The average adult has about 4.5 to 5.5 liters of blood, roughly 10% of their body weight. The exact amount varies based on your size, sex, age, and even your fitness level.
Blood Volume by Body Size and Sex
Blood volume scales with body weight. Adult males carry approximately 75 milliliters of blood per kilogram of body weight, while adult females carry about 65 milliliters per kilogram. That means a 70 kg (154 lb) man has roughly 5.25 liters, while a 60 kg (132 lb) woman has about 3.9 liters. A larger person of either sex will naturally carry more blood.
The American Red Cross estimates that a 150 to 180 pound adult has approximately 1.2 to 1.5 gallons of blood, which converts to about 4.5 to 5.7 liters. As a quick rule of thumb, blood makes up about one-tenth of your total body weight.
Children and Infants Have More Per Pound
Babies and young children carry proportionally more blood relative to their body weight than adults do. Premature infants have roughly 100 milliliters per kilogram, while full-term infants and older children have about 80 milliliters per kilogram (compared to 65 to 75 in adults). A 3.5 kg newborn, for example, has only about 280 milliliters of blood total, less than a can of soda. This is why even small amounts of blood loss can be serious in infants and small children.
What Your Blood Is Made Of
More than half of your blood volume is plasma, a yellowish fluid that carries nutrients, hormones, and waste products through your body. Red blood cells make up about 40% of total volume. The remaining fraction includes white blood cells and platelets. The ratio of red blood cells to total blood volume is called hematocrit, and it’s one of the most common numbers checked in a routine blood test.
When Blood Volume Changes Naturally
Your blood volume isn’t fixed. It shifts in response to pregnancy, physical training, and altitude.
During pregnancy, blood volume begins increasing within the first few weeks and rises progressively until delivery. The total increase varies from 20% to 100% above pre-pregnancy levels, though most women see an expansion close to 45%. For a woman who started with 4 liters, that means carrying nearly 6 liters by late pregnancy. This expansion supports the growing placenta and fetus while also preparing the body for blood loss during delivery.
Endurance athletes also carry significantly more blood than sedentary people. Plasma volume expands quickly, within hours to days of starting a training program, while red blood cell production ramps up more slowly over weeks to months. After several months of consistent training, total blood volume can expand 10% to 20% above baseline. Highly trained endurance athletes carry 20% to 25% more blood than untrained individuals, regardless of age or sex. This extra volume helps the heart pump more blood per beat, which is a major reason trained athletes have better aerobic performance.
Living at high altitude triggers a similar adaptation. With less oxygen available in the air, the body compensates by producing more red blood cells over several months, expanding total blood volume and improving the blood’s ability to carry oxygen.
How Your Body Regulates Blood Volume
Your body actively monitors and adjusts blood volume through a system involving your kidneys, liver, lungs, and several glands. When blood pressure drops, the kidneys release an enzyme that sets off a chain reaction: the liver and lungs help produce a hormone that signals the adrenal glands to retain sodium. Since water follows sodium, your body holds onto more fluid, which increases blood volume and brings pressure back up. This same system also fine-tunes your potassium levels and triggers another hormone that reduces urine output when you’re dehydrated.
This is why dehydration can temporarily reduce blood volume and why drinking fluids helps restore it. It’s also the biological mechanism targeted by several common blood pressure medications.
How Much Blood You Can Safely Lose
Understanding total blood volume puts blood loss in perspective. Trauma guidelines divide hemorrhage into four classes based on the percentage of blood lost:
- Under 15% (less than 750 mL): Typically well-tolerated. This is roughly what happens during a standard blood donation, which collects about one pint (around 470 mL). Most healthy adults recover this volume within a day or two.
- 15% to 30% (750 to 1,500 mL): Heart rate increases and the body starts compensating. You may feel anxious and thirsty.
- 30% to 40% (1,500 to 2,000 mL): Blood pressure drops noticeably. Confusion and rapid breathing set in. This level of blood loss typically requires a transfusion.
- Over 40% (more than 2,000 mL): Life-threatening. Organs begin to fail without immediate intervention.
For context, a person with 5 liters of blood reaches the dangerous 30% threshold after losing just 1.5 liters. That’s less than a large water bottle.