Rule of 72 Calculator

Estimate how long it will take for your investment to double at a given annual rate of return

Investment Information

Annual Rate of Return (%)

Initial Investment

Currency

Rule of 72 Results

Doubling Time

years

Time for investment to double

Doubling Date

Approximate date your investment will double

Doubled Amount

Final amount after doubling

Doubling Timeline

Period	Years	Date	Amount
Calculate to see timeline

About the Rule of 72

The Rule of 72 is a simple way to estimate how long an investment will take to double, given a fixed annual rate of interest. By dividing 72 by the annual rate of return, investors can get a rough estimate of how many years it will take for the initial investment to double.

Key Benefits

• Quick mental calculation

• Simple to understand

• Useful for comparing investments

• Works well for rates between 6% and 10%

Limitations

• Less accurate for very high or low rates

• Doesn't account for taxes or fees

• Assumes compound interest

• Doesn't consider inflation

A Rule of 72 Calculator is a quick mental math tool that estimates how long it will take for an investment to double in value based on a fixed annual rate of return. It’s widely used in finance for its simplicity and practicality.

How the Rule of 72 Works

The Formula

Years to Double = \frac{72}{Annual Interest Rate (%)}

Example:

Interest Rate: 6% per year
Years to Double: $\frac{72}{6} = 12 years$

Key Features

✔ Works for compound interest (not simple interest).
✔ Best for rates between 6% and 10% (less accurate for extreme rates).
✔ Can also estimate required rate to double money in a set time.

How to Use the Rule of 72

1. Estimating Doubling Time

Interest Rate	Years to Double
3%	24 years
6%	12 years
9%	8 years
12%	6 years

2. Reverse Calculation (Finding Required Rate)

Example:

Goal: Double money in 8 years.
Required Rate: $\frac{72}{8} = 9 %$

Why the Rule of 72 Matters

✅ Quick mental math – No calculator needed.
✅ Compare investments – Stocks (7-10%) vs. bonds (3-5%).
✅ Financial planning – Estimate retirement or savings growth.

Limitations

⚠ Approximation only – Not exact (real math uses logarithms).
⚠ Works best for moderate rates (6-12%).
⚠ Assumes constant returns – Real-world investments fluctuate.

Rule of 72 vs. Rule of 70 vs. Rule of 69

Rule	Formula	Best For
Rule of 72	$\frac{72}{r}$	General estimates (6-10% returns)
Rule of 70	$\frac{70}{r}$	Continuous compounding (economics)
Rule of 69	$69.3 / r + 0.35$	High precision (math-heavy)

Real-World Applications

Retirement Planning
- If your portfolio grows at 7%, it doubles every ~10 years.
- $100K → $200K in 10 years → $400K in 20 years.
Debt Management
- A 12% APR credit card debt doubles in 6 years if unpaid.
Business Growth
- A company growing at 9% annually doubles in size in 8 years.

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