We have all been there: you are deep in a technical interview, you use a standard floating point number for a currency calculation, and suddenly your whole architectural plan crumbles because 0.1 + 0.2 === 0.3 is false. Today, we are going to look at how to actually build a US tax calculator using the Next.js App Router without getting roasted by precision drift.

Modeling the US Internal Revenue Code in software is a brutal test of a developer's architectural rigor. You are not just building a UI; you are modeling a localized, highly conditional graph of standard deductions, marginal brackets, and alternative minimum taxes. For engineers targeting Tier 1 US fintechs or FAANG roles in 2026, shipping a high performance financial calculator on the Next.js App Router is a definitive proof of work.…