Simple Interest Calculator with Months & Days
Calculate interest, principal, rate, or time — with support for years, months, and days. 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 with Months & Days – Four Ways to Calculate
Look, I've been there. You're trying to figure out how much interest you'll earn on a savings account, or how much a loan will actually cost you. You pull out your phone, open a calculator app, and then you realize — you have to remember the formula. And then you do the math wrong because you forgot to convert the rate or the time. It's annoying, right? That's why I built this — a simple interest calculator with months and days that does all the heavy lifting for you.
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? This tool gives you four different ways to calculate. Plus, it handles months and days — not just years. Just switch the mode and enter what you know. It's that simple.
And here's the kicker — 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. No signup, no uploads, no data leaving your browser.
Calculate Interest, Principal, Rate, or Time — pick the one you need.
Enter time in years, months, or days — the formula adjusts automatically.
See how principal and interest compare side by side.
Year-by-year breakdown showing the difference between simple and compound interest.
Your numbers never leave your browser. No servers, no storage.
Use it right now. No registration, no email, no credit card.
How to Use This 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 a Simple Interest Calculator with Months & Days?
Here's the thing — most simple interest calculators only work in years. But what if you're calculating interest for 6 months? Or 90 days? You have to do the conversion yourself, and that's where mistakes happen. This tool handles it all — just select the unit and enter the number. The calculator does the rest.
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.
