How I Built 510 Calculator Tools with Next.js SSG and Zero Hosting Cost

I built a site with 510 free calculation tools — and every single page is statically generated at build time. No database. No server-side rendering. Just Next.js SSG and JSON. Here's the architecture behind keisan-tools. The Concept Japanese users search for specific calculators constantly: BMI, mortgage payments, age calculators. Each query has decent search volume but low competition. Instead of building 10 tools manually, I built a system that generates tools from configuration files. Architecture keisan-tools/ ├── data/ │ └── tools.json # 510 tool definitions ├── app/ │ └── [category]/ │ └── [slug]/ │ └── page.tsx # Dynamic route ├── lib/ │ ├── calculator.ts # Calculation engine │ └── tools.ts # Tool loader └── next.config.js # SSG configuration Each tool is defined as a JSON object: { "slug": "bmi-calculator", "category": "health", "title": "BMI Calculator", "inputs": [ { "name": "height", "label": "Height (cm)", "type": "number" }, { "name": "weight", "label": "Weight (kg)", "typ