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Interest Calculator

Simple interest is the fundamental baseline equation of credit and debt, calculating interest earned strictly on the original principal without compou

Finance
$
Initial capital sum borrowed or deposited
%
Stated annual simple interest rate (r)
years
Duration of the loan or deposit in years (t)
Formula & Methodology
I = P × r × t | Total Amount = P + I = P × (1 + r × t)

Under simple interest mechanics, interest I is calculated by multiplying initial principal P by the annual interest rate r and the duration in years t. Because accrued interest is never folded back into the principal base, interest earned in Year 3 is identical to interest earned in Year 1.

Simple Interest Earned over 3.0 Years (Total: $11,500)

$1,500 Simple Interest

Overview

Simple interest is the fundamental baseline equation of credit and debt, calculating interest earned strictly on the original principal without compounding. Common in short-term promissory notes, auto loans, personal loans, and peer-to-peer lending, simple interest maintains a constant linear rate of return throughout the entire loan term.

Educational & Lending Disclaimer: This calculator models linear simple interest. It is not financial or legal advice. Commercial bank loans and revolving credit cards typically compound interest daily or monthly; confirm loan terms with your lender.

Worked Examples

Acceptance Standard: $10,000 at 5% for 3 Years

Principal = $10,000, Annual Rate = 5.0%, Term = 3 years.

  1. Multiply principal by rate: $10,000 × 0.05 = $500.00 interest per year.
  2. Multiply by time in years: $500.00 × 3 = $1,500.00 simple interest.
  3. Calculate total repayment amount: $10,000.00 + $1,500.00 = $11,500.00.
  4. Compare with compound interest: $10,000 × (1.05)³ = $11,576.25 (compounding adds $76.25).
Final Answer: $1,500.00 Simple Interest ($11,500.00 Total)

Short-Term Promissory Note: $5,000 at 8% for 6 Months

Principal = $5,000, Rate = 8%, Term = 0.5 years (6 months).

  1. Evaluate: I = $5,000 × 0.08 × 0.5 = $200.00.
  2. Total due: $5,000 + $200 = $5,200.00.
Final Answer: $200.00 Simple Interest ($5,200.00 Total)

5-Year Auto Loan: $25,000 at 6% Simple Interest

Principal = $25,000, Rate = 6.0%, Term = 5 years.

  1. Annual interest: $25,000 × 0.06 = $1,500.00/year.
  2. Total interest over 5 years: $1,500.00 × 5 = $7,500.00.
  3. Total repaid: $25,000 + $7,500 = $32,500.00.
Final Answer: $7,500.00 Simple Interest

How to Use This Calculator

  1. Enter the initial principal amount (loan amount or deposit).
  2. Enter the stated annual simple interest rate.
  3. Enter the term duration in years (e.g. 0.5 for 6 months, 3 for three years).
  4. Review the simple interest earned, total repayment balance, and compound interest comparison.

Frequently Asked Questions

Where is simple interest typically used in real life?
Simple interest is widely used in short-term personal promissory notes, automobile installment loans, some student loans, and federal treasury bills (T-bills). In contrast, savings accounts, CDs, credit cards, and mortgages usually use compound interest.
Does this calculator compound interest?
No. This tool strictly evaluates linear simple interest (I = P × r × t). It does not compound. However, we display a side-by-side compound interest benchmark in the secondary results so you can see how much simple interest trails compound growth.
How do you calculate simple interest for months or days?
Convert the time period into years before calculating. For months, divide by 12 (e.g. 6 months = 6/12 = 0.5 years). For days, divide by 365 (e.g. 90 days = 90/365 ≈ 0.2466 years).
Why is simple interest advantageous for borrowers?
For a borrower, simple interest is advantageous because unpaid interest does not compound and increase your loan balance. If you pay off a simple interest loan early, you pay interest only for the exact days you borrowed the money.
What is the difference between simple interest and compound interest over 30 years?
Over 30 years on $10,000 at 7%, simple interest yields $21,000 in interest ($31,000 total). Annual compound interest yields $66,123 in interest ($76,123 total)—more than triple the simple interest return.

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Reviewed & Authored by: OnlineConvers Quantitative Finance Editorial Team · Last Updated: 2026-10-07
Calculates linear simple interest strictly using the standard formula I = P × r × t. Does not apply periodic compounding or internal interest capitalization. Compares simple interest against annual compounding (A = P × [1 + r]^t) as a benchmark to illustrate the mathematical compounding advantage. Outputs rounded to 2 decimal places.
Citations: Mathematics of Finance (Cissell, Cissell, and Flaspohler (Houghton Mifflin)) · Consumer Financial Protection Bureau: What is simple interest? (CFPB Consumer Education (consumerfinance.gov))