Interest Rate Calculator
The interest rate calculator solves for the annual interest rate r given the principal P, final amount A (or interest earned I), time t, and interest type (simple or compound). For simple interest, r = (A - P) / (P * t). For compound interest, the formula varies by compounding frequency. This is the inverse of the compound or simple interest calculator.
Formulas
Simple: r = (A - P) / (P * t)
Compound (annual): r = (A / P)^(1/t) - 1
Interest rate calculator: frequently asked questions
What does this calculator do?
This calculator solves for the interest rate r given principal P, final amount A (or interest earned I), time t, and whether the interest is simple or compound.
What is the formula for simple interest rate?
For simple interest: r = (A - P) / (P * t). If you know the interest earned: r = I / (P * t).
What is the formula for compound interest rate?
For compound interest, the formula depends on compounding frequency. For annual: r = (A/P)^(1/t) - 1. For other frequencies: r = n * ((A/P)^(1/(n*t)) - 1).
How do I know if interest is simple or compound?
Check the loan or investment terms. Most savings accounts and investments use compound interest (usually monthly or daily). Many short-term loans use simple interest.
Can the rate be negative?
Yes, if the final amount A is less than the principal P, the rate is negative. This represents a loss rather than a gain.
Official sources
- Wikipedia: Compound Interest.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.