What Is This Interest Rate Calculator?
This calculator works backward from a starting amount, an ending amount, and a time period to find the implied interest rate — useful when you know how much an investment grew but not the rate it grew at.
How to Use the Interest Rate Calculator
- Enter the starting principal or present value.
- Enter the final value.
- Enter the time period in years.
Interest Rate Formulas
Worked Example
Simple rate: ($13,000 − $10,000) / ($10,000 × 5) = 6%
Compound annual rate: (13,000/10,000)^(1/5) − 1 ≈ 5.39%
Understanding Your Results
Simple Interest Rate assumes growth happened linearly and evenly, without compounding. Compound Annual Rate (similar to CAGR) accounts for the compounding effect and is generally the more accurate way to describe actual investment growth over time.
Common Mistakes to Avoid
- Using the simple rate to compare against advertised compound rates (like APY), which aren't directly comparable.
- Forgetting that this calculates an average/implied rate — actual year-to-year rates may have varied.
Frequently Asked Questions
Use the compound annual rate formula: (Final Value ÷ Principal)^(1/Time) − 1, which gives the implied annualized growth rate.
Simple rate assumes linear, non-compounding growth. Compound rate accounts for compounding and is generally the more accurate description of actual investment performance.
It can estimate an implied rate from a loan's starting and ending balance and time period, though actual loan rate calculations depend on the specific payment structure.
The gap grows larger with longer time periods and higher rates, since compounding effects accumulate — for short periods or low rates, the two are often close.
Yes — compound annual growth rate (CAGR) is exactly what the compound annual rate formula calculates.