What it means
The word usually appears in front of another term, such as simple interest, simple return or simple moving average. In each case it signals that no extra layering has been applied.
The figure is easy to calculate by hand and easy to explain. Simple interest is the best-known case.
If you borrow or invest a sum for a number of years at a fixed rate, the interest each year is the same, because it is always based on the starting amount. Many short-term loans, some car finance deals and some bonds calculate interest this way.
Compound interest works differently. Interest earned in one period is added to the balance, so the next period's interest is calculated on a larger sum.
Over long periods the gap between the two grows very large, which is why it matters to know which one applies to a loan or an investment. A simple return is the percentage change in the value of an investment from the start to the end, without adjusting for the timing of cash flows.
A simple average gives every item equal weight, while a weighted average gives some items more influence. Analysts choose between them depending on what they are trying to show.
When reading a contract or an offer, check whether the quoted rate is simple or compound, and how often interest is calculated. Two loans with the same headline rate can cost very different amounts.
If the document does not say, ask before you sign. The same caution applies to averages in management reports.
A simple average of regional margins treats a tiny region the same as the biggest one, which can flatter or damage the picture. When the items differ greatly in size, a weighted average is usually the honest choice.
In practice
Real-world examples.
Example
A small business borrows $20,000 from a supplier for 6 months at 8% simple interest a year. The interest is 20,000 x 0.08 x 0.5 = $800. The owner knows exactly what the loan will cost before signing.
Example
An investor buys shares for $5,000 and sells them for $6,000 a year later. Her simple return is (6,000 - 5,000) / 5,000 = 20%. She uses the figure for a quick comparison with another investment that returned 15%.
Example
A retail analyst calculates the simple average of monthly sales over a year to set a baseline. Each month counts equally, even though the holiday months were far bigger. He later switches to a weighted average to reflect seasonal patterns.
Formula
Calculation
Simple interest = principal x annual rate x time in years
Compound value = principal x (1 + annual rate) ^ years
Suppose you invest $10,000 at 5% a year for 3 years. With simple interest, the interest is 10,000 x 0.05 x 3 = $1,500, so you end with $11,500. With annual compounding, the value is 10,000 x 1.05^3 = 10,000 x 1.157625 = $11,576.25, so the interest is $1,576.25. Compounding earns an extra $76.25 over just three years.Case study
Seen in the real world.
Elmstead Auto Finance is an illustrative, fictional lender that offers two loan products to customers buying used cars. One charges 6% simple interest on the original balance for the whole term, and the other charges 6% a year on the balance still owed as it is repaid each month.
A customer asked the finance manager which was cheaper. The manager showed that on a $15,000 loan over 3 years, the first costs 15,000 x 0.06 x 3 = $2,700 in interest, while the second costs about $1,428, because interest falls as the balance falls.
The customer chose the second loan after seeing the numbers side by side. The illustrative lesson is that the same 6% label can hide very different costs, so the way the rate is applied matters as much as the percentage itself.
Watch out
Common mistakes.
- Comparing a simple rate with a compound rate as if they were the same, when the total cost can differ sharply.
- Forgetting to convert months to years, so that six months is entered as 6 instead of 0.5.
- Using a simple average when the items have different sizes, which gives a misleading picture of overall performance.
Questions
People also ask.
What is the difference between simple and compound interest?
Simple interest is charged only on the original amount, while compound interest is charged on the original amount plus the interest already added.
When is simple interest used?
It is common on short-term loans, some bonds and some consumer finance, although compound interest is more common on savings and long-term borrowing.
Is simple interest always cheaper for a borrower?
Not always, because the total cost also depends on how the balance is repaid, so compare the full repayment schedule.
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