Throughout the Atlantic slave trade, 14.5 percent of captured slaves who embarked on ships in African ports died en route to their destination. Voyages from Africa to the Spanish Americas had the highest mortality rates, where almost one fifth of all captives died during the journey. Voyages between Africa and Brazil had the lowest mortality rates, where approximately one in eight did not survive. There is a clear correlation between voyage lengths and Middle Passage death rates; journeys between Africa and Brazil took an average of 45 days, whereas journeys to the Spanish Mainland Americas took 78 days on average. In addition to voyage lengths, factors such as overcrowding (which increased the rate of disease transmission), violence, insurrection, and high suicide rates also contributed to the high mortality rates along the Middle Passage.

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.

The middle passage was the second leg of the triangular trade network that existed between Europe, Africa and the Americas between the 16th and 19th centuries. Generally speaking, the middle passage was the process of transporting African captives across the Atlantic, where they would begin their new life as a slave in the Americas. Throughout transatlantic slavery, the middle passage had a mortality rate of approximately 15 percent (up to 30 percent in early years), due to a combination of factors such as violence, disease and malnourishment. Each of these factors facilitated one another, and depleted provisions during longer journeys exacerbated this further. Geographical influences In general, voyages to the Spanish mainland American colonies took the longest time, due to a combination of distance and time period (these colonies were the most common destination for slave voyages in early years). For the same reasons, voyages to Brazil were generally the shortest, as these journeys had the shortest distances and were most common in the latter stages of transatlantic slavery when they benefitted from technological and navigational advancements.

Naturally, the region of origin also played just as important a role as destination in determining voyage length. For example, journeys from southeast Africa (present-day Mozambique) and the Indian Ocean islands had the longest voyage times, followed by journeys from around the Gulf of Guinea to North America; journeys from Sierra Leone and West Central Africa (particularly to Brazil) took the shortest time due to their proximity to South America. The correlation between journey length and mortality was also evident in voyages from Southeast Africa to Mainland North America, which had a mortality rate of over 50 percent, while all voyages to Brazil had a mortality rate of 8.6 percent.

Between 1501 and 1866, it is estimated that over 12.5 million people were forced onto ships in Africa, and transported to the Americas as slaves. Furthermore, it is estimated that only 10.7 million of these slaves disembarked on the other side of the Atlantic, meaning that roughly 1.8 million did not survive the journey. The transatlantic slave trade was a part of the triangular trade route between Europe, Africa and the Americas, during the sixteenth and nineteenth centuries. Generally speaking, this route saw European merchants bring manufactured products to Africa to trade for slaves, then transport the slaves to the Americas to harvest raw materials, before taking these materials back to Europe where they would then be consumed or used in manufacturing. Slavery was an integral part in funding the expansion of Europe's colonial empires, which shaped the modern and highly globalized world in which we live today.

The Middle Passage As with trade, the slave journey was also broken into three parts; the First Passage was the stage where slaves were captured and transported to African ports, the Middle Passage was the journey across the Atlantic, while the Final Passage was where the slaves were transported to their place of work. The death toll in the First Passage is thought to be the highest of the three stages, as millions were killed or fatally wounded as they were captured, however a lack of written data and historical evidence has made this number difficult to estimate. In contrast, shipping records from the time give a much more accurate picture of the Middle Passage's death toll, and this data suggest that roughly 14.5 percent of slaves did not survive the journey. The reason for this was the harsh and cramped conditions on board; slave ships were designed in such a way that they could fit the maximum number of slaves on board in order to maximize profits. These conditions then facilitated the spread of diseases, such as smallpox and dysentery, while malnutrition and thirst created further problems. Generally, slavers aimed to keep slaves as healthy (therefore; profitable) as possible, although there are countless examples of mistreatment and punishment of slaves by their captors, and several cases where slaves were exterminated by the crew as provisions ran low.

Rise and fall of the transatlantic slave trade

The European arrival in the Americas also saw the introduction of virgin soil epidemics (new diseases being introduced to biologically defenseless populations) which decimated the indigenous populations. The abundance of natural resources, but lack of available labor led to the rise of the transatlantic slave trade. Until the mid-1600s, Portuguese traders had a near-monopoly on this trade, supplying slaves to the newly expanding Spanish and Portuguese empires in South America. As other European powers began to expand their empires in the Caribbean and North America, the slave trade grew dramatically, and during the eighteenth century, the number of slaves being brought to the New World increased from an annual average of thirty thousand in the 1690s to 87 thousand in the 1790s. The transatlantic slave trade reached its peak between the 1750 and 1850, and an average of 74 thousand slaves were brought to the Americas each year between these dates. The largest decline came as the slave trade was disrupted during the American War of Independence (1775-1783), although the trade became weakened as the abolitionist movement gained momentum in Europe and the Americas around the turn of the century. The most significant impacts came as the slave trade was abolished in Britain and the U.S. in 1807 and Brazil in 1831, and Britain then used its position as the global superpower to impose abolition on other nations and used the Royal Navy to enforce these measures. While most nations abolished the slave trade in the early 1800s, it would take decades before the actual practice of slavery would be abolished; today, slavery is illegal in almost every country, however modern slavery in the forms of forced labor, human trafficking and sexual exploitation continues to be prevalent across the globe.