Logistic Growth Calculator
Our free exponents & logarithms calculator solves logistic growth problems. Get worked examples, visual aids, and downloadable results.
Reviewed by Manoj Kumar, Mathematics Educator
Formula
P(t) = K / (1 + ((K - P0) / P0) * e^(-rt))
Where P(t) is population at time t, K is carrying capacity (maximum population), P0 is initial population, r is intrinsic growth rate, and e is Euler's number. The model produces an S-shaped curve that starts exponentially and levels off at K.
Worked Examples
Example 1: Bacterial Colony Growth
Problem:A bacterial colony starts with 100 organisms in an environment that can support 10,000. The intrinsic growth rate is 0.5 per hour. What is the population after 10 hours?
Solution:P(t) = K / (1 + ((K-P0)/P0) * e^(-r*t))\nK = 10,000, P0 = 100, r = 0.5, t = 10\nA = (10,000 - 100) / 100 = 99\nP(10) = 10,000 / (1 + 99 * e^(-0.5*10))\n= 10,000 / (1 + 99 * 0.00674)\n= 10,000 / (1 + 0.6671)\n= 10,000 / 1.6671 = 5,998
Result:P(10) = 5,998 organisms (59.98% of carrying capacity)
Example 2: Technology Adoption Forecast
Problem:A new app has 1,000 users with a potential market of 500,000. If the growth rate is 0.3 per month, when will it reach 250,000 users (inflection point)?
Solution:Inflection occurs at P = K/2 = 250,000\nt_inflection = ln((K-P0)/P0) / r\n= ln((500,000-1,000)/1,000) / 0.3\n= ln(499) / 0.3\n= 6.2126 / 0.3\n= 20.71 months
Result:Inflection point at 20.71 months | Peak growth rate = 37,500 users/month
Frequently Asked Questions
What is logistic growth and how does it differ from exponential growth?
Logistic growth describes population increase that starts exponentially but slows as resources become scarce, eventually leveling off at a maximum called the carrying capacity. In exponential growth, the population increases by a constant percentage indefinitely, which is unrealistic because no environment has unlimited resources. Logistic growth introduces a self-limiting mechanism: as the population approaches the carrying capacity K, the growth rate decreases toward zero. The logistic model produces an S-shaped (sigmoid) curve, while exponential growth produces a J-shaped curve. Most real biological populations follow logistic-type growth over long periods.
What does the growth rate parameter r represent?
The intrinsic growth rate r represents how fast the population would grow if there were no resource limitations. It is the per-capita growth rate when the population is far below the carrying capacity. A higher r means faster initial growth and a steeper S-curve. The parameter r is measured in inverse time units: if r = 0.5 per year, the population would grow approximately 50% per year in the early exponential phase. In practice, r is estimated from the observed doubling time during early growth, from birth and death rate data, or by fitting the logistic model to observed population data. Typical values vary enormously across species and contexts.
What is the inflection point in logistic growth?
The inflection point is where the growth rate reaches its maximum and the curve changes from concave up (accelerating) to concave down (decelerating). It occurs exactly at half the carrying capacity, when P = K/2. At this point, the population is growing as fast as it ever will, with the growth rate equal to rK/4. The inflection point time is calculated as t = ln((K-P0)/P0) / r. This midpoint is significant in epidemiology (peak infection rate), marketing (peak adoption rate), and ecology (optimal harvesting point). Understanding where the inflection point falls helps predict when growth will begin slowing down.
How is the logistic growth model used in epidemiology?
In epidemiology, the logistic growth model helps predict the spread of infectious diseases. The carrying capacity represents the total susceptible population, the growth rate reflects the transmission rate, and the inflection point indicates when new infections peak. During the early phase of an outbreak, cases grow approximately exponentially. As more people become infected (or vaccinated), the susceptible pool shrinks and growth decelerates. Public health officials use logistic models to forecast hospital capacity needs, plan resource allocation, and evaluate the impact of interventions like social distancing. The COVID-19 pandemic saw extensive use of logistic and modified logistic models.
References
Reviewed by Manoj Kumar, Mathematics Educator · Editorial policy