Calculate simple interest, principal sums, rates, or time frames with day-count conventions and compound comparison.
Math Audited
Principal Amount (P)$10,000.00
$
$100$100,000
Annual Interest Rate (R)5.00%
%
0%25%
Time Duration (T)3 years
030 years
Simple Interest Earned
$1,500.00
Total Accumulated Value
$11,500.00
Principal Investment (P)$10,000.00
Interest Earned (I)$1,500.00
Annual Nominal Rate (R)5.00%
Time Duration (T)3.00 years
Accrued Balance: Simple vs. Compound Growth
Simple InterestCompound Interest
๐ก Compounding would have earned you an additional $76.25 (+0.66%) over this period.
Verification and Calculations Standard: Linear Simple Interest model based on SEC and FINRA educational standards. Formula: I = P * R * T.
Assumptions & Limitations:
Assumes nominal interest rate remains constant across the entire timeframe.
No intermediate draws or deposits are accounted for.
Interest is not compounded (does not earn interest on interest).
Formula Sources: SEC Investor.gov, Federal Reserve 12 CFR ยง 1026.17Last Verified: 2026-07-10
About the Simple Interest Calculator
Simple interest is a straightforward method of calculating the interest charge on a loan or the investment yield on a principal sum. Historically dating back to ancient Babylonian and Roman commerce, simple interest remains a foundational concept in banking and retail finance. Unlike compound interest, which calculates interest on both the principal and prior accumulated interest, simple interest is computed exclusively on the original principal sum. This makes it a linear calculation where the interest charge remains constant over each period. Modern applications of simple interest include short-term consumer credit, auto loans, single-payment bank notes, and certain types of certificate of deposit products. Because it does not compound, simple interest is highly predictable and easier for consumers to calculate, though it yields significantly lower returns for long-term investors compared to compounding accounts.
Mathematical Formula & Logic
The mathematical formula for calculating simple interest is:
I = P ร r ร t
Where:
- I = The total interest earned or charged
- P = The initial principal amount (investment or loan balance)
- r = The annual nominal interest rate (expressed as a decimal, e.g., 6.5% = 0.065)
- t = The time duration the money is borrowed or invested, expressed in years
To compute the final accumulated amount (A), use the formula:
A = P + I = P ร (1 + r ร t)
Note: If the time period is given in months, t = months / 12. If in days, t = days / 365 (exact interest method) or t = days / 360 (ordinary interest/Banker's Rule).
Step-by-Step Example
Calculate the simple interest and final amount for a $50,000 principal at an annual interest rate of 6.5% for a time period of 5 years:
1. Identify the variables: P = 50,000, r = 0.065, t = 5
2. Compute interest: I = 50,000 ร 0.065 ร 5 = 16,250
3. Calculate total amount: A = P + I = 50,000 + 16,250 = $66,250
4. The total simple interest earned is $16,250, resulting in a final accumulated value of $66,250.
Reference Data & Values
term
interest earned
total value
interest ratio
1 Year
$3,250
$53,250
6.1%
2 Years
$6,500
$56,500
11.5%
3 Years
$9,750
$59,750
16.3%
4 Years
$13,000
$63,000
20.6%
5 Years
$16,250
$66,250
24.5%
Frequently Asked Questions
The basic formula for simple interest is I = P * r * t, where I is the interest earned, P is the principal amount, r is the annual interest rate as a decimal, and t is the time duration in years.
Simple interest is computed solely on the initial principal amount. In contrast, compound interest calculates interest on both the principal and any previously accumulated interest, causing the balance to grow exponentially over time.
The Actual/360 convention (also called the Banker's Rule) calculates interest by dividing the actual number of calendar days elapsed by a 360-day year. This convention is standard in commercial lending and money markets, and results in slightly higher interest payments compared to the Actual/365 convention.
Modern banks use compound interest because it incentivizes depositors by offering interest on accumulated earnings and maximizes lending yields. Simple interest is usually reserved for short-term commercial paper, demand loans, and car loans.
Yes, if the time is given in months, divide it by 12. If it is in days, you must divide by the number of days in the year (365 for Actual/365, or 360 for Actual/360 and 30/360 conventions) to convert it to years.
The 30/360 convention assumes every calendar month contains exactly 30 days and the year contains 360 days. This makes manual interest calculations easier and is widely used for corporate and municipal bond interest calculations.
If the interest rate is zero, the interest earned will be zero, and the final accrued balance will be exactly equal to the initial principal amount.
While simple interest adds a fixed dollar amount to your balance over time, inflation continually erodes the purchasing power of that money. Over a long period, inflation can easily exceed simple interest returns, leading to a loss in real value.