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💰 Simple Interest Calculator — compute interest step by step

Determine simple interest easily. Input your principal amount, interest rate, and time period in years, months, or days, and see a step-by-step breakdown of how the interest is calculated.

Finance Free · no sign-up Added 2026-08-05
Input
Result
Step-by-Step Calculation 

  


        

      

    

    

      
      Private by design. This tool runs entirely in your browser — nothing you enter or add is uploaded to any server.
    

  


  

    
How it works

    
    
      
  1. Enter the starting principal amount (the money invested or borrowed).
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  2. Enter the annual interest rate as a percentage.
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  3. Enter the time period and choose the unit: Years, Months, or Days.
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  4. The calculator converts the time period to years and applies the formula: Interest = (Principal * Rate * Time) / 100.
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  5. The step-by-step math, final interest amount, and total accumulated amount are displayed instantly.
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Examples

    
    
      
  • Investing $10,000 at a 5% annual interest rate for 3 years yields exactly $1,500 in simple interest.
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  • Borrowing $5,000 at a 12% annual interest rate for 6 months yields exactly $300 in simple interest.
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Frequently asked questions

    

  

    What is simple interest?
    
Simple interest is a quick and easy method of calculating interest charge on a loan or investment. It is determined by multiplying the daily or annual interest rate by the principal amount and by the number of periods that elapse.

  

  

    What is the formula for simple interest?
    
The formula is: I = P * R * T, where I is Interest, P is Principal, R is Rate (as a percentage), and T is Time (expressed in years).

  

  

    How does simple interest differ from compound interest?
    
Simple interest is calculated only on the initial principal amount. Compound interest is calculated on the principal plus any accumulated interest from previous periods (interest on interest), causing money to grow at a faster rate over time.

  



  


  

    

      
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