We've all been there: you're trying to plan your monthly budget or figure out how much a Systematic Investment Plan (SIP) will yield over the next decade. You search for a mutual fund calculator, click on a top result, and are immediately hit with cookie banners, sticky ads, and a pop-up asking for your phone number to "unlock your results". If you manage to bypass that, you're left fighting with clunky slider controls that jump from 5 years to 20 years on a mobile screen.
I got tired of giving away my contact details just to run a simple compound interest calculation. I wanted a tool that was fast, ad-free, and worked instantly on any device without trackers. So, I decided to build one myself: https://sipcalculator.getinfotoyou.com.
Why I Built It
The goal was straightforward: create a tool that does one thing exceptionally well. For Indian investors and finance enthusiasts, calculating SIP returns is a frequent task. The mathematics behind it isn't complex, yet existing tools wrap it in layers of marketing bloat. I wanted to see if I could build a calculator that loads under 500ms, stores no user data, and offers an intuitive visual breakdown of wealth gained versus principal invested.
The Tech Stack
To keep the page load times minimal and avoid running up hosting costs, I opted for a serverless, static approach:
- HTML5 & Vanilla CSS: No heavy UI frameworks. CSS grid and flexbox handled the layout, while CSS custom variables made styling the sliders and toggle states straightforward.
- Vanilla JavaScript: Used for all the calculations, chart updates, and DOM manipulation.
- Lightweight SVG Canvas: Instead of pulling in a massive charting library like Chart.js or D3, I wrote a custom SVG renderer to draw the pie chart showing the investment breakdown. This saved about 100kb in bundle size.
Technical Challenges
Slider Usability on Mobile
Standard HTML sliders (<input type="range">) can be frustrating to use on touch screens. If you want to select exactly 12% expected return, a tiny swipe might jump from 10% to 15%. I solved this by implementing a dual-control input: users can either drag the slider or tap the output number to type in an exact value. Syncing these two states in vanilla JS without creating infinite loops required careful event listener handling.The Compounding Formula
The formula for SIP returns is:
M = P * [ ( (1 + i)^n - 1 ) / i ] * (1 + i)
Where:
-
Mis the maturity amount -
Pis the monthly investment -
iis the periodic rate of interest (annual rate / 12 / 100) -
nis the number of payments (tenure in years * 12)
Translating this into JavaScript is straightforward, but formatting large numbers in the Indian numbering system (Lakhs and Crores) rather than the standard Western millions/billions format required implementing Intl.NumberFormat('en-IN') to ensure localized readability for the target audience.
Lessons Learned
Building this project reinforced a valuable web development lesson: you don't always need a framework. By keeping the app client-side, the load time is virtually instantaneous, and the user's financial inputs never leave their browser.
If you want to calculate your investment planning without the noise, you can try out the tool here: https://sipcalculator.getinfotoyou.com. I'd love to hear your feedback on the slider behavior and calculations.
