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Simple Interest Calculator

Interest on the principal only, and what compounding would add.

Inputs
%
Total interest
2,500.00
Final balance
12,500.00
Per year
500.00
If compounded annually
2,762.82
Compounding would add
262.82
interest = principal × rate × years

Simple interest is charged on the original principal only. Most real products compound.

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Simple interest is principal times rate times years. It is charged on the original amount only, so it grows in a straight line — unlike compound interest, which charges interest on interest already earned.

How to use Simple Interest Calculator

  1. Enter the principal. The original amount borrowed or invested.
  2. Set the annual rate and term. As a percentage per year, and the number of years.
  3. Compare. The compound figure for the same inputs is shown alongside.

About calculating simple interest

Simple interest is the model most people carry in their heads and it describes surprisingly few real products. Its defining property is linearity: the same amount is added every period, because the calculation always refers back to the original principal. That makes it easy to reason about and is exactly why it is used for statutory late-payment interest and some short-term consumer credit, where predictability matters more than accuracy about the time value of money. Almost everything long-term compounds, and the divergence is dramatic rather than marginal. Ten thousand at five percent for thirty years earns fifteen thousand in simple interest and thirty-three thousand compounded — more than double, from the same rate and the same principal. That is the entire argument for starting to save early, and equally the reason credit card debt is so punishing, since the same mechanism runs against you and typically compounds monthly rather than annually. Two cautions when comparing products. A quoted rate means little without knowing the compounding frequency, which is why regulators require an annualised figure such as APR or AER. And every figure here is nominal: a five percent return with three percent inflation is a two percent real gain, and money in an account paying less than inflation is losing value while the balance rises.

Frequently asked questions

What is the difference between simple and compound interest?
Simple interest applies only to the original principal, so it adds the same amount every year. Compound interest applies to the principal plus interest already added, so each year adds more than the last. Over long periods the difference is enormous.
Where is simple interest actually used?
Short-term consumer loans in some jurisdictions, some car finance, certain bonds that pay a fixed coupon without reinvestment, and most statutory interest on late payments. Savings accounts and mortgages almost always compound.
How much does compounding add?
Over five years at five percent it is a few percent of the principal; over thirty years it more than doubles the interest. That gap is why the distinction matters far more for long-term saving than for a two-year loan.
Does this account for inflation?
No. The figures are nominal. If inflation runs at three percent while your money earns five, the real return is closer to two — and a return below inflation is a loss in purchasing power despite the balance going up.

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