Simple Interest Calculator

Overview

What is a Simple Interest Calculator?

A Simple Interest Calculator finds the interest earned or owed on a principal that doesn't compound. Interest = Principal × Rate × Time (I = P × r × t). Enter the principal, annual rate, and time and it shows the interest and the final total, with the formula — and you can solve for any missing variable. It runs in your browser.

Simple interest is interest calculated only on the original principal, not on accumulated interest. It's used for some short-term loans, bonds, and quick estimates, and it's the easiest interest formula to understand.

This calculator applies I = P × r × t, shows the working, and can solve for whichever value you don't know. It runs entirely in your browser, so your figures stay private.

Guide

How to calculate simple interest

Step 1: Enter the principal amount and the annual interest rate

Enter the principal amount and the annual interest rate.

Step 2: Enter the time period (years, months, or days)

Enter the time period (years, months, or days).

Step 3: Read the interest and the final total, with the formula shown

Read the interest and the final total, with the formula shown.

Screens

Simple Interest Calculator in action

Simple Interest Calculator showing interest and total in the browser. — Interest + total

Simple interest solved for a variable with the formula. — Solve-for

Calculating simple interest on a phone, no app installed. — Mobile (stacked)

Details

Simple interest vs compound interest

Simple interest uses the formula I = P × r × t, where P is the principal, r is the annual rate as a decimal, and t is the time in years; the final amount is P + I. For example, $1,000 at 5% for 3 years earns 1000 × 0.05 × 3 = $150, for a total of $1,150. The defining feature is that interest is always charged on the original principal only — it never earns interest on itself. That's the key contrast with compound interest, where the balance (principal plus accrued interest) earns interest, making it grow faster over time. Because of this, simple interest is the lower-growth option for savings and the cheaper option for borrowing over long periods. It's most common in short-term or specific instruments; many real-world savings accounts and loans actually compound. The calculator lets you solve for any variable (find the rate, time, or principal given the others) and handles time in years, months, or days. It's not financial advice — it's the textbook formula made instant. Everything runs locally in your browser. For interest that compounds, use the Compound Interest Calculator.

Support

Frequently Asked Questions

What is the simple interest formula?

Interest = Principal × Rate × Time (I = P × r × t), with the rate as a decimal and time in years. The final amount is Principal + Interest. For example, $1,000 at 5% for 3 years is 1000 × 0.05 × 3 = $150 interest, totalling $1,150. The calculator shows this working.

How is simple interest different from compound interest?

Simple interest is charged only on the original principal, so it grows in a straight line. Compound interest is charged on the principal plus accumulated interest, so it grows faster over time. For the same rate and term, compound interest yields more (for savings) or costs more (for loans) than simple interest.

Can it solve for the rate, time, or principal?

Yes. Because I = P × r × t can be rearranged, the calculator can find any one variable when you know the other three — for instance the rate needed to earn a target interest, or how long a principal takes to earn a given amount. Just leave the unknown blank.

How do I handle time in months or days?

Convert the time to a fraction of a year for the formula: 6 months is 0.5 years, 90 days is 90/365. The calculator accepts years, months, or days and does this conversion for you, so you can enter the period in whatever unit your problem uses.

When is simple interest actually used?

For some short-term loans, certain bonds, car loans in some markets, and quick estimates or homework. Many everyday savings accounts and long-term loans use compound interest instead, so check which applies to your situation — using the simple formula on a compounding product will understate the real growth or cost.

Is the result financial advice?

No. It's the simple-interest formula applied to your numbers. Real products may compound, include fees, or use different day-count conventions, and the tool doesn't consider your overall finances. Use it for calculations and learning, and consult a financial professional for decisions about saving or borrowing.

Is it free, and do I need to sign up?

Yes, it's completely free with no account, no trial, and no limit on how many times you use it. It runs entirely in your browser, so there's no sign-in wall — just open the page and calculate.

Is my data private?

Yes. Everything is calculated in your browser — the numbers you enter are never sent to a server, and nothing is stored once you close the tab. That keeps any personal figures (salary, loan, health) on your own device.

Does it work on mobile?

Yes. It runs in your phone's browser with nothing to install, so you can calculate on Android or iOS. The layout adapts to small screens, so entering numbers and reading the result is easy on the go.

Does it work offline?

Once the page has loaded, the calculation runs entirely on your device, so it keeps working without a connection. You only need to be online to open the page the first time — which is also why nothing you enter is uploaded.

Can I copy or share the result?

Yes. You can copy the result to your clipboard with one click, and many results can be shared. Because everything is calculated locally, copying is instant with no upload step.

How accurate is it, and is this professional advice?

The math is accurate for the inputs you give, and the formula is shown so you can verify it. But results are estimates for general use, not professional advice — finance calculators use simplified assumptions and ignore fees and rate changes, and health calculators are general estimates, not medical advice. For important decisions, consult a qualified professional.

Still have questions?

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