Rex says
Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.
Try a scenario
Click to load — tweak from there.Inputs
Result
Interest
$750
Total balance
$5,750
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter principal ($).
- 2Enter annual rate (%).
- 3Enter years.
- 4Read your interest on the right — it updates as you type.
- 5Hit Share to keep the scenario or send it to someone.
About this calculator
Compute simple interest on a principal over a number of years.
Worked example
Using the values the calculator loads with:
Inputs
- Principal: 5000 $
- Annual rate: 5 %
- Years: 3
Results
- Interest: $750.00
- Total balance: $5,750.00
What each field means
Inputs
- Principal ($)
- The principal used in the calculation, measured in $. Starts at 5000 $ so you have a working example on load.
- Annual rate (%)
- The annual rate used in the calculation, measured in %. Starts at 5 % so you have a working example on load.
- Years
- The years used in the calculation. Starts at 3 so you have a working example on load.
Results
- Interest
- Returned as a money amount in US dollars and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Total balance
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the simple interest calculator work?
Compute simple interest on a principal over a number of years. The underlying maths is: I = P × r × t
What do I need to enter?
3 values: principal, annual rate, and years. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the interest result mean?
It is returned as a money amount in US dollars and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
Who is this tool for?
It's built for anyone comparing money scenarios before committing — budgeting a payment, sanity-checking a quote, or seeing what a change in rate or term actually costs.
Accuracy and limitations
- Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
- Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
- This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Simple Interest Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/simple-interest
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/simple-interest" target="_blank" rel="noopener">Simple Interest Calculator — RevenueLab</a> (2026).</p>
Source: [Simple Interest Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/simple-interest) (2026).