The average adult has about 4.5 to 5.7 liters of blood, assuming a body weight between 65 and 80 kilograms (roughly 143 to 176 pounds). A quick way to estimate your own blood volume: multiply your weight in kilograms by 70 milliliters. For a 70 kg person, that works out to about 4.9 liters, or just over 1.3 gallons.
How Body Size and Sex Affect Blood Volume
Blood volume scales with body size, not just height or weight alone. The standard estimate of 70 milliliters per kilogram works well for adults at a healthy weight, but it becomes less accurate at the extremes. In people with obesity, blood volume per kilogram decreases in a nonlinear way, meaning you can’t simply multiply a higher weight by the same 70 ml figure and get a reliable number.
Sex also plays a role, largely because of differences in body composition. Men tend to carry more muscle mass and less body fat, and muscle tissue is more densely supplied with blood vessels. This is reflected in hematocrit, the percentage of blood made up of red blood cells. In healthy men, hematocrit typically ranges from 40% to 54%, while in women it falls between 36% and 48%. The remaining volume is mostly plasma, the pale yellow liquid that carries nutrients, hormones, and waste products throughout the body.
Blood Volume in Babies and Children
Babies have far less total blood than adults, but proportionally more relative to their size. A child’s blood volume runs about 75 to 80 milliliters per kilogram. Newborns start even higher, at around 85 ml/kg, and this rises to a peak of roughly 105 ml/kg by the end of the first month of life before gradually dropping over the following months. For perspective, a 3.5 kg newborn has only about 300 ml of blood, less than a can of soda. This is one reason even small amounts of blood loss can be serious in infants.
When Blood Volume Changes Naturally
Your blood volume isn’t fixed. Several normal physiological situations can shift it significantly.
Pregnancy causes the most dramatic change. A pregnant person’s blood volume increases by about 45%, adding roughly 1,200 to 1,600 milliliters above non-pregnant levels. Most of this increase comes from plasma expanding by 50 to 60%, with the bulk of it happening by around 34 weeks of gestation. This extra volume supports the placenta and growing fetus, and it also provides a buffer against blood loss during delivery.
Altitude is another factor, though it works differently than you might expect. When someone who lives at sea level travels to high altitude, plasma volume actually decreases in the short term. This concentrates the existing red blood cells, raising the blood’s oxygen-carrying capacity as a quick adaptation to thinner air. Over weeks and months, the body produces new red blood cells, but the maximum rate of that expansion is only about 50 ml of red cell volume per week.
What Your Blood Is Made Of
When a blood sample is spun in a centrifuge, it separates into three distinct layers. The bottom layer, making up the largest portion, consists of red blood cells. A thin middle layer contains white blood cells and platelets. The top layer is plasma, which accounts for roughly half to just over half of total blood volume depending on your hematocrit. Plasma is mostly water, with dissolved proteins, electrolytes, and other molecules that keep your tissues functioning.
How Much Blood You Can Safely Lose
Knowing your total blood volume puts blood loss into context. A standard blood donation removes about 450 ml, which is roughly 8 to 10% of an average adult’s supply. Your body replaces the plasma portion within 24 hours, which is why you’re encouraged to drink fluids after donating. The red blood cells take longer: bone marrow restores them over three to four weeks, while the iron used to build those cells takes six to eight weeks to fully replenish.
In medical emergencies, blood loss is classified into four stages. Losing less than 15% of your blood volume (under about 750 ml for most adults) is Class I hemorrhage, which the body handles with minimal symptoms. Losing 15 to 30% is Class II, where you’ll notice a faster heart rate and may feel anxious. Class III, at 30 to 40% loss, brings confusion, a significant drop in blood pressure, and rapid breathing. Losing more than 40%, over 2 liters for an average adult, is Class IV hemorrhage, which is immediately life-threatening without intervention.
Estimating Your Own Blood Volume
The simplest method is the 70 ml/kg rule. Weigh yourself in kilograms (divide your weight in pounds by 2.2), then multiply by 70. A 60 kg person carries roughly 4.2 liters. A 90 kg person carries about 6.3 liters, though this estimate becomes less precise at higher body weights because fat tissue requires less blood supply than muscle.
For most everyday purposes, remembering that you carry between 4.5 and 5.5 liters is close enough. That’s roughly the volume of one and a quarter gallon jugs of milk, continuously circulating through about 60,000 miles of blood vessels.