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Simple Interest Calculator

Enter principal, annual rate, and term to instantly calculate simple interest and the total amount.

Last updated: 2026/10/16 Calculator Feedback & bug reports
Input
%
- Interest (simple)
- Total amount
- Interest (compound, ref.)
- Difference vs. compound
These results are general-information estimates only. They exclude taxes and fees and do not guarantee the performance of any financial product. This tool is not investment advice; make decisions at your own risk.

How to Use

1

Enter the principal

Enter the principal amount. Choose the unit: JPY or 10,000 JPY (man).

2

Enter rate and term

Enter the annual rate (%) and the term. The term can be set in years, months, or days.

3

View the results

Simple interest and the total amount are calculated automatically, with the difference vs. compound shown for reference.

What is Simple Interest?

Simple interest is calculated only on the original principal. Because interest is not added back to the principal (not reinvested), the interest earned each period stays constant. The formula is: Interest = Principal x Annual Rate x Term (in years). Compound interest, by contrast, earns interest on previously earned interest, so the gap between the two widens over longer terms. This tool shows the simple interest and total amount, plus the difference against compound interest for reference.

Features

  • Real-time CalculationInterest and total update instantly as you type. No waiting for results.
  • Compound ComparisonSee the compound interest for the same conditions and the difference vs. simple at a glance.
  • Flexible Term InputEnter the term in years, months, or days to cover short- and long-term cases.
  • Privacy ProtectedAll calculations run in your browser. The amounts you enter are never sent to a server.

Use Cases

  • Estimating interest on fixed deposits or bonds
  • Calculating simple interest on personal loans
  • Comparing simple- vs. compound-interest products
  • Interest simulations for financial education
  • Estimating interest received on short-term holdings

FAQ

What is the difference between simple and compound interest?
Simple interest is earned only on the original principal, while compound interest also earns interest on previously earned interest. The difference is small over short terms but grows larger the longer the term.
How are months and days converted?
Months are divided by 12 and days by 365 to convert into years. For example, 6 months is treated as 0.5 years and 73 days as 0.2 years.
Are taxes and fees included?
No. This tool shows a pre-tax estimate without fees. In Japan, interest is usually taxed at 20.315%, so the actual amount received will be less than shown.
Can I use these results for investment decisions?
This tool provides general-information estimates only and is not investment advice. Real financial products may use different rates and terms, so make any final decision at your own responsibility.
Are the amounts I enter saved or transmitted?
No. All calculations run entirely within your browser, and the values you enter are never sent to or stored on any server, so you can use it with peace of mind.

Related Tools

Compound Interest Calculator Calculate compound interest with savings simulation, reverse calculation, comparison chart & Rule of 72 Loan Prepayment Simulator Estimate interest and term saved by prepaying a loan Percent Calculator Calculate percentages easily

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