I Built 21 Free APIs in 3 Months — Here's What I Learned

I'm a self-taught developer from China. Three months ago I had zero passive income. Today I have 21 APIs live on RapidAPI, all running on $0/month infrastructure.

Here's exactly how I did it, what worked, and what I wish I knew earlier.

The Stack (All Free)
Backend: FastAPI — $0

Hosting: Render Free Tier — $0

Health Check: UptimeRobot — $0

Monetization: RapidAPI — 20% commission

Total monthly infrastructure cost: $0.00

The Playbook

1. Pure Algorithm APIs Are Cheat Codes No external dependencies. No API keys. No rate limits. Write once, run forever.

QR Code Generator — Pure Python — Zero maintenance

Password Checker — Entropy math — Zero maintenance

UUID Generator — Python uuid — Zero maintenance

Markdown to HTML — Regex — Zero maintenance

These took 4 hours each. They've never broken once.

1. Free Public Data Sources Are Goldmines Wikidata → Company info for any business → Company Info API

RDAP Protocol → Domain registration data → Domain WHOIS API

OpenStreetMap → Business listings worldwide → Places Search API

All free. All global coverage. No API keys needed.

1. Caching Prevents Cold Starts Render's free tier sleeps after 15 minutes of inactivity. First request takes 50+ seconds.

Fix: UptimeRobot pings /health every 5 minutes. 50 monitors, all free.

The Numbers
After 3 months: 21 APIs, $0 monthly cost, 3+ APIs with growing traffic.

Best performer: Domain WHOIS API — 7.6K calls per week from a single user in Germany.

People are willing to pay for convenience. RDAP is free and well-documented, but wrapping it in a clean JSON API is worth real money.

What I'd Do Differently
Start with health endpoints. Lost weeks debugging cold starts that turned out to be missing /health routes.
Vary your categories. Data, Finance, Tools, Security — different search terms catch different buyers.
Write longer descriptions. RapidAPI search favors APIs with detailed markdown descriptions.
The APIs
Data: Domain WHOIS, Company Info, Email Validator, IP Geolocation, Places Search

Finance: Currency Converter, Stock Price, EU VAT Validator

Security: Password Checker

Tools: QR Code, Barcode, UUID, Markdown, Slug, User Agent Parser, PDF to Text

Search "buigialy30206" on RapidAPI to find all of them.

The real lesson: you don't need venture capital, a team, or a SaaS product. You need one free hosting provider, one free data source, one afternoon per API, and the willingness to ship.

The best passive income is the one that costs $0 to run.