Simple Interest Formula
Simple interest is interest calculated only on the original principal amount, using the formula I = Prt.
The Formula
When to use: You lend someone \$100 and they pay you \$5 every year as a thank-you — the \$5 never changes because it is always based on the original \$100.
Quick Example
Notation
What This Formula Means
Simple interest is interest calculated only on the original principal amount, using the formula .
You lend someone \$100 and they pay you \$5 every year as a thank-you — the \$5 never changes because it is always based on the original \$100.
Worked Examples
Example 1
easyAnswer
First step
See the full worked solution + why-it-works coaching
SetupKey insightWhy it worksCommon pitfallConnection
Example 2
mediumExample 3
mediumCommon Mistakes
- Compounding by accident - simple interest uses the original every year: , not interest on a growing balance
- Leaving the rate as a whole percent - convert to a decimal first (5% becomes )
- Mismatching the time units - is in years to match an annual rate; 6 months is
Why This Formula Matters
Simple interest is the entry point to financial math and shows the contrast that makes compound interest meaningful: when interest never gets added to the base, growth is steady and linear, not accelerating. Recognizing it by "Is each period's interest computed on the original principal alone (so the yearly interest never changes)?" — rather than by familiar numbers — is what lets a student tell it apart from compound interest and percent of a number and percent increase in a mixed problem set.
Frequently Asked Questions
What is the Simple Interest formula?
Simple interest is interest calculated only on the original principal amount, using the formula .
How do you use the Simple Interest formula?
You lend someone \$100 and they pay you \$5 every year as a thank-you — the \$5 never changes because it is always based on the original \$100.
What do the symbols mean in the Simple Interest formula?
= principal, = annual interest rate (as decimal), = time in years, = interest earned, = total amount
Why is the Simple Interest formula important in Math?
Simple interest is the entry point to financial math and shows the contrast that makes compound interest meaningful: when interest never gets added to the base, growth is steady and linear, not accelerating. Recognizing it by "Is each period's interest computed on the original principal alone (so the yearly interest never changes)?" — rather than by familiar numbers — is what lets a student tell it apart from compound interest and percent of a number and percent increase in a mixed problem set.
What do students get wrong about Simple Interest?
The procedure for simple interest is the easy part; the trap is compounding by accident. Asking "Is each period's interest computed on the original principal alone (so the yearly interest never changes)?" first is what keeps a correct-looking calculation from being attached to the wrong concept.
What should I learn before the Simple Interest formula?
Before studying the Simple Interest formula, you should understand: percentages, decimal operations, multiplication.