Simple Interest Calculator Pro
Calculate interest, principal, rate, or time. Compare simple vs compound interest year by year. 100% free, private, and local.
Enter your numbers, select a mode, and click "Calculate" to see the results.
Your numbers are processed entirely in your browser. They never leave your device. No uploads, no tracking, no signup.
Simple Interest Calculator Pro – Four Ways to Calculate
Most simple interest calculators only give you the interest. This one gives you four different ways to calculate. Need to find the principal? The rate? The time? Just switch the mode and enter what you know. It's that simple.
Plus, you get a Principal vs Interest breakdown and a Simple vs Compound Interest year-by-year comparison. You can see exactly how simple interest stacks up against compound interest with the same inputs — year by year.
Calculate Interest, Principal, Rate, or Time — pick the one you need.
See how principal and interest compare side by side.
Year-by-year breakdown showing the difference between simple and compound interest.
Choose your time unit — the formula adjusts automatically.
Your numbers never leave your browser. No servers, no storage.
Use it right now. No registration, no email, no credit card.
How to Use the Simple Interest Calculator
- Pick a mode – Click Interest, Principal, Rate, or Time at the top.
- Enter what you know – The fields update based on your mode.
- Choose your time unit – Years, Months, or Days.
- Click Calculate – Get your result instantly.
- Compare – See Principal vs Interest, and Simple vs Compound year by year.
That's it. No hidden steps. No "sign up to see results" nonsense. Just calculate and go.
Why Use This Simple Interest Calculator?
Here's the thing — most calculators only give you one answer. You put in principal, rate, and time, and you get interest. But what if you already know the interest and need to find the principal? Or the rate? Or the time? That's where this tool stands out. Four modes, one tool. Plus, the year-by-year comparison table is something you won't find in most simple calculators.
I've used this to check loan terms, estimate savings growth, and help friends figure out if they're getting a fair deal. It's one of those tools you don't think you need until you need it, and then you're really glad it exists.
Frequently Asked Questions
Learn how to use this tool correctly by reading our step-by-step tutorial.
