Doubling Time Calculator

Calculate how long it takes for your investment to double

Instantly convert between growth rate and doubling time with high precision

Last updated: November 30, 2025

CreatorFrank Zhao

Growth Rate (%)

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% per period

Doubling Time (Periods)

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periods

Doubling Time Visualization

Initial Amount

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📈 What is Doubling Time?

Doubling time is the period required for a quantity to double in size or value, assuming a constant growth rate. It is a fundamental concept in exponential growth, widely applied in finance (compound interest), demography (population growth), and biology (bacteria reproduction).

The concept relies on a constant percentage increase over time. If the growth rate remains stable, the time it takes to double remains constant, regardless of the starting amount. The opposite concept is "half-life," which measures exponential decay (e.g., in radioactive materials).

🧮 Doubling Time Formula

To calculate the exact doubling time, we use the natural logarithm. The formula is derived from the compound interest equation:

Doubling Time = ln(2) / ln(1 + r)

Where:

ln is the natural logarithm.
r is the growth rate per period expressed as a decimal (e.g., 5% = 0.05).

💡 Quick Estimation - Rule of 72

For a quick mental estimate, divide 72 by the percentage growth rate. For example, at 6% growth, the doubling time is approximately 72 / 6 = 12 periods.

⚠️ Limitations

The main limitation of the doubling time formula is its assumption of a constant growth rate. In the real world, growth rates often fluctuate due to market conditions, resource constraints, or environmental factors.

Therefore, doubling time should be viewed as a theoretical projection based on current trends, rather than a guaranteed prediction of the future.

📝 Calculation Example

Let's say you have an investment portfolio growing at a constant rate of 8% per year. How long will it take to double your money?

Given: r = 8% = 0.08

Time = ln(2) / ln(1 + 0.08)
Time ≈ 0.693 / 0.077
Time ≈ 9.006 years

So, it will take approximately 9 years for your investment to double at an 8% annual growth rate.

❔ Frequently Asked Questions

Q: Does the initial amount affect doubling time?

A: No. The time it takes to double depends only on the growth rate, not the starting value. It takes the same amount of time for $100 to become $200 as it does for $1 million to become $2 million, provided the growth rate is the same.

Q: What is the doubling time for a population?

A: It is the time required for a population to become twice its current size. It is calculated using the same formula: T = ln(2) / ln(1 + r). For example, a country with a 2% annual population growth rate will double its population in about 35 years.

Q: How accurate is the Rule of 72?

A: The Rule of 72 is a very good approximation for growth rates between 6% and 10%. For very high or very low rates, the divergence from the exact logarithmic formula increases, but it remains a useful tool for quick estimates.

Q: What is the difference between doubling time and half-life?

A: Doubling time measures exponential growth (how long until a value doubles), while half-life measures exponential decay (how long until a value reduces to half). They are inverse concepts used in different contexts.

Q: Can I use this calculator for any type of growth?

A: Yes, this calculator works for any scenario involving constant exponential growth: investments, population growth, bacterial reproduction, viral spread, or any other process with a constant percentage increase per time period.

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