Algebra
Simple Interest Calculator
Compute interest, final amount, rate, or time. Solve for any variable in the simple interest formula I = Prt — find interest earned, principal, rate, or duration.
Examples:
Formula Reference
Linear Equation
ax + b = c → x = (c−b)/a
Quadratic Formula
x = (−b ± √(b²−4ac)) / 2a
Discriminant
Δ = b² − 4ac
Vertex Form
y = a(x−h)² + k
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Solution Methodology
01
Identify Known Variables
Determine which three of P, r, t, I are known.
02
Rearrange Formula
Algebraically isolate the unknown variable from I = Prt.
03
Compute & Compare
Output the result and show A = P + I alongside compound equivalent.
Common Questions
What is the simple interest formula?
I = P × r × t, where P is the principal (starting amount), r is the annual interest rate expressed as a decimal, and t is the time in years. The total amount after the period is A = P + I = P(1 + rt).
How do I find the interest rate if I know the other values?
Rearrange I = Prt to r = I / (P × t). For example, if $500 earns $75 over 3 years, r = 75 / (500 × 3) = 0.05 = 5% per year.