How I Built 100+ Client-Side E-Commerce & Dev Utilities With Zero Server Storage

Most online calculators and PDF/image utilities upload your sensitive data—unit costs, ad spend, or private documents—to a backend server and clutter the screen with intrusive ads. I built SwiftKitHub (swiftkithub.com) to solve this: a fast, 100% client-side utility workbench running entirely inside browser memory using React, TypeScript, and WebAssembly. Architecture Highlights 0 KB Cloud Payload: All calculations, PDF merging, and image conversions run locally in the browser with zero server storage or tracking. Deterministic 2026 Math: Verified against 400+ automated unit tests so sellers and media buyers get exact cent-accurate numbers. Static Pre-Rendering: Built with Vite and statically pre-rendered across 136 routes for sub-second First Contentful Paint. Flagship Tools You Can Try Free (No Signup) E-Commerce & Marketplace Fees: Etsy Fee Calculator 2026 — Models transaction fees, listing fees, regulatory operating fees, and net margin. TikTok Shop US Fee Calculator — Calculates 6.0% referral tiers and affiliate commissions. Product Margin Calculator — Real landed COGS vs. gross/net margin breakdown. Ad Spend & ROAS: Break-Even ROAS Calculator — Finds your exact survival ROAS floor and max CPA before spending on ads. Media & Developer Utilities: 4:3 to 16:9 Aspect Ratio Converter — Instant pixel resolution scaling for video and screens. CSS Gradient Generator — Live linear and radial CSS code builder. I'd love feedback from fellow builders on the UI, speed, and any edge-case calculators you'd find useful!