Simple Interest Calculator

Name: Simple Interest Calculator
Author: Jitendra Kumar

Calculate interest, total amount, and a period-by-period growth schedule using the I = P × r × t formula for any principal, rate, and time.

Last Updated: April 2026

Principal

Annual interest rate

Time period

Time unit

Educational and Estimation Notice

This calculator is designed for financial education and estimation. Actual loan interest may differ based on lender-specific conventions, payment timing, origination fees, and regulatory requirements. Always review your loan agreement or consult a financial professional for binding calculations.

Reviewed For Methodology, Labels, And Sources

Every CalculatorWallah calculator is published with visible update labeling, linked source references, and founder-led review of formula clarity on trust-sensitive topics. Use results as planning support, then verify institution-, policy-, or jurisdiction-specific rules where they apply.

Reviewed By

Jitendra Kumar, Founder & Editorial Standards Lead, oversees methodology standards and trust-sensitive publishing decisions.

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Topic Ownership

Sales tax and tax-sensitive estimate tools, Education and GPA planning calculators, Health, protein, and screening-formula pages, Platform-wide publishing standards and methodology

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Methodology & Updates

Page updated April 2026. Trust-critical pages are reviewed when official rates or rules change. Evergreen calculator guides are checked on a recurring quarterly or annual cycle depending on topic volatility.

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How to Use This Calculator

Step 1: Enter the principal
Type the starting loan or investment amount.
Step 2: Set the annual interest rate
Enter the annual rate as a percentage (for example, 5 for 5%).
Step 3: Enter the time period
Type the duration and choose years, months, or days from the dropdown.
Step 4: Review results and schedule
Check the interest, total amount, accrual rates, and the period-by-period growth schedule.

How This Calculator Works

The calculator applies the simple interest formula: I = P × r × t. Principal is your starting amount, r is the annual rate expressed as a decimal, and t is time in years. If you select months or days, the tool converts automatically before applying the formula.

Because simple interest grows linearly, the same dollar amount of interest accrues every period. The calculator displays this as a daily, monthly, and yearly rate so you can reason about accrual at any timescale. The growth schedule breaks the full period into steps so you can see the running balance at each checkpoint.

The effective rate for the period shows the total interest as a percentage of principal over the entire term. For a 5% annual rate over 3 years, the effective period rate is 15% — meaning the total interest equals 15% of the original principal, regardless of how many payments are made.

What You Need to Know

The simple interest formula

Simple interest is calculated with one equation: I = P × r × t. P is the principal (the amount borrowed or invested), r is the annual interest rate expressed as a decimal (divide the percentage by 100), and t is time measured in years. Total repayment or total savings is then A = P + I.

Formula	Expression	Description
Simple Interest	I = P × r × t	Interest earned or owed
Total Amount	A = P + I = P × (1 + r × t)	Principal plus interest
Daily Interest	I/day = P × r / 365	Interest added per calendar day
Monthly Interest	I/month = P × r / 12	Interest added per month
Effective Period Rate	r × t	Total rate applied over the full time period

The formula is linear in all three variables. Doubling the principal doubles the interest. Doubling the rate doubles the interest. Doubling the time doubles the interest. This predictability is one reason simple interest is used for short-term lending and educational examples.

Worked examples

Scenario	Calculation	Total amount
$5,000 at 6% for 2 years	5,000 × 0.06 × 2 = $600	$5,600
$15,000 at 4.5% for 18 months	15,000 × 0.045 × 1.5 = $1,012.50	$16,012.50
$1,000 at 8% for 90 days	1,000 × 0.08 × (90/365) = $19.73	$1,019.73

Simple interest vs compound interest

The key difference is what the interest rate is applied to each period. Simple interest always applies to the original principal. Compound interest applies to the growing balance — principal plus any interest already earned. Over short terms, the two are close. Over long terms, compound interest produces substantially larger totals.

Factor	Simple interest	Compound interest
Interest base	Principal only	Principal + accumulated interest
Growth pattern	Linear — same amount each period	Exponential — grows faster over time
Short-term cost/gain	Very similar to compound	Slightly higher than simple
Long-term cost/gain	Much lower than compound	Much higher than simple
Typical products	Auto loans, some personal loans, T-bills	Mortgages, savings accounts, credit cards
Calculation ease	One multiplication	Requires exponentiation

For a quick comparison, enter your values here and then run the same numbers through the compound interest calculator. The difference grows visibly with longer time periods and higher rates.

Where simple interest appears in real life

Product	How simple interest applies
Auto loan	Banks apply your monthly payment first to interest, then principal, using simple interest on the remaining balance
Treasury bills	US T-bills use simple interest because the term is under one year
Savings bonds (Series EE/I)	Simple interest before maturity; compound interest applies at maturity under certain conditions
Short-term personal loans	Many credit unions and community banks offer simple interest personal loans
Promissory notes	Informal loans between individuals typically use simple interest for clarity

Using time in months or days

When a loan term or investment period is expressed in months or days, convert to years before applying the formula. For months, divide by 12. For days, divide by 365. Some financial products use a 360-day year (the bank year or commercial year) instead of 365. In those cases the daily interest will be slightly higher. This calculator uses 365 days per year.

Example: A 90-day T-bill at 5% on $10,000 earns: I = 10,000 × 0.05 × (90/365) = $123.29. If the same calculation used a 360-day year: I = 10,000 × 0.05 × (90/360) = $125.00. The difference is small but matters for precise financial work.

Solving for principal, rate, or time

The formula rearranges easily for any unknown. To find the principal needed to earn a target interest: P = I / (r × t). To find the rate required for a given interest over a known period: r = I / (P × t). To find the time needed to earn a target amount at a known rate: t = I / (P × r).

This calculator focuses on the standard forward calculation from principal, rate, and time. For deeper savings and investment planning, the compound interest calculator and CAGR calculator extend this analysis to scenarios with recurring contributions and compounding periods.

Keep the research moving with Compound Interest Calculator, CAGR Calculator, Loan EMI Calculator, and Inflation Calculator.

Frequently Asked Questions

Simple interest is calculated as I = P × r × t, where P is the principal (starting amount), r is the annual interest rate as a decimal, and t is the time in years. The total amount owed or accumulated is A = P + I = P × (1 + r × t).

Simple interest is calculated only on the original principal for every period. Compound interest is calculated on the principal plus accumulated interest, so earnings grow faster over time. For short time periods or small rates, the difference is small. Over many years or with high rates, compound interest grows significantly larger than simple interest.

Simple interest is commonly used for short-term loans, auto loans, some personal loans, Treasury bills, and savings bonds. Many payday loans and installment loans also use simple interest calculations. It is also the standard method taught in financial literacy and math education.

Convert the time to years before applying the formula. For months, divide by 12. For days, divide by 365. So a 6-month loan at 8% on $5,000 would be: I = 5,000 × 0.08 × (6/12) = $200. This calculator handles the conversion automatically when you select months or days as the time unit.

Simple interest generally favors the borrower compared to compound interest because interest is not charged on accumulated interest. For savings, compound interest favors the saver because returns grow on top of prior earnings. Simple interest savings accounts or bonds grow more slowly than compounded alternatives.

The effective rate for the period is the total interest as a percentage of principal over the entire time span. For example, a 10% annual rate over 3 years produces an effective rate of 30% for the period (10% × 3 years). This number shows the total cost or gain without dividing by individual years.

This calculator computes interest, total amount, and a schedule from the principal, rate, and time you enter. To find the principal, rate, or time from a known interest amount, rearrange the formula: P = I / (r × t), r = I / (P × t), t = I / (P × r). You can verify results by plugging the derived value back in.