The average adult human body contains about 5 liters (1.3 gallons) of blood, making up roughly 7 to 8 percent of total body weight. That number shifts depending on your size, sex, age, and even where you live or how active you are.
Average Blood Volume in Adults
Adult men carry about 5.5 liters of blood on average, while adult women carry about 4.5 liters. The difference comes down mostly to body size and composition, since blood volume scales with lean body mass. A larger person simply has more tissue to supply with oxygen and nutrients, which requires a bigger blood supply.
A quick way to estimate your own blood volume: it runs about 70 milliliters per kilogram of body weight. So a 180-pound (82 kg) man would have roughly 5.7 liters, while a 140-pound (64 kg) woman would have about 4.5 liters. These are estimates, not exact figures, but they’re the same calculations used in clinical settings.
What Blood Is Made Of
Blood is not a uniform fluid. Roughly half of it is plasma, a pale yellow liquid made mostly of water that carries proteins, hormones, nutrients, and waste products. The other portion is made up of cells: red blood cells, white blood cells, and platelets. The share of blood occupied by red blood cells, called hematocrit, ranges from 35 to 54 percent in healthy adults and tends to run higher in men than in women.
Red blood cells are by far the most abundant, numbering around 25 trillion in the body at any given time. They give blood its color and do the heavy lifting of oxygen transport. White blood cells and platelets make up a much smaller fraction but handle immune defense and clotting.
Blood Volume in Babies and Children
Babies have far less total blood than adults, but proportionally more relative to their body weight. A full-term newborn carries about 80 to 85 milliliters of blood per kilogram, compared to roughly 70 ml/kg in adults. Premature infants have even more, around 90 to 100 ml/kg, because a higher proportion of their body weight is water and blood.
By the end of the first month of life, a baby’s blood volume per kilogram actually peaks at around 105 ml/kg before gradually declining. As children grow, their ratio settles to about 70 to 80 ml/kg, approaching adult proportions by the time they reach school age. In absolute terms, though, a 10 kg (22-pound) infant has only about 800 ml of blood total, which is less than a fifth of an adult’s supply. This is one reason blood loss that would be minor in an adult can be serious in a small child.
How Pregnancy Changes Blood Volume
Pregnancy triggers one of the most dramatic shifts in blood volume the body naturally undergoes. Blood volume begins increasing as early as six to eight weeks of gestation and rises progressively until about 28 to 30 weeks. The total increase typically lands around 45 percent above pre-pregnancy levels, though it can range anywhere from 20 to 100 percent depending on the individual.
For a woman who started with 4.5 liters, a 45 percent increase means she’s carrying closer to 6.5 liters by the third trimester. Most of that extra volume is plasma rather than red blood cells, which is why pregnant women often have lower hematocrit readings even though they’re not actually anemic. The expansion serves a clear purpose: it supports the placenta, cushions against blood loss during delivery, and meets the metabolic demands of a growing fetus.
Athletes and High-Altitude Populations
Your blood volume isn’t fixed for life. It responds to physical demands. High-intensity endurance athletes carry blood volumes 20 to 25 percent larger than untrained individuals, regardless of age or sex. That extra volume, mostly plasma, improves the heart’s pumping efficiency and helps regulate body temperature during prolonged exercise. It’s one of the earliest adaptations to consistent aerobic training, beginning within days of starting a program.
People who live at high altitudes also show distinctive blood profiles. Andean highlanders produce more red blood cells to compensate for lower oxygen levels, resulting in a higher concentration of hemoglobin. Sherpas in the Himalayas take a different approach: they also produce more red blood cells than lowlanders, but they expand their plasma volume significantly as well. The result, according to research published in PNAS, is that both populations end up with comparable total blood volumes but achieve oxygen delivery through different biological strategies. This is a striking example of how the same environmental pressure can produce different physiological solutions.
How Much Blood You Can Lose
Losing a small amount of blood is something the body handles easily. A standard blood donation removes about one pint (roughly 470 ml), which is less than 10 percent of your total supply. Your body replaces the plasma portion within about 24 to 48 hours, though rebuilding red blood cells takes several weeks.
Beyond that comfortable range, blood loss becomes increasingly dangerous. Hemorrhage is classified into four stages based on the percentage of total blood volume lost:
- Class 1 (up to 15%): Usually produces minimal symptoms. Your heart rate may rise slightly, but most people wouldn’t notice.
- Class 2 (15 to 30%): You’d feel anxious, thirsty, and notice a faster heartbeat. For someone with 5 liters of blood, this means losing 750 ml to 1.5 liters.
- Class 3 (30 to 40%): Blood pressure drops noticeably. Confusion sets in, skin becomes pale and cool, and the body is under serious stress.
- Class 4 (over 40%): Life-threatening. Losing more than 2 liters puts the body into hemorrhagic shock, where organs begin to fail from lack of oxygen.
Dehydration Shrinks Your Blood Volume
You don’t have to bleed to lose blood volume. Dehydration pulls water out of your plasma, effectively concentrating your blood. Research from the Journal of Applied Physiology found that when men exercised hard enough to lose 4 percent of their body weight through sweat, their blood volume dropped by nearly 10 percent. For a 5-liter blood supply, that’s about half a liter gone, enough to strain the cardiovascular system and explain why dehydration causes dizziness, rapid heartbeat, and fatigue.
This is also why rehydrating after intense exercise or illness isn’t just about quenching thirst. You’re restoring the fluid your blood needs to circulate efficiently and deliver oxygen to your tissues.