Compound vs Simple Interest: The Formula, a Free Calculator, and Its Limits
Compound interest adds earned returns to the amount that earns future returns, while simple interest is paid on the original principal only, so it grows by the same amount each year. With a constant annual rate, future value is principal × (1 + rate)^years. Compare both in Theeb Library’s free calculator; real returns can change or turn negative.
Compound vs simple interest calculator — free, instant, no account
Enter an amount, an annual rate and a period to see the difference between the two methods.
- With compound interest
- 1,628.89
- With simple interest
- 1,500.00
- Difference from compounding
- 128.89
- Years to double, rule of 72 (estimate)
- 14.4
An educational model with a fixed rate added once a year, without fees, taxes or inflation. It is not financial advice or a forecast of any investment return.
Create a free Zayenha Hub account: we bring you back to this result with the same numbers, and you can read the Arabic book “Emotional Intelligence Wins!” for free.
Calculate a transparent example
If 1,000 units earn a fixed 5% once each year and all returns stay invested, the model gives 1,000 × 1.05² = 1,102.50 after two years. Simple interest on the original 1,000 would produce 1,100 under the same simplified assumptions. The difference starts small and grows as the number of periods rises. The units here are illustrative; this is not an offer, forecast, or personal investment recommendation.
What the rule of 72 can and cannot tell you
Dividing 72 by an annual percentage rate gives a rough estimate of years to double: at 6%, about 12 years. It is a shortcut, not a replacement for the full calculation, and it becomes less accurate when the rate changes or contributions and withdrawals occur. Inflation also changes what a doubled nominal amount can buy.
A useful way to read the book
Before applying a worked example, list its assumptions: starting amount, rate, compounding interval, fees, and time. Change one assumption at a time and compare the result. Dr. Alaa Theeb’s Arabic book “سحر الفائدة المركبة” also uses compounding as an analogy for repeated habits. That analogy can motivate reflection, but human behavior does not follow a guaranteed financial equation.
Common questions
What is the method where a fixed rate applies only to the principal, so the amount grows by the same sum every year?
That is simple interest, sometimes called fixed interest: the return is calculated on the original principal only, so the balance grows by the same amount each year. Compound interest is calculated on the principal plus earlier returns, so each year’s increase gets larger. The rule of 72 is not a method of calculation but a rough estimate of years to double.
How does compound interest differ from simple interest?
Simple interest is calculated on the original principal. Compound interest includes accumulated returns in the amount used for later calculations.
Does the formula guarantee an investment result?
No. It produces a result for the rate and timing you enter. Actual returns, costs, and purchasing power may differ substantially.
View the Arabic book about compounding
The linked book and reader are in Arabic. This English guide is editorial context, not a translation of the book or a promise of a particular result.