The average adult carries about 4.5 to 5.5 liters of blood, which works out to roughly 1.2 to 1.5 gallons. That number isn’t fixed, though. Your blood volume depends on your body size, sex, age, and even where you live.
Blood Volume by Body Size and Sex
Blood volume scales with body weight, but not equally for everyone. Adult men carry about 75 milliliters of blood per kilogram of body weight, while adult women carry about 65 milliliters per kilogram. So a 180-pound (82 kg) man has roughly 6.1 liters of blood, while a 140-pound (64 kg) woman has about 4.2 liters. The difference between sexes comes largely from body composition: muscle tissue is more heavily supplied with blood vessels than fat tissue, and men on average carry more muscle mass.
Children and newborns actually have more blood relative to their size. Infants carry about 80 milliliters per kilogram, and newborns about 85 milliliters per kilogram. Premature babies have the highest ratio at roughly 95 milliliters per kilogram. Despite that higher concentration, a newborn’s total blood volume is still tiny, only about one cup. By contrast, a full-grown adult weighing 150 to 180 pounds has around 10 standard units of blood (about 5 liters total).
What Blood Is Made Of
About 55% of your blood is plasma, a pale yellow fluid that’s mostly water with dissolved proteins, salts, and hormones. Plasma is the transport medium: it carries nutrients to your cells and waste products away. Red blood cells make up about 44% of your blood volume and are responsible for carrying oxygen from your lungs to every tissue in your body. The remaining 1% consists of white blood cells and platelets, which handle immune defense and clotting.
The ratio of red blood cells to total blood volume is called hematocrit, and it’s one of the most common values checked in a blood test. A higher hematocrit means more oxygen-carrying capacity. This ratio naturally varies between individuals and can shift based on hydration, altitude exposure, and health conditions.
What Changes Your Blood Volume
Pregnancy is one of the most dramatic natural shifts in blood volume. A pregnant person’s blood volume increases by almost 50%, peaking in the third trimester. This expansion supports the growing placenta and fetus, and it also provides a buffer against blood loss during delivery. The extra volume is mostly plasma, which is why pregnant women often have slightly lower red blood cell concentrations even though their total red cell count goes up.
Living at high altitude also reshapes your blood. When the air contains less oxygen, your kidneys produce more of a hormone that accelerates red blood cell production. This process kicks in at elevations above roughly 1,600 meters (about 5,200 feet). Over weeks and months of living at altitude, your total red blood cell mass increases, which raises overall blood volume. Athletes training at altitude see their oxygen-carrying red cell mass climb by about 3.4% after just two weeks above 2,100 meters.
Dehydration works in the opposite direction. Since plasma is mostly water, losing fluid through sweat, illness, or inadequate intake shrinks your plasma volume and concentrates your blood. Severe dehydration can meaningfully reduce total blood volume and strain the cardiovascular system.
How Much Blood You Can Lose
Your body tolerates small blood losses without much trouble. Losing up to 15% of your blood volume (about 750 milliliters for an average adult) is classified as a Stage 1 loss. At this level, your blood pressure and heart rate typically stay normal, and your body compensates on its own. This is why blood donation is safe: a standard donation draws 350 to 450 milliliters, roughly 8% to 12% of your total volume.
Things get more serious at Stage 2, which is a loss of 15% to 30% (750 to 1,500 milliliters). At this point, your heart rate rises and your body starts redirecting blood flow to protect vital organs. Losses beyond 30% become life-threatening and require emergency intervention. The body can regenerate lost plasma volume within a day or two through fluid intake, but replacing the red blood cells takes weeks.
Blood Donation and Replacement
When you donate blood, the 450 milliliters drawn represents a small enough fraction that most people feel fine afterward. Your body replaces the plasma portion within about 24 to 48 hours, which is why you’re encouraged to drink extra fluids after donating. The red blood cells take longer to replenish, typically four to six weeks, which is why donation centers require at least an eight-week gap between whole blood donations.
Your blood volume at any given moment is a balance of production and loss. Bone marrow continuously manufactures new red blood cells at a rate of roughly 2 million per second, while old cells are broken down by the spleen and liver after about 120 days of circulation. This constant turnover keeps your blood volume remarkably stable under normal conditions.