Simple Interest Calculator: Loans & IOUs

Q: What is the simple interest formula?

The simple interest formula is I = P × r × t, where I is interest, P is principal, r is the annual interest rate as a decimal, and t is time in years. The total maturity value is A = P + I (principal plus interest).

Q: When is simple interest used?

Simple interest is commonly used for car loans, personal loans, short-term IOUs, and some certificates of deposit (CDs). It's predictable and easy to calculate, making it ideal for fixed-term agreements. For savings accounts and long-term investments, compound interest is typically used instead.

Simple Interest Calculator 2026: Calculate Interest Without Compounding

Calculate simple interest on loans and investments. How to calculate simple interest formula I = P × r × t. Supports years, months, days, and date ranges.

Featured Snippet: What is the simple interest on $1,000 at 5% for 2 years?

Simple Interest Calculation

The simple interest is $100 ($1,000 × 0.05 × 2), bringing your total value to $1,100.

Formula:

$I = P \times r \times t$
Where:

I = Interest, P = Principal ($1,000), r = Rate (5% = 0.05), t = Time (2 years)
Calculation:

$1,000 × 0.05 × 2 = $100
Total Value:

$1,000 + $100 = $1,100

Simple interest is calculated only on the original principal amount, not on previously earned interest. This makes it predictable and easy to calculate.

Understanding Simple Interest

The Simple Interest Formula

Simple interest is calculated using a straightforward formula that multiplies principal, rate, and time:

Formula:

$I = P \times r \times t$
I

Interest earned or owed
P

Principal amount (original sum)
r

Annual interest rate as a decimal (5% = 0.05)
t

Time in years

The total maturity value (amount you'll receive or pay) is

A = P + I

(principal plus interest)

Time Conversion

The calculator automatically converts different time units to years for accurate calculations:

Months to Years:

Months ÷ 12
Days to Years:

Days ÷ 365
Date Range:

Calculates days between start and end dates, then converts to years

How to Use This Calculator

Enter a principal, interest rate, and time period to calculate simple interest and the total maturity value.

Principal (P):

The original sum of money borrowed or invested. This is the base amount on which interest is calculated.
Annual Interest Rate (r):

Enter as a percentage (e.g., 5 for 5%). The calculator converts it to a decimal (0.05) for the formula.
Time Period (t):

Select years, months, or days and enter the duration. Months are divided by 12 and days by 365 to convert to years. Or switch to Date Range mode and pick specific start and end dates.
Results:

Interest earned or owed (I = P × r × t), total maturity value (A = P + I), and a step-by-step breakdown of the calculation with your values substituted into the formula.

Simple Interest vs. Compound Interest

Key Differences

Understanding the difference between simple and compound interest helps you choose the right calculation method:

Simple Interest:

Calculated only on the original principal. Grows linearly and predictably. Best for car loans, personal IOUs, and short-term agreements.
Compound Interest:

Calculated on principal plus previously earned interest. Grows exponentially over time. Best for savings accounts and long-term investments.
Growth Pattern:

Simple interest: Linear growth (straight line). Compound interest: Exponential growth (curved line).
Predictability:

Simple interest is more predictable and easier to calculate. Compound interest accelerates over time.

For savings accounts and investments, use our Compound Interest Calculator to see how your money can grow faster with compounding.

Common Use Cases for Simple Interest

When to Use Simple Interest

Simple interest is commonly used in these scenarios:

Car Loans:

Many auto loans use simple interest, making monthly payments predictable and easy to calculate.
Personal Loans:

Short-term personal loans often use simple interest for transparency and simplicity.
IOUs and Promissory Notes:

Personal agreements between individuals typically use simple interest calculations.
Short-Term Investments:

Some certificates of deposit (CDs) and short-term bonds use simple interest.

Simple interest provides transparency and predictability, making it ideal for fixed-term agreements where you need to know exactly what you'll pay or receive.

Date-to-Date Calculations

Calculating Interest Between Specific Dates

Many real-world scenarios require calculating interest for a specific date range, such as a loan that starts on one date and ends on another.

How It Works:

Enter your start date and end date. The calculator calculates the number of days between them, converts to years (days ÷ 365), and applies the simple interest formula.
Example:

A $5,000 loan at 6% from January 1 to March 31 (90 days) earns $5,000 × 0.06 × (90/365) = $73.97 in interest.
Use Cases:

Short-term loans, bridge financing, seasonal investments, and any agreement with specific start and end dates.

FAQ

What is the simple interest on $1,000 at 5% for 2 years?

The simple interest is $100 ($1,000 × 0.05 × 2), bringing your total value to $1,100. Simple interest is calculated as Principal × Rate × Time, where the rate is expressed as a decimal (5% = 0.05) and time is in years.

How is simple interest different from compound interest?

Simple interest is calculated only on the original principal amount, while compound interest is calculated on the principal plus previously earned interest. Simple interest grows linearly (predictable), while compound interest grows exponentially (faster over time). Simple interest is common for car loans and personal IOUs, while compound interest is used for savings accounts and investments.

Can I calculate simple interest for days or months?

Yes! The calculator supports years, months, and days. It automatically converts to years for calculation: months are divided by 12, and days are divided by 365. You can also use the date range feature to calculate interest between two specific dates, which is useful for short-term loans or investments.

What is the simple interest formula?

The simple interest formula is I = P × r × t, where I is interest, P is principal, r is the annual interest rate as a decimal, and t is time in years. The total maturity value is A = P + I (principal plus interest).

When is simple interest used?

Simple interest is commonly used for car loans, personal loans, short-term IOUs, and some certificates of deposit (CDs). It's predictable and easy to calculate, making it ideal for fixed-term agreements. For savings accounts and long-term investments, compound interest is typically used instead.

How do I calculate simple interest for a date range?

Use the "Date Range" mode in the calculator. Enter your start date and end date, and the calculator will automatically calculate the number of days between them, convert to years (days ÷ 365), and apply the simple interest formula. This is perfect for loans or investments with specific start and end dates.

Simple Interest Calculator

Time input

Principal

Interest rate

Time

Key metrics

Principal vs. interest breakdown

Predictability

Best use cases

Simple Interest: Predictability and Clarity

Strategic Simple Interest Insights

The Predictability Advantage

Transparency in Lending

Linear Growth Pattern

Choosing the Right Tool

When Predictability Matters Most