What to Calculate
Select the value you want to find
Enter Values
Input the known variables
Results
Calculated values and breakdown
Interest Rate
8.1368%
Interest Earned
$5,000
Effective Rate (APY)
8.447%
Compounding
monthly
Calculation
r = 12 × [(15000/10000)^(1/12×5) - 1] = 8.1368%
Common Interest Rates (Reference)
Typical rates by investment type
| Type | Typical Range | $10K in 5 Years |
|---|---|---|
| Savings Account | 0.5% - 5% | $11,330 |
| CD | 4% - 5.5% | $12,834 |
| Money Market | 3% - 5% | $12,210 |
| Treasury Bonds | 4% - 5% | $12,518 |
| Corporate Bonds | 5% - 8% | $13,489 |
| S&P 500 (Historical) | 7% - 10% | $14,898 |
* Future values calculated at mid-range rate with monthly compounding. Actual returns may vary.
Rule of 72 - Time to Double
Quick estimate for doubling time
Divide 72 by your interest rate to estimate how many years it takes to double your money.
At 3%
24.0
years to double
At 5%
14.4
years to double
At 7%
10.3
years to double
At 10%
7.2
years to double
Compound Interest Formulas
Mathematical formulas used
Standard Compound Interest
A = P(1 + r/n)^(nt)
A = Final Amount, P = Principal, r = Annual Rate, n = Compounds/Year, t = Years
Continuous Compounding
A = Pe^(rt)
e ≈ 2.71828 (Euler's number)
Effective Annual Rate (APY)
APY = (1 + r/n)^n - 1
For continuous: APY = e^r - 1
Find Interest Rate
r = n × [(A/P)^(1/nt) - 1]
Rearranged from compound interest formula
What if the time period changed?
See how time affects your interest calculations
Personalized Insights
2 insights based on your inputs
8.14% Annual Rate
This is a strong return rate - typical of equity investments.
Compounding Boost
monthly compounding adds 0.31% to your effective annual rate (APY).
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Frequently Asked Questions
What is compound interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. This creates exponential growth over time. The formula is A = P(1 + r/n)^(nt), where P is principal, r is annual rate, n is compounding frequency, and t is time in years.
What is continuous compounding?
Continuous compounding assumes interest is calculated and added to the principal continuously, rather than at discrete intervals. The formula uses the mathematical constant e: A = Pe^(rt). This gives the maximum possible compound interest for a given rate.
How do I calculate the interest rate I'm earning?
Use "Find Interest Rate" mode. Enter your initial principal, final amount, time period, and compounding frequency. The calculator will solve for the rate. For example, if $10,000 grew to $12,000 in 3 years with monthly compounding, the rate is about 6.04%.
What's the difference between APR and APY?
APR (Annual Percentage Rate) is the stated nominal rate without considering compounding. APY (Annual Percentage Yield) is the effective rate including compounding. A 5% APR compounded monthly has an APY of 5.116%. This calculator shows both.
How long to double my money?
The Rule of 72 provides a quick estimate: divide 72 by the interest rate. At 6% interest, money doubles in about 12 years (72/6=12). For precise calculations, use this calculator's "Find Time" mode with your principal doubled as the final amount.
Does compounding frequency really matter?
Yes, but the effect is smaller than many expect. Going from annual to daily compounding on 5% APR increases the effective rate from 5% to 5.127% - a 0.127% difference. For $10,000 over 10 years, that's about $160 extra. More frequent is always better, but the gains diminish.
How this works
Calculations are run entirely in your browser. No inputs are sent to our servers and no account is required. Formulas follow standard US definitions from the IRS and the CFPB where applicable; international users should confirm local tax and regulatory rules apply.
What this tool can’t do
When to consult a professional
This is a software engineering tool, not financial advice. Run the math here, then take the result to a certified financial planner, CPA, or your bank before making a decision that materially affects your money.
Sources
- [1]Consumer Financial Protection Bureau (CFPB)Official sourceconsumerfinance.govAccessed Apr 21, 2026
US consumer finance regulator; authoritative on mortgage disclosures, APR rules, credit cards.
Full-stack software engineer specializing in embedded systems, web architecture, and AI/ML. Founder of Practical Web Tools. Built the gesture-controlled drone IP acquired by KD Interactive (Aura Drone, sold on Amazon).