Simple Interest Calculator – Compute SI Online Instantly

Calculate simple interest, total amount, principal, rate, or time with our free online simple interest calculator. Uses the SI = PRT formula with clear step-by-step results.

Principal (P)

Interest Rate (%)

Time Period

Examples:

Understanding Simple Interest

Simple interest is the most straightforward way to calculate interest on money. Unlike compound interest, which grows exponentially, simple interest grows linearly – you earn or pay the same amount of interest each period based on the original principal.

Banks use simple interest for some loans and savings accounts. Car loans often use simple interest calculations. When you lend money to a friend with a verbal agreement to pay back "plus 5%," you're talking about simple interest.

The Simple Interest Formula

SI = (P × R × T) / 100

Where:

P = Principal (initial amount)
R = Annual interest rate (as a percentage)
T = Time period (in years)

Total Amount:

A = P + SI

Or combined: A = P(1 + RT/100)

Worked Examples

Example 1: Basic savings calculation

Problem: You deposit $10,000 in a savings account paying 5% simple interest per year. How much interest do you earn in 3 years?

Solution: SI = (10000 × 5 × 3) / 100 = 150,000 / 100 = $1,500

Total amount after 3 years: $10,000 + $1,500 = $11,500

Example 2: Short-term loan with months

Problem: You borrow $5,000 at 8% annual simple interest for 18 months. What's the total repayment?

Solution: Convert 18 months to years: 18/12 = 1.5 years

SI = (5000 × 8 × 1.5) / 100 = 60,000 / 100 = $600

Total repayment: $5,000 + $600 = $5,600

Example 3: Finding the rate

Problem: $2,000 grows to $2,400 in 4 years with simple interest. What's the rate?

Solution: Interest earned = $2,400 - $2,000 = $400

Using SI = PRT/100: 400 = (2000 × R × 4) / 100

R = (400 × 100) / (2000 × 4) = 40,000 / 8,000 = 5% per year

Example 4: Finding the time

Problem: How long will it take $8,000 to earn $1,200 at 6% simple interest?

Solution: Using SI = PRT/100: 1200 = (8000 × 6 × T) / 100

T = (1200 × 100) / (8000 × 6) = 120,000 / 48,000 = 2.5 years (or 30 months)

Example 5: Comparing investment options

Problem: Option A pays 7% for 3 years. Option B pays 5% for 5 years. Which earns more on $10,000?

Solution: Option A: SI = (10000 × 7 × 3) / 100 = $2,100

Option B: SI = (10000 × 5 × 5) / 100 = $2,500. Option B earns $400 more despite the lower rate.

Quick Fact

Simple interest has been used for over 4,000 years. Ancient Babylonian clay tablets from 1800 BCE show interest calculations on grain loans. The Romans formalized interest rate laws, capping rates at 8.33% (1/12 per month). The word "interest" comes from Latin "interesse," meaning "to make a difference" – the difference between what you borrowed and what you repay.

Simple vs Compound Interest

Simple Interest

• Interest calculated on original principal only
• Linear growth – same interest each period
• Formula: SI = PRT/100
• Used for: short-term loans, car loans, some bonds
• Easier to calculate mentally

Compound Interest

• Interest calculated on principal + accumulated interest
• Exponential growth – interest grows each period
• Formula: A = P(1 + r/n)^(nt)
• Used for: savings accounts, mortgages, credit cards
• "Interest on interest" effect

Frequently Asked Questions

When is simple interest better than compound interest?

Simple interest benefits borrowers on short-term loans – you pay less overall. For savers, compound interest is better because your money grows faster. Banks typically use compound interest for savings accounts but simple interest for some personal loans.

Do car loans use simple interest?

Most car loans use simple interest calculated daily on the remaining balance. As you pay down the principal, you pay less interest each month. This is different from "add-on interest" loans where total interest is calculated upfront and added to the loan amount.

How do I convert monthly rate to annual rate?

Multiply the monthly rate by 12. A 1% monthly rate equals 12% annual simple interest. Be careful – this doesn't work for compound interest, where you'd use (1 + monthly rate)^12 - 1.

Can simple interest be calculated for partial years?

Yes. Express time as a fraction of a year. For 90 days, use 90/365 = 0.247 years. For 6 months, use 6/12 = 0.5 years. The formula works with any time value as long as it's in years.

What's a good simple interest rate?

"Good" depends on context. For savings, anything above 4-5% is decent. For personal loans, under 10% is favorable. Credit cards charge 15-25% (usually compound). Government bonds typically pay 3-5%. Compare rates relative to current market conditions and inflation.

Is simple interest used for mortgages?

Most mortgages use compound interest (monthly compounding). However, some interest-only loans or short-term bridge loans may use simple interest. Always check your loan documents to understand how interest is calculated.

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