DocomintSimple Interest Calculator
Calculate simple interest on a principal amount over time, at a fixed annual rate.
What is a Simple Interest Calculator?
Simple interest is calculated only on the original principal, never on interest already earned — a flat, linear growth rate rather than compound interest's accelerating curve. Some short-term loans and older savings products still use it, so it's worth being able to check directly.
How to use the Simple Interest Calculator
- Enter the principal, annual interest rate, and time in years
- Press Calculate
- The interest earned and total amount appear instantly, computed on your device
Formula
SI = P × r × t ÷ 100
A flat percentage of the original principal for every time period, with no interest earned on interest already accrued.
Example
$10,000 at 5% simple interest for 3 years earns exactly $1,500 (5% of $10,000, three times) for a total of $11,500 — compound interest at the same rate and term would earn slightly more, since it factors interest on top of interest.
Common mistakes
- Using this for a savings account or loan that actually compounds — most modern financial products do, which this tool intentionally doesn't model.
- Entering the rate as a decimal (0.05) instead of a percentage (5).
- Reading the result as the total amount owed/earned rather than just the interest — total = principal + SI, not SI alone.
Frequently asked questions
When is simple interest used instead of compound?
Some short-term loans, certain bonds, and a handful of older savings products calculate interest as simple rather than compound — it's less common today, but worth checking directly for any product that specifies it.
How is simple interest calculated?
SI = Principal × Rate × Time / 100 — a flat percentage of the original principal for every year (or other time period), with no compounding on interest already earned.
Which financial products actually use simple interest?
Some short-term personal loans, a handful of bonds, and certain older savings products calculate interest this way — it's less common today than compounding, so check your specific product's terms rather than assuming.
Can I use months instead of years for the time period?
Enter a fractional year instead — 6 months is 0.5 years, 18 months is 1.5 years — since the time field expects years, not a separate unit selector.
Is my numbers uploaded anywhere?
No — Simple Interest Calculator runs entirely in your browser using JavaScript/WebAssembly. Your numbers is never sent to a server.