Understanding demographics requires analyzing the fundamental inputs of human population change. One of the most common questions students and analysts ask is how the rate of increase is determined. Specifically, when calculating global population growth the death rate is subtracted from the birth rate.
Unlike local population calculations, which must account for immigration and emigration, the global population is a closed system (assuming no interplanetary migration). Therefore, the formula relies entirely on the rate of natural increase (RNI).
The Formula for Natural Increase
To determine the percentage growth rate of the global population, demographers use the "Crude Birth Rate" (CBR) and the "Crude Death Rate" (CDR). Both figures are typically expressed as the number of events per 1,000 individuals in the population.
The calculation follows these steps:
Step 1: Determine the Crude Birth Rate (e.g., 18 births per 1,000).
Step 2: Determine the Crude Death Rate (e.g., 8 deaths per 1,000).
Step 3: Subtract the Death Rate from the Birth Rate to find the Natural Increase per 1,000 people.
Step 4: Divide the result by 10 to convert it into a percentage.
For example, if the birth rate is 20 and the death rate is 8, the calculation is: (20 – 8) = 12 per 1,000. Converting this to a percentage yields a 1.2% annual growth rate.
Why the Death Rate Matters
When calculating global population growth, the death rate acts as the primary counterbalance to fertility. Historically, population explosions occurred during the "demographic transition," a period where death rates plummeted due to medical advancements (vaccines, antibiotics, sanitation) while birth rates remained high.
Conversely, in some modern developed nations, the death rate is approaching or exceeding the birth rate, leading to zero or negative population growth. This calculator allows you to model these dynamics by inputting different scenarios for fertility and mortality over specific timeframes.
function calculatePopulationGrowth() {
// Get Input Values
var popCurrent = document.getElementById('popCurrent').value;
var birthRate = document.getElementById('birthRate').value;
var deathRate = document.getElementById('deathRate').value;
var timeFrame = document.getElementById('timeFrame').value;
// Validation
if (popCurrent === "" || birthRate === "" || deathRate === "" || timeFrame === "") {
alert("Please fill in all fields to calculate population growth.");
return;
}
// Parse numbers
var P = parseFloat(popCurrent);
var B = parseFloat(birthRate);
var D = parseFloat(deathRate);
var T = parseFloat(timeFrame);
if (isNaN(P) || isNaN(B) || isNaN(D) || isNaN(T)) {
alert("Please enter valid numbers.");
return;
}
if (P < 0 || B < 0 || D < 0 || T = 0 ? "+" : "";
document.getElementById('resIncrease').innerText = sign + formatNumber(numericChange);
document.getElementById('resTotalPop').innerText = formatNumber(futurePop);
// Show result box
document.getElementById('popResult').style.display = 'block';
}