Simple Interest Formula
The Formula
A = P + I = P(1 + rt)
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 I = Prt.
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.
Common Mistakes
- Forgetting to convert the interest rate from a percentage to a decimal — using 5 instead of 0.05
- Using months for t when r is an annual rate — time and rate must use the same period
- Confusing simple interest (I = Prt) with compound interest (A = P(1+r)^t)
Why This Formula Matters
Introduces the time value of money. Contrasts with compound interest and helps students understand loans, savings, and financial literacy.
Frequently Asked Questions
What is the Simple Interest formula?
Simple interest is interest calculated only on the original principal amount, using the formula I = Prt.
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?
P = principal, r = annual interest rate (as decimal), t = time in years, I = interest earned, A = total amount
Why is the Simple Interest formula important in Math?
Introduces the time value of money. Contrasts with compound interest and helps students understand loans, savings, and financial literacy.
What do students get wrong about Simple Interest?
Students confuse simple interest (linear growth) with compound interest (exponential growth) and apply the wrong formula.
What should I learn before the Simple Interest formula?
Before studying the Simple Interest formula, you should understand: percentages, decimal operations, multiplication.