How to Calculate Simple Interest Manually
While our tool does the heavy lifting, knowing the math is essential for financial literacy. The simple interest formula is straightforward and linear, unlike compound interest which grows exponentially.
I = P × r × t
- I = Interest Amount
- P = Principal (Starting amount)
- r = Annual Interest Rate (in decimal form)
- t = Time (in years)
Calculation Example: Personal Loan from a Friend
Let's say you borrow $5,000 from a friend. They agree to a flat 6% interest rate over 2 years.
- Convert rate to decimal: 6% ÷ 100 = 0.06
- Apply the formula: 5,000 × 0.06 × 2
- Calculate: $600 in total interest.
- Total Repayment: $5,000 (Principal) + $600 (Interest) = $5,600.
Simple Interest vs. Compound Interest: What's the Difference?
The key difference lies in what the interest is calculated on. This distinction can save (or cost) you thousands of dollars depending on whether you are the borrower or the lender.
| Feature | Simple Interest | Compound Interest |
|---|---|---|
| Calculation Base | Principal Amount Only | Principal + Accumulated Interest |
| Growth | Linear (Steady) | Exponential (Accelerating) |
| Common Uses | Car Loans, Personal Loans, Bonds | Credit Cards, Mortgages, Savings |
Daily Simple Interest Explained
Some loans, particularly auto loans, accrue interest daily. This is calculated using the daily periodic rate. To find this, you divide your annual rate by 365 (or sometimes 360, depending on the bank).
Why does this matter? If you pay your loan early, you save money on interest for every single day the principal is lower. Our calculator above features a "Days" mode specifically to help you estimate these costs.