Formula

Simple Interest Formula

The simple interest formula relates four quantities associated with borrowing or investing money:

I=PrtI = Prt

where

  • II = the amount of interest (earned on an investment or owed on a loan),
  • PP = the principal (the original amount of money invested or borrowed),
  • rr = the annual interest rate expressed as a decimal (not as a percent),
  • tt = the time the money is invested or borrowed, measured in years.

Because the rate must be in decimal form, a percent such as 5%5\% is first converted to 0.050.05 before being substituted into the formula. To apply this formula in a word problem, identify which three of the four quantities are given, substitute them into I=PrtI = Prt, and solve for the remaining unknown using basic algebra. For example, to find the interest earned on a $3,000\$3{,}000 investment at 4.5%4.5\% annual interest for 22 years, substitute P=3000P = 3000, r=0.045r = 0.045, and t=2t = 2 to obtain I=3000×0.045×2=270I = 3000 \times 0.045 \times 2 = 270, so the interest is $270\$270. This formula models situations in which the interest is computed only on the original principal and does not compound.

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Updated 2026-05-02

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