How I Built 3 Professional Calculators in One Weekend with Next.js 14 – ██FR█████ █INTELL███████████

This content originally appeared on DEV Community and was authored by Daniel Zaharia

Ever found yourself switching between different calculator websites for percentage calculations, financial planning, and real estate analysis? I did too. So I decided to build a comprehensive solution that combines all three into one sleek, modern platform.

The Problem

As a developer, I often need quick calculations for:

Percentage changes and markups for pricing
Compound interest and ROI for investment decisions
Real estate commissions and rental yields

But existing solutions were either too basic, had annoying ads, or required multiple tabs. I wanted something fast, accurate, and beautiful.

The Solution: Advanced Percentage Calculator

I built a complete calculator suite with three specialized tools:

Percentage Calculator

Basic percentage calculations (what is X% of Y?)
Reverse percentage finding
Percentage change analysis
Markup and discount calculations
Compound interest over time

Finance Calculator

Compound interest with different compounding periods
Loan payment calculations
ROI and APR analysis
Investment return projections
Tax calculations

Real Estate Calculator

Commission calculations
Rental yield analysis
Property appreciation tracking
Mortgage payment breakdowns
Property tax estimations

Tech Stack & Architecture

Frontend:

Next.js 14 with App Router for optimal performance
TypeScript for type safety and better DX
Tailwind CSS for rapid, responsive styling
ShadCN UI for beautiful, accessible components
Lucide React for consistent iconography

Key Features:

Client-side calculations – No server requests, instant results
Local storage – Save calculation history
Dark mode – Built with next-themes
Mobile-first – Responsive design with touch-friendly interface
Keyboard shortcuts – Power user features
Export functionality – Download calculation history

Implementation Highlights

1. Reusable Header Component

// Modern responsive navigation with mobile menu
<Header />

2. Real-time Calculations

// Instant updates as user types
const calculate = useCallback(() => {
  // Complex calculation logic
  setResult(calcResult);
}, [input1, input2, type]);

3. Local Storage Integration

// Persistent calculation history
useEffect(() => {
  const saved = localStorage.getItem('calc-history');
  if (saved) setHistory(JSON.parse(saved));
}, []);

Design Decisions

Why Next.js 14?

App Router for better routing and layouts
Server Components for improved performance
Built-in optimizations for images and fonts
Easy deployment to Vercel

Why Client-side Only?

Privacy – No data sent to servers
Speed – Instant calculations
Offline capability – Works without internet
Cost-effective – No server costs

Why ShadCN UI?

Accessible – Built with ARIA standards
Customizable – Easy to theme
Modern – Beautiful default design
Type-safe – Full TypeScript support

Performance & SEO

Lighthouse Score: 95+ across all metrics
Core Web Vitals: Excellent
SEO Optimized: Meta tags, structured data, sitemap
PWA Ready: Manifest and service worker ready

Challenges & Solutions

1. Mobile Navigation

Challenge: Creating a smooth mobile menu experience
Solution: Used ShadCN Sheet component with Radix UI primitives

2. Calculation Precision

Challenge: Handling floating-point arithmetic errors
Solution: Implemented custom precision controls and proper rounding

3. State Management

Challenge: Managing complex form states across multiple calculators
Solution: Used React hooks with proper dependency arrays and useCallback

Results

3 calculators in one platform
100% client-side – No server costs
Mobile responsive – Works on all devices
Fast loading – Under 2 seconds
SEO optimized – Ready for search engines

Live Demo

Check out the live application: Advanced Percentage Calculator

Key Takeaways

Next.js 14 is powerful – The App Router and server components make development much faster
Client-side can be robust – With proper state management, you can build complex applications without a backend
Design matters – Good UX can make technical tools accessible to everyone
Performance is crucial – Users expect instant results from calculators