+ free financial calculators across five countries using Next.js for SEO and client-side React for privacy. The post reveals how complex tax rules interact non-obviously; the UK State Pension alone adds £170K to FIRE targets that most calculators miss. All calculations run in-browser with zero tracking, verified against official sources (HMRC, IRS, SEBI, ATO, CRA).

  • •Built 20+ free financial calculators for UK, India, US, Canada, Australia using Next.js + client-side React
  • •Discovered non-obvious interactions in complex tax rules across jurisdictions; UK State Pension adds £170K to FIRE targets
  • •Browser-based calculations verified against regulatory sources, zero tracking or login required

Generated with AI, which can make mistakes.

#product-launch
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