About Our Simple Interest Calculator
Calculate simple interest quickly and accurately with our free online calculator. Whether you're planning savings, evaluating a loan, or understanding investment returns, our simple interest calculator provides instant results with a detailed breakdown.
With our calculator, you can:
Calculate total interest earned on your savings or investments
Determine the final maturity amount after a specified period
Add deposits and withdrawals to see how they affect your returns
View a month-by-month breakdown of your interest growth
Visualize results with interactive charts
What is Simple Interest?
Simple interest is a method of calculating interest where the interest is computed only on the original principal amount throughout the entire loan or investment period. Unlike compound interest, simple interest does not earn "interest on interest."
Simple interest is commonly used for:
- Short-term personal loans
- Auto loans
- Certificates of deposit (CDs)
- Some savings accounts
- Treasury bills and bonds
Simple Interest Formula
The formula for calculating simple interest is straightforward:
SI = P × R × T
Where:
- SI = Simple Interest (the interest earned or paid)
- P = Principal (the initial amount of money)
- R = Rate of Interest (annual interest rate as a decimal)
- T = Time (duration in years)
To find the total amount (A) after interest:
A = P + SI = P × (1 + R × T)
Simple Interest Example
Let's say you deposit $10,000 in a savings account that pays 5% simple interest per year for 3 years.
SI = P × R × T
SI = $10,000 × 0.05 × 3
SI = $1,500
Total Amount = $10,000 + $1,500 = $11,500
After 3 years, you would earn $1,500 in interest, and your total balance would be $11,500.
When is Simple Interest Used?
Simple interest is commonly applied in the following scenarios:
Auto Loans: Many car loans use simple interest calculated on the remaining balance
Personal Loans: Short-term personal loans often use simple interest
Certificates of Deposit: Some CDs pay simple interest rather than compound
Treasury Bills: Government securities often use simple interest calculations
Consumer Credit: Some store financing and installment plans use simple interest
Simple interest formula: the three conventions
You'll see the same formula written three ways. They all give the same answer — the only difference is whether the rate is entered as a decimal or as a whole percentage, and whether the time is measured in years or in shorter periods.
Standard (rate as a decimal)
SI = P × R × T ; total A = P(1 + RT)
R is the annual rate written as a decimal (5% = 0.05) and T is the time in years.
India / school (rate as a whole %)
SI = (P × R × T) / 100
Here R is a whole number (5 instead of 0.05), so you divide by 100 to compensate. This is the form taught in most textbooks.
Per-period (daily / monthly)
I = P × r × n
r is the rate per period and n is the number of periods. Useful when interest accrues per month or per day rather than per year.
All three are the same equation. The /100 form simply lets you type the rate as a whole number instead of a decimal.
How to calculate simple interest (step by step)
Take $10,000 at 5% for 5 years. Multiply the principal by the rate (as a decimal) and by the number of years, then add the interest back to the principal for the total repayment.
SI = P × R × T
SI = 10,000 × 0.05 × 5 = $2,500
Total repayment = 10,000 + 2,500 = $12,500
So the interest is $2,500 and the total repayment is $12,500. Because the interest is charged only on the original $10,000, the same $500 is added every year (5 × $500 = $2,500).
Simple interest for months or by date
For a period shorter than a year, convert the time into a fraction of a year before using the formula. A term of 6 months becomes T = 0.5, and 90 days becomes T = 90/365.
$2,000 at 10% for 9 months
T = 9/12 = 0.75
SI = 2,000 × 0.10 × 0.75 = $150
So nine months of interest on $2,000 at 10% is $150. Counting by exact dates? Use the actual number of days over 365 (or 360 for some bank conventions) as your value of T.
Simple vs compound interest: the dollar gap
Take the same $10,000 at 5% for 5 years. With simple interest the total is $12,500. Compounded monthly it grows to $12,833.59. The $333.59 difference is interest earned on interest — small here, but large over decades.
| Year | Simple (linear) | Compound monthly (exponential) |
|---|---|---|
| 1 | $10,500.00 | $10,511.62 |
| 2 | $11,000.00 | $11,049.41 |
| 3 | $11,500.00 | $11,614.72 |
| 4 | $12,000.00 | $12,208.95 |
| 5 | $12,500.00 | $12,833.59 |
Simple interest adds a flat $500 every year (a straight line). Compound interest curves upward because each period earns on the growing balance. See the Compound Interest Calculator to explore the difference over longer terms.
Simple interest: quick answers
References & methodology
Results use the standard simple interest equation SI = P × R × T (total A = P(1 + RT)). For education only — not financial advice.