Simple Interest

Calculate simple interest with I = Prt, solve for principal, rate, or time, and compare it with compound interest

About this calculator

The Simple Interest Calculator applies I = P × r × t: interest is earned only on the original principal, so it grows by the same amount every period. Enter any three of principal, annual rate, time, and interest to solve for the fourth, use years, months, or days (on a 365- or 360-day basis), and see how the result compares with the same money compounded annually.

How to use the Simple Interest calculator

  1. Enter your amounts, rates and terms into the input fields.
  2. Read the result — it updates instantly as you type.
  3. Adjust the inputs to compare scenarios side by side.
  4. Copy the page URL to share the exact calculation.

Common examples

  • $10,000 at 5% for 3 years → interest $1,500.00, final amount $11,500.00 (compounded annually: $11,576.25)
  • $2,500 at 4.5% for 18 months → interest $168.75, final amount $2,668.75
  • Solve for rate: $8,000 earning $960 of interest over 4 years → 3% per year
  • $1,000 at 6% for 90 days on a 365-day basis → interest $14.79 (14.7945…); on a 360-day basis → $15.00

Frequently asked questions

How does the Simple Interest calculator work?

Enter your amounts, rates and terms, and the calculator applies the standard financial formulas to show the outcome, including intermediate figures where they help you understand the result. All arithmetic runs at high precision, so rounding errors do not build up across periods.

When would I use the Simple Interest calculator?

Use it when planning a purchase, loan, investment or budget, to compare options before committing, or to sanity-check figures quoted to you. It is an educational tool, not financial advice — confirm important decisions with your provider or a professional.

How accurate are the results?

The formulas are the standard ones used in finance and run at 30 significant digits internally. Real products may differ in fees, compounding conventions and rounding rules, so treat the result as a close estimate rather than a contractual figure.

Can I share a specific calculation with someone else?

Yes. Every value you enter updates the URL, so copying the address bar and sharing the link reopens the exact same calculation for whoever clicks it.