The average adult has about 4,500 to 5,500 ml of blood, or roughly 1.2 to 1.5 gallons. The exact amount depends on your body size, sex, and age. A simple way to estimate your own blood volume: multiply your weight in kilograms by 70 (for women) or 75 (for men).
Average Blood Volume by Sex and Size
Blood volume scales with body weight. Adult males carry approximately 75 ml of blood per kilogram of body weight, while adult females carry about 65 ml per kilogram. That difference is largely because men tend to have more lean muscle mass, which is more densely supplied with blood vessels than fat tissue.
To put that in practical terms: a 70 kg (154 lb) man has roughly 5,250 ml of blood. A 60 kg (132 lb) woman has about 3,900 ml. A larger person of either sex will have proportionally more. Someone who weighs 100 kg might carry over 7,000 ml.
For a more precise individual estimate, clinicians sometimes use the Nadler formula, which factors in both height and weight rather than weight alone. But for most purposes, the ml-per-kg shortcut gets you close enough.
Blood Volume in Babies and Children
Newborns and young children actually have more blood relative to their size than adults do, even though their total volume is much smaller. A premature baby carries about 90 to 100 ml per kilogram, and a full-term newborn has around 80 to 85 ml per kilogram. That means a 3.5 kg newborn has only about 280 to 300 ml of total blood, less than a standard can of soda.
By infancy, the ratio drops to about 70 to 80 ml per kilogram, and in older children it settles around 70 to 75 ml per kilogram before reaching adult values. These small total volumes are one reason blood loss in children is treated so urgently. What looks like a modest amount of bleeding to an adult can represent a significant percentage of a child’s entire supply.
What Your Blood Is Made Of
About 55% of blood volume is plasma, the pale yellow liquid that carries nutrients, hormones, and waste products. The remaining 45% is made up of blood cells. Red blood cells account for 40% to 45% of total volume on their own, which is the measurement known as hematocrit. White blood cells and platelets together make up roughly 1%.
This ratio matters because it shifts in various conditions. Dehydration concentrates your blood cells into less plasma, raising your hematocrit without actually adding red blood cells. Conversely, overhydration dilutes everything.
Where Blood Sits in Your Body
Your blood isn’t evenly distributed. At any given moment, about 18% of your total blood volume is pooled in the large veins, which act as a reservoir. The lungs hold around 10.5%, and the heart chambers contain about 9%. The rest circulates through arteries, smaller veins, and the capillary networks that feed your organs and tissues. Your body can redirect blood from these reservoirs when needed, like shunting more to your muscles during exercise or to your skin when you’re overheated.
How Pregnancy Changes Blood Volume
Pregnancy triggers one of the most dramatic natural shifts in blood volume. Plasma volume increases by an average of about 1,250 ml, nearly 50% above normal levels. Red blood cell volume rises too, but more modestly, by about 250 to 450 ml depending on iron intake. The net result: a pregnant woman near the end of her third trimester may be carrying 1,500 ml or more of extra blood.
Most of this expansion happens between the end of the first trimester and around weeks 34 to 36, when it plateaus. The increase is the body’s way of supporting the placenta, cushioning against blood loss during delivery, and meeting the oxygen demands of a growing fetus.
High Altitude and Other Adjustments
Living at high altitude reshapes your blood composition over time. Because there’s less oxygen in the air, your body produces more red blood cells to compensate. Research from the American Society of Hematology shows that high-altitude residents carry measurably higher red blood cell mass compared to people at sea level. At the same time, plasma volume tends to decrease. So the total volume may not change dramatically, but the balance between red cells and plasma shifts toward a thicker, more oxygen-carrying mix.
Other factors that influence blood volume include fitness level (endurance athletes tend to have higher plasma volume), obesity (more tissue requires more blood supply), and chronic conditions affecting the heart or kidneys.
How Much Blood You Can Safely Lose
Your body tolerates small blood losses without much trouble. The Advanced Trauma Life Support system classifies hemorrhage into four stages. Losing less than 15% of your blood volume (under about 750 ml in an average adult) is Class I, and your body compensates so effectively that heart rate and blood pressure barely change. This is roughly the range of a standard blood donation, which draws 350 to 450 ml.
Class II hemorrhage, 15% to 30% loss (roughly 750 to 1,500 ml), causes a faster heart rate and narrowing blood pressure. Class III, at 30% to 40%, brings confusion, significant drops in blood pressure, and rapid breathing. Class IV, above 40%, is immediately life-threatening. For an adult with 5,000 ml of blood, that threshold is around 2,000 ml.
After a standard blood donation, your body replaces the plasma within about 24 to 48 hours. The red blood cells take longer, typically four to six weeks to fully regenerate, which is why donation centers space visits at least eight weeks apart.