The average adult has roughly 4.5 to 5.5 liters of blood, or about 1.2 to 1.5 gallons. That number isn’t fixed. It shifts based on your size, sex, age, and even whether you’re pregnant. Blood makes up about 7% to 8% of your total body weight, so a 70 kg (154 lb) man carries around 5.25 liters, while a 60 kg (132 lb) woman carries closer to 3.9 liters.
How Body Size and Sex Affect Blood Volume
The most reliable way to estimate blood volume is by body weight. Adult males average about 75 milliliters of blood per kilogram of body weight, while adult females average about 65 mL/kg. That 10 mL/kg gap exists largely because of differences in body composition. Fat tissue has fewer blood vessels running through it than muscle does, so a kilogram of fat requires less blood supply than a kilogram of lean tissue. Since women on average carry a higher percentage of body fat relative to lean mass, their blood volume per kilogram tends to be lower.
This also means that two people who weigh the same can have meaningfully different blood volumes depending on how muscular they are. In highly trained athletes of both sexes, blood volume normalized to lean body mass lands in a similar range of about 110 to 120 mL per kilogram of lean mass, only about 5% to 10% higher than in untrained people. The takeaway: muscle mass is a stronger predictor of blood volume than total body weight alone.
Blood Volume in Babies and Children
Infants and children carry more blood relative to their size than adults do, but far less in absolute terms. Premature newborns have the highest ratio at roughly 90 to 100 mL per kilogram. Full-term newborns come in around 80 to 85 mL/kg, peaking near 105 mL/kg by the end of the first month before gradually declining. By the time a child reaches school age, the ratio drops to about 70 to 80 mL/kg, approaching adult levels.
To put this in practical terms, a 3.5 kg (about 7.7 lb) newborn has only about 280 to 300 mL of blood in their entire body, less than the volume of a standard can of soda. That’s why even small amounts of blood loss during birth or surgery can be significant for a newborn.
How Pregnancy Changes Blood Volume
Pregnancy triggers one of the most dramatic natural increases in blood volume. Starting as early as 6 to 8 weeks of gestation, plasma volume begins to rise and continues climbing until about 28 to 30 weeks. The total increase typically lands around 45% above pre-pregnancy levels, though it can range anywhere from 20% to 100% depending on the individual. For a woman who started with about 4 liters, that means carrying close to 5.5 to 6 liters of blood by the third trimester.
This expansion serves two purposes: it supplies the placenta and growing fetus with adequate blood flow, and it provides a buffer against the blood loss that naturally occurs during delivery.
What Your Blood Is Made Of
About 55% of your blood volume is plasma, a pale yellow liquid made mostly of water along with dissolved proteins, salts, and hormones. The remaining 45% is cellular: red blood cells, white blood cells, and platelets. Red blood cells alone account for 40% to 45% of total blood volume. That proportion, known as hematocrit, is one of the most common values measured in a routine blood test.
The balance between plasma and cells isn’t just a medical curiosity. It directly affects how easily blood flows through your vessels. A higher percentage of red blood cells makes blood thicker and more viscous, which can strain the heart. Populations that have adapted to high altitudes illustrate this tradeoff well. Andean natives living at elevation have a noticeably higher packed cell volume than people at sea level, while Sherpa populations achieve similar oxygen-carrying capacity through a different strategy: they produce more red blood cells but also expand their plasma volume, keeping blood viscosity lower and protecting circulation.
How Much Blood You Can Lose
Your body is remarkably tolerant of small losses. A standard blood donation removes about 450 to 500 mL, roughly 10% of an average adult’s supply. Most people handle this without any trouble. Your body replaces the liquid plasma portion within about 24 to 48 hours, though rebuilding the red blood cells takes several weeks.
Losing 15% to 30% of blood volume produces a faster heart rate and mild anxiety but is usually manageable without transfusion. Once losses cross 30% to 40%, the body struggles to maintain blood pressure, and confusion, rapid breathing, and cold skin set in. Losses beyond 40%, roughly 2 liters in an average adult, are immediately life-threatening without emergency intervention. This is why trauma care focuses so heavily on controlling bleeding early.
Quick Estimates by Body Weight
- 150 lb (68 kg) woman: approximately 4.4 liters (1.2 gallons)
- 150 lb (68 kg) man: approximately 5.1 liters (1.3 gallons)
- 200 lb (91 kg) man: approximately 6.8 liters (1.8 gallons)
- 8 lb (3.6 kg) newborn: approximately 290 mL (about 10 oz)
- 40 lb (18 kg) child: approximately 1.4 liters (0.37 gallons)
These are rough estimates using the standard 65 mL/kg for women, 75 mL/kg for men, 80 mL/kg for newborns, and 75 mL/kg for children. Your actual blood volume will vary with fitness level, body composition, and health conditions, but for most people these numbers land in the right ballpark.