Once Africans boarded the slave ships for the journey to the Americas, an estimated 10 to 20 percent did not survive the six to eight week voyage known as the Middle Passage

. More detailed historical estimates suggest that about 1.5 million out of approximately 12 million Africans died during the voyage, which translates to roughly a 12.5 percent mortality rate on board the ships

. Mortality rates were higher in the earlier decades of the trade, averaging above 20 percent, but declined to about 10 percent by 1800

. The high death toll was caused by brutal conditions including overcrowding, disease (such as dysentery, scurvy, smallpox, and measles), starvation, and psychological trauma

. The conditions below deck were squalid, with many enslaved people shackled in close quarters, leading to rapid spread of illness and death

. The death rate increased with the length of the voyage due to worsening conditions and diminishing supplies

. In summary, between about 10 and 20 percent of enslaved Africans died during the Middle Passage voyage to the Americas, with an average estimate near 12.5 percent mortality during the crossing itself