How Much Blood is in the Human Body?

You may already know that blood is a key body fluid that delivers oxygen and nutrients. But have you ever wondered how much blood is in your body—and how much of it do you really need? The total amount of blood in the human body, known as blood volume, is largely dependent on one’s size. In adults, there is an average of 65 to 70 milliliters of blood for every kilogram of body weight. That makes blood about 7 percent of a person’s total weight, meaning the average person has about 4.5 to 5.5 liters of blood in their body. However, this estimate differs from person to person, as the total amount of blood in one’s body depends on several other factors. Blood volume tends to decrease as people age, and males have a slightly higher blood volume than females. Two commonly cited equations may be used to calculate one’s blood volume. Nadler’s equation combines one’s height, weight, and sex to estimate blood volume, while the Lemmens-Bernstein-Brodsky equation uses BMI to provide more accurate estimates for patients of higher body weight. The large amount of blood in one’s body minimizes the impact of minor blood loss. The human body experiences minimal effects from losses up to around 15 percent of total blood volume, which means most adults can donate a pint of blood, or about 500 milliliters, with ease. Such a loss is regenerated rapidly: it takes only a couple days to replace the lost plasma, and the red blood cells are regenerated within weeks. However, major drops in blood volume are significant, and losses over 40 percent are typically life-threatening.