Early marriages are leading to high levels of school dropouts - with only four in 10 African girls finishing secondary school, says the World Bank.
Child marriage is costing African countries at least $60 billion in lost lifetime earnings, more than what the world gives the continent in aid each year, the World Bank has said.
Be it high school dropout rates, teen pregnancy or poor health outcomes, the cost of child marriage is far from just monetary, the Bank said in its report.
But the vast sums lost might just be the headline that helps provoke long-awaited change, activists said.
"When it comes to policy-making, money talks. What this research shows is that ending child marriage is not only the right thing to do, it is also the smart thing to do," said Lakshmi Sundaram, Executive Director of Girls Not Brides.
Sub-Saharan Africa has the highest child marriage rates in the world, with more than three million - or one in three - girls marrying before they turn 18.
The report said early marriages were leading to high levels of school dropouts - with only four in 10 African girls finishing secondary school, lowering their job horizons.
"On average, women who have a secondary education are more likely to work and they earn twice as much as those with no education," said Quentin Wodon, World Bank economist and lead author of the report.
"Estimates for 12 countries - which account for half of the African continent's population - suggest that through its impact on girls' education, child marriage is costing these countries $63 billion in lost earnings and human capital wealth."
Assuming similar losses continent-wide, the Bank estimated lost lifetime earnings could even be twice as large – and three times more than the $40 billion foreign aid given to sub-Saharan Africa in 2016.
Child marriage was also leading to high fertility rates and population growth, whilst an end to child marriage would boost standards of living, especially for the poorest, it said.
Eighteen of the world's top 20 countries for early marriage are in Africa. Niger, Central African Republic, Chad, Burkina Faso, Mali, South Sudan and Guinea top the list with more than half of girls married off before 18.
Gender experts say early marriage not only stifles a girl's progress in education, health and employment, but also hampers the development of her children - creating a vicious cycle of malnutrition, poor health and ignorance.
A child bride is more likely to drop out of school, endure serious complications during pregnancy and childbirth and is at high risk of domestic violence. Her children are also more likely to be underweight and lucky to survive beyond five.
Campaigners said the World Bank's findings demonstrated the need for African nations to act on commitments and increase investment to end child marriage.
"By ridding Africa of child marriage, we can help alleviate poverty and ensure that girls everywhere have access to a brighter future," said Sundaram of Girls Not Brides.