# Exponential Decay Calculator

> Calculate exponential decay with initial value, decay rate, and time. Find remaining amount, half-life, and decay constant.

**Category:** Math
**Keywords:** exponential, decay, half-life, radioactive, depreciation, rate, constant
**URL:** https://complete.tools/exponential-decay-calculator

## How it calculates

The formula for exponential decay is given by the equation: N(t) = N0 × e^(-kt), where N(t) is the remaining quantity at time t, N0 is the initial quantity, k is the decay constant, and e is the base of the natural logarithm (approximately equal to 2.71828). In this equation, N0 represents the starting amount of the substance, k is determined by the decay rate expressed as a fraction (e.g., 0.1 for a 10% decay rate), and t is the elapsed time. The negative exponent indicates that the quantity decreases as time increases. The decay constant k is linked to the half-life (t1/2) by the relation: k = ln(2) ÷ t1/2, allowing for conversions between these two measures of decay.

## Who should use this

Nuclear physicists assessing the decay of radioactive isotopes in experiments, environmental scientists estimating the degradation of pollutants in ecosystems, pharmacologists calculating the elimination rates of drugs from the body, and archaeologists using carbon dating to determine the age of organic materials.

## Worked examples

Example 1: A radioactive substance has an initial mass of 80 grams and a decay rate of 25% per hour. To find the remaining mass after 3 hours, use the formula: N(3) = 80 × e^(-0.25 × 3) = 80 × e^(-0.75) ≈ 80 × 0.4724 ≈ 37.79 grams. Therefore, after 3 hours, approximately 37.79 grams remain.

Example 2: A sample of a chemical compound starts with 100 mg and has a decay rate of 10% per day. To find the amount left after 5 days, we calculate: N(5) = 100 × e^(-0.1 × 5) = 100 × e^(-0.5) ≈ 100 × 0.6065 ≈ 60.65 mg. After 5 days, about 60.65 mg remains.

Example 3: A certain bacteria culture has an initial count of 1,000 cells and decays at a rate of 5% per hour. To find the cell count after 4 hours: N(4) = 1000 × e^(-0.05 × 4) = 1000 × e^(-0.2) ≈ 1000 × 0.8187 ≈ 818.70 cells. After 4 hours, approximately 818.70 cells are left.

## Limitations

The tool assumes a constant decay rate throughout the specified time period, which may not be accurate for all substances. It relies on the exponential decay model, which is not applicable to materials that do not follow this pattern. The precision of calculations may be limited by rounding errors, particularly when dealing with very small or large initial values. Additionally, the tool does not account for external factors that could influence decay rates, such as temperature or pressure changes, which may affect certain processes.

## FAQs

**Q:** How does the decay constant relate to the half-life?


**A:** The decay constant (k) is related to the half-life (t1/2) by the formula k = ln(2) ÷ t1/2. This relationship allows for the conversion between these two measures, indicating how quickly a substance decays.


**Q:** Can this tool be used for any type of decay process?


**A:** This tool is specifically designed for processes that follow exponential decay. Non-exponential decay processes, such as linear decay or those influenced by external factors, may yield inaccurate results.


**Q:** What happens if the decay rate is zero?


**A:** If the decay rate is zero, the quantity remains constant over time, meaning N(t) will equal N0 for any value of t. This indicates no decay is occurring.


**Q:** How can I determine the decay rate from given half-life?


**A:** The decay rate can be determined using the formula k = ln(2) ÷ t1/2. By inputting the half-life into this equation, one can find the decay constant, which can then be expressed as a decay rate.

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