The average adult has about 4.5 to 5.5 liters of blood, though the exact amount depends on body size and sex. A quick way to estimate your own blood volume: multiply your weight in kilograms by 70 (for women) or 75 (for men) to get milliliters, then divide by 1,000 for liters. A 70 kg (154 lb) man carries roughly 5.25 liters, while a 60 kg (132 lb) woman has closer to 3.9 liters.
How Body Size and Sex Affect Blood Volume
Blood volume scales with body weight, not height alone. The standard clinical estimate is 75 mL per kilogram for adult men and 65 mL per kilogram for adult women. The difference 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 relative to their weight.
This means two people of the same height can have meaningfully different blood volumes. A 90 kg man has roughly 6.75 liters circulating through his body, while a 50 kg woman has about 3.25 liters. That nearly two-fold difference matters in medicine, which is why surgeons and anesthesiologists calculate estimated blood volume before procedures rather than relying on a single “average” number.
Blood Volume in Babies and Children
Newborns actually have more blood relative to their size than adults do. Premature infants carry about 95 mL per kilogram of body weight, full-term newborns about 85 mL per kilogram, and older infants around 80 mL per kilogram. In absolute terms, though, a newborn’s total blood volume is tiny, often just 250 to 300 mL for a baby weighing about 3.5 kg.
Children gradually settle toward adult ratios as they grow, reaching approximately 75 to 80 mL per kilogram through childhood. By adolescence, the adult estimates of 65 to 75 mL per kilogram apply.
What Your Blood Is Made Of
More than half of your blood volume is plasma, the straw-colored liquid that carries nutrients, hormones, and waste products. Red blood cells make up about 40% of total volume. The remaining fraction includes white blood cells and platelets, which together account for less than 1% by volume but play outsized roles in immunity and clotting.
The ratio of red blood cells to total blood volume is called hematocrit. It runs higher in men (typically 40 to 54%) than in women (36 to 48%), which partly explains the sex-based difference in total blood volume. People living at high altitude develop higher hematocrit over time because their bodies produce more red blood cells to compensate for lower oxygen levels, expanding total blood volume compared to people living at sea level.
How Pregnancy Changes Blood Volume
Pregnancy triggers one of the most dramatic natural increases in blood volume. A woman carrying a single baby of average size adds roughly 1,250 mL of plasma, nearly a 50% jump over her non-pregnant volume. There’s little change during the first trimester, then a steady rise that peaks around weeks 34 to 36.
Red blood cell volume also increases, but by a smaller proportion, around 18% without iron supplements and closer to 30% with them. This uneven expansion is why pregnant women often have lower hematocrit readings even though they have more blood overall. Women carrying twins or triplets see even larger increases in plasma volume.
How Much Blood You Can Lose Safely
Your body tolerates small blood losses remarkably well. Donating blood removes about 470 mL, roughly 8 to 10% of an average adult’s supply, and most people feel fine within minutes. The body replaces the plasma portion within 24 to 48 hours, though red blood cells take several weeks to fully regenerate.
Problems begin as losses climb. Hemorrhagic shock is classified in four stages:
- Class 1 (up to 15% loss): About 750 mL in an average adult. Heart rate and blood pressure stay mostly normal.
- Class 2 (15 to 30% loss): Roughly 750 mL to 1.5 liters. Heart rate rises, skin may feel cool, and anxiety sets in.
- Class 3 (30 to 40% loss): Between 1.5 and 2 liters. Blood pressure drops significantly, confusion develops, and the situation becomes life-threatening.
- Class 4 (over 40% loss): More than 2 liters. Without immediate intervention, this stage is frequently fatal.
Notably, changes in heart rate or blood pressure don’t become obvious in healthy people until 15 to 20% of blood volume is lost, which is why significant internal bleeding can go unrecognized in its early stages.
Factors That Shift Your Blood Volume Day to Day
Your blood volume isn’t fixed. It fluctuates based on hydration, physical activity, altitude, and even body position. Dehydration reduces plasma volume, which concentrates red blood cells and thickens the blood. Endurance athletes, conversely, develop expanded blood volumes over time, sometimes 20 to 25% above average, as their bodies adapt to sustained cardiovascular demand.
Chronic exposure to high altitude also expands blood volume. Over several months at elevation, both red blood cell mass and plasma volume increase, improving the blood’s ability to carry oxygen in thinner air. This adaptation is one reason elite runners sometimes train at altitude before competing at sea level.
Even something as simple as standing up shifts blood distribution. Roughly 500 mL of blood pools in your legs when you move from lying down to standing, which is why some people feel lightheaded if they rise too quickly, especially when dehydrated.