The average adult carries about 1.2 to 1.5 gallons of blood, or roughly 4.5 to 5.7 liters. That works out to about 10% of your body weight. A 150-pound person has less blood than a 200-pound person, so the total scales with your size.
Blood Volume by Age and Size
Adults are often estimated at around 70 milliliters of blood per kilogram of body weight. But the ratio is actually higher in younger, smaller bodies. Premature newborns carry about 90 to 100 ml per kilogram, while full-term newborns have roughly 80 to 90 ml per kilogram. By the end of the first month of life, blood volume briefly peaks at about 105 ml per kilogram before gradually declining. Infants settle around 70 to 80 ml per kilogram, and older children carry about 70 to 75 ml per kilogram, approaching adult proportions.
To put this in real terms: a 7-pound newborn has less than a cup of blood in their entire body. A 70-pound child has roughly 2.5 liters. These differences matter in medical settings, where even small amounts of blood loss can be significant in children.
What Blood Is Made Of
Blood is roughly 55% plasma and 45% cells. Plasma is the liquid portion: mostly water, plus proteins, salts, and nutrients that get carried to tissues throughout your body. The remaining 45% is dominated by red blood cells, which carry oxygen. White blood cells and platelets make up a tiny fraction of that cellular portion but play outsized roles in immune defense and clotting.
The percentage of your blood that consists of red blood cells is called your hematocrit. A hematocrit of 40% to 45% is typical for a healthy adult. This number fluctuates with hydration, altitude, fitness level, and certain medical conditions.
How Pregnancy Changes Blood Volume
During pregnancy, maternal plasma volume expands by about 45% on average to meet the increased circulatory demands of the growing uterus, placenta, and fetal blood supply. That’s a substantial increase, equivalent to adding roughly a liter or more of plasma. Individual variation is wide, though. Some women experience minimal expansion, while others nearly double their plasma volume. Red blood cell production also increases during pregnancy, but not as fast as plasma volume, which is why many pregnant women develop mild anemia even when perfectly healthy.
How Much Blood You Can Lose
Your body tolerates small blood losses without much trouble. Losing less than 15% of your total blood volume (under about 750 ml in an average adult) causes minimal symptoms. You might feel slightly anxious, and your heart rate stays under 100 beats per minute. This is roughly what happens during a blood donation, which collects about 470 ml.
Losing 15% to 30% starts producing noticeable effects: a faster heart rate between 100 and 120, quicker breathing, and a drop in pulse pressure even though overall blood pressure may still read normal. At 30% to 40% loss, the body can no longer compensate effectively. Heart rate climbs above 120, blood pressure drops measurably, and confusion sets in. Beyond 40%, which represents roughly 2 liters in an average adult, the situation becomes immediately life-threatening, with severely low blood pressure, a heart rate above 140, and potential loss of consciousness.
What Happens After a Blood Donation
A standard whole blood donation collects about 450 to 475 ml, roughly one pint. That’s close to 10% of the total blood volume in a typical adult, which is why donation centers screen for minimum weight requirements.
Your body replaces the different components of blood on very different timelines. Plasma volume recovers quickly once you rehydrate, which is why donation centers encourage drinking fluids immediately afterward. White blood cells and platelets also bounce back within days. Red blood cells take considerably longer. Your bone marrow ramps up production, but it typically takes 6 to 12 weeks for hemoglobin levels to fully return to pre-donation levels. This is why most blood services require a minimum of 8 to 12 weeks between whole blood donations.
If you’re curious whether you personally fall on the higher or lower end of blood volume, the simplest estimate is to multiply your weight in kilograms by 70 ml. A 70 kg (154-pound) adult, for example, carries roughly 4.9 liters. Men tend to have slightly higher blood volumes than women of the same weight, partly because of differences in body composition and red blood cell counts.