The average adult human body contains about 4.5 to 5.5 liters of blood, roughly 1.2 to 1.5 gallons. Women typically carry around 4.5 liters, while men carry closer to 5.5 liters. The difference comes down to body size: blood volume scales with weight, running about 65 milliliters per kilogram in women and 75 milliliters per kilogram in men.
How Body Size Determines Blood Volume
Blood volume isn’t a fixed number for everyone. A quick estimate is to multiply your weight in kilograms by 70 (a rough average between the male and female figures). A 90-kilogram man would carry approximately 6.75 liters, while a 55-kilogram woman would have about 3.6 liters. Height matters too, since taller people have more tissue that needs blood supply.
Blood accounts for about 7 to 8 percent of total body weight. For a 150-pound person, that works out to roughly 10 to 12 pounds of blood circulating at any given moment.
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 full-term newborn carries about 80 to 85 milliliters per kilogram of body weight. Premature babies have even more, around 90 to 100 milliliters per kilogram, partly because they retain a higher proportion of fluid from the womb.
A typical 3.5-kilogram newborn (about 7.7 pounds) has only around 280 to 300 milliliters of blood total, barely more than a cup. As children grow, the ratio gradually decreases to adult levels. By school age, children carry roughly 70 to 80 milliliters per kilogram.
What Blood Is Made Of
About 55 percent of your blood volume is plasma, a pale yellow fluid that’s mostly water (92 percent). Plasma carries proteins like antibodies and clotting factors, along with hormones, vitamins, salts, and enzymes. The remaining 45 percent is cellular: red blood cells, white blood cells, and platelets. Red blood cells make up the vast majority of that cellular fraction, which is why blood appears red rather than the straw color of plasma alone.
The ratio of cells to plasma (called hematocrit) varies between individuals and shifts in response to hydration, altitude, and health conditions. People living at high altitude gradually produce more red blood cells to compensate for thinner air. This adaptation takes several weeks, during which the red cell mass increases while plasma volume adjusts proportionally.
When Blood Volume Changes Naturally
Pregnancy triggers one of the most dramatic shifts in blood volume. Total blood volume begins rising as early as six to eight weeks of gestation and climbs progressively until around 28 to 30 weeks. By that point, most pregnant women have about 45 percent more blood than before pregnancy, though the increase can range anywhere from 20 to 100 percent. This extra volume supports the placenta and the growing fetus, and it also provides a buffer against blood loss during delivery.
Blood donation offers another common example. A standard donation removes one pint (about 470 milliliters), which is roughly 8 to 10 percent of an average adult’s supply. Your body replaces the plasma portion within a day or two, mostly by pulling in water from surrounding tissues. The red blood cells take longer: about four to six weeks to fully regenerate, which is why donation centers require at least eight weeks between whole blood donations.
How Much Blood Loss Is Dangerous
Your body tolerates small losses well. Losing up to 15 percent of your blood volume (roughly 750 milliliters in an average adult) is classified as the mildest category of hemorrhage. At this level, your heart rate may barely change. You might feel slightly lightheaded but otherwise normal.
Problems escalate quickly beyond that. Losing 30 to 40 percent of blood volume causes a rapid heart rate, confusion, pale skin, and a dangerous drop in blood pressure. At around 50 percent loss, the body can no longer maintain blood flow to vital organs. Without intervention, losing half your blood volume is typically fatal. This is why trauma care focuses heavily on controlling bleeding early, since the difference between survivable and unsurvivable blood loss can be just a few minutes.
For context, the average adult has roughly 4.7 to 5.5 liters circulating. Fifty percent of that is about 2.5 liters, or a little over half a gallon. It sounds like a lot, but rapid injuries can cause losses at that scale in a surprisingly short time.