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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