Percentage of Girls marrying below 18 years of Age (X₄): Early marriage of girls, particularly below 18 years may result in more births, high infant mortality rate and pregnancy complications. Districts with higher proportion of girls marrying below 18 years of age are socially worse of than districts with low proportion of girls marrying below 18 years of age. In India the proportion of girls marrying below 18 years is quite high. Data from RCH survey shows that 36.8 percent of the girls in India married below 18 years of age. In the states like Bihar, Madhya Pradesh and Rajasthan this proportion was quite high, 50 to 60 percent of the girls in these states got married before 18 years of age. In Tripura, Uttar Pradesh and West Bengal 45-50 percent of the girls got married below 18 years of age. Chandigarh, Goa, Himachal Pradesh, Jammu & Kashmir and Pondicherry recorded the lowest proportion of girls marrying below 18 years of age. As low as 5 per cent of the girls in these states married below 18 years of age

Lalitpur district in Uttar Pradesh recorded the highest percentage (83.80 per cent) of girls marrying below 18 years of age closely followed by the Sahajpur district in Madhya Pradesh (83.70 per cent), Mahrajganj in Uttar Pradesh (82.20 per cent), Bundi in Rajasthan (80.6), Panschim Champaran in Bihar (79.4). None of the girls from four districts in Kerala (Alappuzha, Eranakulam Kottayam, and Pathanamthitta), Kanyakummari district in Tamil Nadu, Chandigarh, South Garo Hills district in Meghalaya and Lahaul district in Himachal Pradesh got married below 18 years of age. 

The index of marriage is computed by fixing an upper limit of 83.80 and lower limit of zero observed in the data. The index is computed as 

Index₄ = 100*(83.8 -X₄)/(83.8-0) 

            = 100*(83.8 - X₄)/83.8 

Where X₄ is the percentage of girls married below 18 years of age in a particular district.