Calculate the NRR based on age-specific fertility and mortality rates.
This represents the ratio of female births to total births.
Age Group
Interval (Years)
ASFR (Births per 1,000 women)
Survival Probability (lx/l0)
15-19
20-24
25-29
30-34
35-39
40-44
45-49
Net Reproductive Rate
0.00
(Gross Reproductive Rate: 0.00)
What is Net Reproductive Rate (NRR)?
The Net Reproductive Rate (NRR) is a key demographic metric used to measure population growth potential. It is defined as the average number of daughters that would be born to a female (or a group of females) if she passed through her lifetime conforming to the age-specific fertility and mortality rates of a given year.
Unlike the Gross Reproductive Rate (GRR), which assumes all women survive to the end of their reproductive years, the NRR accounts for mortality. It recognizes that some women will die before reaching reproductive age or during their reproductive years, reducing the potential for population replacement.
How to Interpret NRR Results
The NRR provides a clear indication of whether a population is replacing itself:
NRR = 1 (Replacement Level): Each woman is replaced by exactly one surviving daughter on average. The population will remain stable in the long run.
NRR < 1 (Below Replacement): The population is not producing enough daughters to replace the mothers. If this persists, the population will eventually decline.
NRR > 1 (Above Replacement): The population is growing, as each generation of mothers produces more than enough daughters to replace themselves.
Understanding the Inputs
To calculate NRR accurately, three specific data points are required for each age group (cohort):
1. Age-Specific Fertility Rate (ASFR)
This is the number of births occurring to women in a specific age group. In this calculator, we enter the rate as births per 1,000 women, which is the standard format in demographic reports. The calculator automatically converts this to a per-woman basis.
2. Survival Probability (lx)
This represents the probability of a female surviving from birth to the midpoint of the specific age group. It is derived from life tables. For example, a value of 0.98 means there is a 98% chance a female newborn survives to reach that age group.
3. Sex Ratio at Birth
Since NRR is concerned with the replacement of mothers by daughters, we must account for the fact that not all births are female. The standard biological ratio is approximately 105 boys for every 100 girls, resulting in a proportion of female births of roughly 0.488.
The NRR Formula
The mathematical formula used in this calculation is:
NRR = Σ (ASFRx × 5 × Pf × Sx)
Where:
ASFRx: Fertility rate per woman in age group x.
5: The width of the age interval (5 years).
Pf: Proportion of births that are female.
Sx: Probability of survival to age x.
function calculateNRR() {
// 1. Get Global Variables
var sexRatioInput = document.getElementById('sexRatio');
var sexRatio = parseFloat(sexRatioInput.value);
// Validation for Sex Ratio
if (isNaN(sexRatio) || sexRatio 1) {
alert("Please enter a valid Proportion of Female Births (between 0 and 1).");
return;
}
var totalSumNRR = 0;
var totalSumGRR = 0;
var isValid = true;
// 2. Iterate through the 7 standard age groups
for (var i = 1; i <= 7; i++) {
// Get elements by ID
var asfrElement = document.getElementById('asfr_' + i);
var survElement = document.getElementById('surv_' + i);
var intElement = document.getElementById('int_' + i);
// Parse values
// ASFR is usually per 1000, so we treat input as such unless user enters decimals 1) {
alert("Survival probability cannot be greater than 1 (Row " + i + ").");
isValid = false;
break;
}
// Calculation logic
// Convert ASFR per 1000 to ASFR per woman
var asfrPerWoman = asfrPer1000 / 1000;
// GRR Contribution (Fertility * Interval) * Sex Ratio happens later
// Just Fertility * Interval for now
totalSumGRR += (asfrPerWoman * interval);
// NRR Contribution (Fertility * Survival * Interval)
// Note: Standard formula often puts SexRatio outside summation, or inside.
// Result is same. Sum(ASFR * Surv * Interval) * SexRatio
totalSumNRR += (asfrPerWoman * survivalProb * interval);
}
if (!isValid) return;
// 3. Finalize Calculations
var finalNRR = totalSumNRR * sexRatio;
var finalGRR = totalSumGRR * sexRatio;
// 4. Display Results
var resultBox = document.getElementById('resultBox');
var nrrValue = document.getElementById('nrrValue');
var grrValue = document.getElementById('grrValue');
var interpretationText = document.getElementById('interpretationText');
resultBox.style.display = "block";
nrrValue.innerHTML = finalNRR.toFixed(3);
grrValue.innerHTML = "(Gross Reproductive Rate: " + finalGRR.toFixed(3) + ")";
// 5. Interpretation
var html = "";
if (finalNRR < 0.9) {
html = "Declining PopulationThe NRR is significantly below 1.0. This indicates that the population is not replacing itself and will decline in the long term unless fertility or survival rates improve.";
} else if (finalNRR >= 0.9 && finalNRR < 1.0) {
html = "Slight DeclineThe NRR is just below replacement level (1.0). The population is contracting slowly.";
} else if (finalNRR >= 1.0 && finalNRR < 1.1) {
html = "Replacement LevelThe NRR is roughly 1.0. The population is stable, with each generation replacing itself.";
} else {
html = "Growing PopulationThe NRR is above 1.0. The population is reproducing at a rate greater than replacement, leading to population growth.";
}
interpretationText.innerHTML = html;
}