What is simple interest?
Simple interest is charged or earned only on the original principal, never on interest already added. This Simple Interest Calculator can solve for the end balance, principal, rate or term: choose the tab for what you want to find, enter the other values (with the rate per year, month or day and the term in years, months or days) and click Calculate. It shows the steps, a graph and a breakdown of principal and interest. It is useful for short-term loans, some car loans, Treasury bills and classroom problems.
Simple Interest Formula
The standard formula for calculating simple interest is:
I = P × r × t
Where:
- I = Total simple interest
- P = Principal amount
- r = Annual interest rate expressed as a decimal
- t = Time in years
For example, a $10,000 loan at a 5% annual rate for 5 years would produce:
I = $10,000 × 0.05 × 5 = $2,500
The total repayment would therefore be:
Total Amount = Principal + Interest
$10,000 + $2,500 = $12,500
Simple Interest for Different Time Periods
The simple interest formula can also be used when the loan or investment period is measured in months, days, or other intervals. The important point is that the interest rate and time period must use the same frequency. For instance, when calculating monthly interest, the annual rate should be converted to a monthly rate, and the number of months should be used as the number of periods.
Simple interest can be useful for comparing borrowing costs, estimating returns, and understanding how much interest will accumulate over a fixed period. However, many modern financial products use compound interest instead, where accumulated interest can also generate additional interest over time.
Simple Interest vs. Compound Interest
The main difference between simple and compound interest is the balance on which interest is calculated. Simple interest applies the rate only to the original principal, while compound interest calculates interest on the principal plus previously accumulated interest. As a result, compound interest can produce higher earnings for investors but may also result in higher borrowing costs.
For borrowers, simple interest can be easier to predict because the interest calculation does not grow as previously accumulated interest is added to the balance. For savings and investments, compound interest can provide greater long-term growth because returns are reinvested and can themselves generate additional returns.
Tips
- Make sure the rate and the term use the same unit: a monthly rate needs a term in months.
- Many car loans and personal loans use simple daily interest, so paying early in the month lowers the interest you pay.
- For savings that pay interest on interest, use the Compound Interest Calculator instead.
Frequently Asked Questions
Simple interest is interest calculated only on the original principal amount. It does not include previously accumulated interest when calculating future interest.
The basic formula is I = P × r × t, where I represents interest, P is the principal, r is the interest rate, and t is the time period in years.
It depends on the situation. Simple interest can be less expensive for borrowers because interest is calculated only on the original principal, while compound interest can provide greater growth for investors over time.
Yes. Simple interest can be calculated monthly or using another period, provided that the interest rate and time period are expressed using the same frequency.
A Simple Interest Calculator uses the principal, interest rate, and time period to calculate the total interest. It can then add the interest to the principal to determine the total amount payable or accumulated.
Disclaimer: This calculator gives estimates for general information only. It is not financial, tax or legal advice. Results depend on the numbers you enter and on simplified assumptions, so your actual figures may differ. Check important decisions with a qualified professional or the official source.
Related Calculators
To compare simple interest with other kinds of interest, try these calculators.