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Simple Interest Calculator

Calculate simple interest accrued over years, months, or days with exact principal and total repayment breakdowns.

Quick Answer:Simple interest calculates finance charges purely on the original principal balance without interest compounding on interest: Interest = Principal × Rate × Time.

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How to Use the Simple Interest Calculator with Amortization Schedule

1Enter your starting principal balance in dollars.
2Enter the annual interest rate as a percentage (e.g., 5.5 for 5.5%).
3Specify the investment or loan term and select years, months, or days.
4View the total accrued interest, overall repayment amount, and average monthly rate.

Formula & How It Works

Simple Interest Formula

I = P × (r / 100) × t, Total = P + I

Computed using linear accrual where time is converted to years: I = P × r × t, where P is principal, r is the decimal interest rate, and t is time in years.

Worked Examples

Personal Loan over 3 Years

Input: $10,000 at 5% for 3 years

Explanation: $10,000 × 0.05 × 3 = $1,500 interest, giving a total repayment sum of $11,500.

Result: Interest: $1,500.00 | Total: $11,500.00

Short-Term Promissory Note (6 Months)

Input: $5,000 at 8% for 6 months

Explanation: $5,000 × 0.08 × (6 / 12) = $200 total accrued simple interest.

Result: Interest: $200.00 | Total: $5,200.00

90-Day Commercial Note

Input: $25,000 at 6% for 90 days

Explanation: $25,000 × 0.06 × (90 / 365) = $369.86 interest accrued on exact day count.

Result: Interest: $369.86 | Total: $25,369.86

Frequently Asked Questions

What is the difference between simple interest and APR?▼

Simple interest calculates the raw interest rate applied directly to the principal balance without fees or compounding. Annual Percentage Rate (APR) incorporates mandatory upfront fees, points, and compounding frequency to reflect the true annual cost of borrowing.

Are auto loans simple interest or compound interest?▼

Most auto loans in the United States and Canada are structured as simple interest loans. Interest accrues daily based on the remaining principal balance, meaning paying ahead of schedule reduces overall interest paid.

How is day-count calculated for simple interest?▼

Standard actual/365 convention divides the exact number of calendar elapsed days by 365 (or 366 during leap years). Certain commercial commercial contracts use actual/360 or 30/360 conventions.

Can I pay off a simple interest loan early to save money?▼

Yes, assuming there is no prepayment penalty clause. Because simple interest accrues daily on remaining principal, reducing principal early cuts all future interest obligations immediately.

Does simple interest compound monthly?▼

No. Pure simple interest never compounds. It accrues linearly based on original or outstanding principal.

What happens if a payment is made late on a simple interest loan?▼

More interest accrues because more days elapsed with a higher principal balance. A larger portion of your next installment goes toward accrued interest, extending the payoff timeline.

Quick Reference & Conversion Table

Principal	Annual Rate	Term	Accrued Interest	Total Repayment
$1,000	5.0%	1 Year	$50.00	$1,050.00
$5,000	6.5%	2 Years	$650.00	$5,650.00
$10,000	7.0%	3 Years	$2,100.00	$12,100.00
$20,000	8.5%	4 Years	$6,800.00	$26,800.00
$50,000	9.0%	5 Years	$22,500.00	$72,500.00

⚠️ Common Pitfalls to Avoid

•Confusing simple interest with compound interest, where unpaid interest is added to the balance and earns subsequent interest.
•Entering monthly interest rates instead of nominal annual percentage rates (APR).
•Using 360-day bank convention when the agreement specifies 365-day exact annual day count.

💡 Practical Real-World Uses

•Calculating interest due on private promissory notes and family loans.
•Evaluating short-term installment contracts and retailer bridge loans.
•Understanding dealer interest charges on simple interest retail vehicle installment contracts.

Simple Interest vs Compound Interest

Feature	Simple Interest	Compound Interest
Interest Calculation	Calculated solely on principal	Calculated on principal plus accumulated interest
Growth Trajectory	Linear growth over time	Exponential compounding growth
Typical Products	Auto loans, personal notes, bonds	Savings accounts, mortgages, credit cards
Borrower Cost	Lower overall cost for identical rate/term	Higher finance charges over extended loan horizons