Blood makes up about 7% to 8% of your total body weight. For an average adult, that works out to roughly 4.5 to 5.5 liters (about 1.2 to 1.5 gallons) circulating through your body at any given moment. The exact amount depends on your size, sex, age, and body composition.
How Blood Volume Differs by Sex and Size
Men and women carry different amounts of blood relative to their weight. Adult men average about 75 milliliters of blood per kilogram of body weight, while adult women average about 65 mL/kg. For a 180-pound man, that comes out to roughly 6.1 liters. For a 140-pound woman, it’s closer to 4.1 liters.
The difference comes down largely to body composition. Blood volume tracks closely with lean body mass, not total weight. Each kilogram of lean mass corresponds to roughly 84 mL of additional blood volume. Since men typically carry more muscle and less fat, they have proportionally more blood. Fat tissue is much less metabolically active than muscle and doesn’t require the same blood supply, so a person with a higher body fat percentage will have a lower blood-to-weight ratio than someone of the same weight with more muscle.
This is also why using a flat percentage like “10% of body weight” (a figure you’ll sometimes see) can overestimate blood volume in people with more body fat and underestimate it in very lean or muscular individuals.
Blood Volume in Children and Infants
Babies and young children actually have a higher blood-to-weight ratio than adults. According to data from Boston Children’s Hospital, newborns at 24 hours old carry about 83 mL of blood per kilogram. Infants at three months peak at around 87 mL/kg. By age 10, the ratio drops to about 75 mL/kg, and by age 15, it settles near adult levels at roughly 71 mL/kg.
In practical terms, though, a small child’s total blood volume is still very small. A 10-kilogram toddler has only about 800 mL of blood total, which is why even minor blood loss during injuries or surgery can be proportionally significant in young children.
What’s Actually in Those 5 Liters
A little more than half your blood volume is plasma, a yellowish liquid made mostly of water, proteins, and dissolved salts. The rest is cellular: red blood cells, white blood cells, and platelets. The ratio of cells to total blood volume is measured as hematocrit, and it differs between sexes. Men typically have a hematocrit of 42% to 52%, meaning cells make up roughly half their blood. Women run lower, at 37% to 47%. These ranges shift with age. Newborns start high, around 44% to 64%, and values may dip slightly in older adults.
When Blood Volume Changes
Your blood volume isn’t fixed. Several normal conditions can shift it substantially.
Pregnancy causes the most dramatic change. Plasma volume starts rising as early as 6 to 8 weeks and climbs progressively until 28 to 30 weeks. The total increase varies widely but averages about 45% above pre-pregnancy levels, with some women seeing increases as high as 100%. This extra volume supports the placenta and growing fetus, and it also provides a buffer against blood loss during delivery.
Endurance training also expands blood volume. After several months of consistent aerobic exercise, total blood volume increases roughly 10% to 20% above baseline. Elite endurance athletes carry 20% to 25% more blood than untrained individuals, regardless of age or sex. The extra volume improves the heart’s ability to pump efficiently and helps regulate body temperature during prolonged effort.
Dehydration works in the opposite direction, reducing plasma volume and concentrating your blood. Severe dehydration can noticeably decrease your total circulating volume, which is part of why it causes dizziness, rapid heart rate, and fatigue.
How Much Blood You Can Safely Lose
Understanding your total blood volume puts blood loss into perspective. Clinicians classify hemorrhage into four stages based on the percentage of blood volume lost:
- Under 15% (roughly 750 mL in an average adult): Your body compensates well. Heart rate may barely change, and most people feel fine. This is in the range of a standard blood donation, which takes about 470 mL.
- 15% to 30%: You’ll notice a faster heart rate, anxiety, and possibly some lightheadedness as the body works harder to maintain blood pressure.
- 30% to 40%: This is a critical threshold. Blood pressure drops, heart rate spikes, and confusion or altered consciousness can set in. Medical intervention becomes urgent.
- Over 40%: Life-threatening. Without rapid treatment, this level of blood loss can be fatal.
For someone with about 5 liters of blood, that 30% tipping point is around 1.5 liters, roughly three times what you’d give during a blood donation. The body has a remarkable ability to compensate for moderate losses by constricting blood vessels and increasing heart rate, but those mechanisms have limits.