From 1501 until 1866, it is estimated that the transatlantic slave trade saw more than 12.5 million African people forcefully put on slave ships and transported to the Americas. Of these 12.5 million, only 10.7 million disembarked on the other side of the Atlantic, meaning that approximately 1.8 million (14.5 percent) did not survive the journey, known as the Middle Passage. Throughout most of the the sixteenth century, the mortality rate was around thirty percent, it then fell below twenty percent in the late seventeenth century, and below fifteen percent in the late eighteenth century. There was a slight increase in the mid-1800s, before the transatlantic slave trade effectively ended in the 1860s. The overall average mortality rate is lower than the rate in most decades, due to the larger numbers of captives transported in the late 1700s; a significant number of these voyages were between Africa and Brazil, which was generally the shortest of the major routes.

https://www.statista.com/statistics/1143458/annual-share-slaves-deaths-during-middle-passage/