And every year, 10 million children become brides, notes the study, which urges the British government to do more to help end child marriage.
Pushing children to get married breaks UN conventions and it is illegal in most countries. Many, especially poorer countries turn a blind eye.
But child marriage is child abuse says Marie Staunton from Plan UK, the development charity behind the report.
"Marriage often spells the end of a girl's education,” she said. “She's also less likely to send her own children to school perpetuating the poverty cycle. Why is the international community so silent when so many girls are forced to marry when they are still children?"
Sub-Saharan Africa has the second highest rate of early and forced marriage with 39 per cent. In total, 14.3 million girls are married in the region before they hit 18, the report says. The rate of early marriage differs across the continent with west and central Africa at 43 per cent and eastern and Southern Africa at 36 per cent. In countries where the rate of early and forced marriage is more than 70 per cent – Niger, Chad, and Mali – there’s also a high rate of mums dying in childbirth.
In countries where the legal age of marriage is different for boys and girls, the age for women is always lower. In Benin, Cameroon, Gabon, Mali, Niger, and the Democratic Republic of Congo, the legal age of marriage is 18 for boys and only 15 for girls.
Researchers interviewed girls as young as 10 who had been married off, often to much older men. "I was forced to leave school in order to get married,” said Madina, now 14, from Sudan. “I was divorced after eight months. I wish other girls don't suffer like me.''
Child marriage has nasty consequences for both the child and their future children. It keeps them trapped in a cycle of poverty, creates health problems and raises the risk of death – a girl of 15 is five times more likely to die in childbirth than a woman in her 20s.
The charity is asking the government to put together a more joined up approach to the practice and to use its influence as a major funder of basic education to help tackle child marriage.