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Devendra Kumar Dixit
Devendra Kumar Dixit

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Building a Simple Interest Calculator with HTML, CSS, and JavaScript

Why I Built This

I wanted to build another small front-end project that focuses on logic and clarity rather than features or design polish.
An interest calculator felt like a good choice because it uses simple math but is still useful in real life.
The goal was to practice writing clean JavaScript and building something understandable without frameworks.

This is a learning-focused build log.

What the Interest Calculator Does

The calculator keeps things intentionally simple:

Takes principal amount, interest rate, and time as input

Calculates simple interest instantly

Displays the result clearly

Works well on mobile screens

No charts, no compound logic, no advanced options.

Tech Stack

I intentionally kept the stack minimal:

HTML – structure and input fields

CSS – clean, responsive layout

JavaScript – interest calculation and live updates

Using vanilla JavaScript helped me focus on fundamentals instead of abstractions.

Interest Calculation Logic
The Formula (Plain English)

Simple interest is calculated using:

Simple Interest = (P × R × T) / 100

Where:

P = Principal amount

R = Annual interest rate

T = Time (in years)

In this project:

Principal is entered as a number

Interest rate is annual

Time is entered in years

Core JavaScript Logic

Here’s the main calculation function:

function calculateInterest(principal, rate, time) {
if (principal <= 0 || rate <= 0 || time <= 0) {
return null;
}

const interest = (principal * rate * time) / 100;
return interest.toFixed(2);
}

To keep the calculator responsive, the result updates whenever the user changes an input:

inputs.forEach(input => {
input.addEventListener("input", updateInterest);
});

function updateInterest() {
const principal = Number(principalInput.value);
const rate = Number(rateInput.value);
const time = Number(timeInput.value);

const interest = calculateInterest(principal, rate, time);
result.textContent = interest ? Interest: $${interest} : "";
}

The focus here was readability and correctness rather than compact code.

UI & UX Decisions

I kept the UI simple and functional:

Mobile-first layout

Clearly labeled input fields

Instant feedback instead of a calculate button

Minimal styling to avoid distraction

The calculator is designed to be usable without any instructions.

Edge Cases & Possible Improvements

Things handled or noticed:

Empty inputs show no output

Zero or negative values are ignored

Assumes simple interest only

Possible improvements for a future version:

Total amount calculation (principal + interest)

Option to switch between years and months

Better validation and accessibility support

I avoided adding these to keep the scope small.

Demo & Source Code

If you want to try it or look at the code:

Live demo: https://yuvronixstudio.github.io/interest-calculator/

Source code: https://github.com/YuvronixStudio/interest-calculator

Feedback

I’d appreciate feedback from other developers, especially on:

Is the interest calculation logic correct?

Any UI or UX improvements you’d recommend?

Better ways to structure JavaScript?

This was built as a learning exercise, so constructive feedback is welcome.

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