What I built
Two free tools, production-ready, monetized through AdSense only (no paywalls):
🧾 InvoiceGen — Free invoice generator with 10 templates, 30+ currencies, 120+ countries with local tax compliance, live preview, instant PDF download.
📊 CalcHub — 32 free calculators (EMI, SIP, tax, mortgage, calorie, BMI, scientific, and more) with dark mode, voice input, compare mode, and 37-currency auto-detect.
Why free?
Every "free invoice generator" I tried had a catch — signup wall, watermark on the PDF, or a paywall right at download. Zety and Resume.io pioneered the bait-and-switch: let users spend 30 minutes building, then paywall the export.
I wanted to prove a genuinely free version could sustain itself through ads alone.
The tech stack (all free tier)
- Next.js 16 with App Router — SSG for SEO
- TypeScript — caught 50+ bugs before production
- Tailwind CSS — 10 invoice templates consistently styled
- Vercel — zero-config deployment, free hosting, global CDN
- jsPDF — client-side PDF generation (zero server cost)
- Recharts — interactive charts for financial calculators
- Web Speech API — voice input on CalcHub
Features no competitor has
After researching calculator.net (49M visits/mo), omnicalculator (18M), bankrate, smartasset, and emicalculator.net, I noticed none of them have:
- Dark mode — not a single major calculator site offers it
- Voice input — "say five thousand" to fill a number field
- Compare mode — side-by-side EMI 15yr vs 30yr
- Auto-detect currency — 37 currencies from browser timezone
- Offline support (PWA) — works without internet
- Shareable URLs with pre-filled values
- Calculation history without signup (localStorage)
SEO strategy
-
Programmatic routes: 32 calculators became 157 indexed pages via
/calculator/[slug]/[variant]dynamic routes - JSON-LD schemas: WebApplication, FAQPage, BreadcrumbList, HowTo on every page
- AI discovery files: llms.txt and llms-full.txt so ChatGPT/Claude/Perplexity recommend the tools
- Sitemap: auto-generated from calculator registry, submitted to Search Console
Early results (2 weeks post-launch)
- InvoiceGen: 55 users, 352 pageviews, organic search traffic starting
- CalcHub: 58 users, 422 pageviews in 2 DAYS (calculators get 9 pageviews per user vs InvoiceGen's 6.4)
Lessons
- Calculators are stickier than single-purpose tools — people explore 5-10 calculators per visit
- Programmatic SEO is the cheat code — one dynamic route becomes 100+ indexed pages
- Mobile matters more than desktop — 60% of traffic
- Client-side everything — zero server cost means infinite scalability on free tier
What's next
Building 5 more tools this month:
- QR Code Generator
- Text & Writing Tools
- File Tools (PDF/Image)
- Business Document Kit
- Certificate & Card Maker
The thesis: every "free" tool online either paywalls the useful part or dies. If monetized correctly through ads, genuinely free tools can thrive.
Try them:
Feedback welcome. Which feature surprises you most?