Blood makes up about 7 to 8 percent of your total body weight. For an average adult weighing around 70 kg (154 pounds), that translates to roughly 5.5 liters, or just under 1.5 gallons. The exact percentage varies depending on your sex, age, and body composition.
How the Numbers Break Down by Sex
Men and women carry slightly different amounts of blood relative to their size. Adult men average about 75 milliliters of blood per kilogram of body weight, while adult women average about 65 milliliters per kilogram. That means a 180-pound man has roughly 6.1 liters of blood, while a 140-pound woman has closer to 4.1 liters.
The difference comes down mainly to body composition. Men typically carry more lean muscle tissue, which is heavily supplied with blood vessels. Fat tissue, by contrast, is less vascular. Someone with a higher body fat percentage will generally have a lower blood-to-weight ratio than someone of the same weight with more muscle. This is why using a flat “8 percent” estimate can be slightly off for people at either end of the body composition spectrum.
Babies Have a Higher Blood-to-Weight Ratio
Newborns and infants carry proportionally more blood than adults. A full-term newborn has about 80 to 90 milliliters of blood per kilogram, and premature babies can have as much as 90 to 100 milliliters per kilogram. That’s roughly 10 percent of body weight, compared to the adult average of 7 to 8 percent.
As children grow, the ratio gradually decreases. By the time a child weighs around 10 kg (22 pounds), the ratio settles to about 80 milliliters per kilogram, and it continues dropping until it reaches adult levels in the mid-60s to mid-70s range. This higher proportion in early life is one reason even small amounts of blood loss during pediatric surgery require careful monitoring.
What Your Blood Is Made Of
That 7 to 8 percent of your body weight isn’t a uniform fluid. By volume, plasma makes up about 54 percent of your blood. It’s mostly water (92 percent) with dissolved proteins, salts, and hormones. Red blood cells account for roughly 45 percent, and white blood cells and platelets fill in the remaining fraction of a percent.
The ratio of red blood cells to total blood volume is what doctors call hematocrit. A typical hematocrit of 45 percent means that nearly half your blood volume consists of oxygen-carrying red blood cells. The total plasma volume in an average person runs between 2.7 and 3.0 liters, while the rest is cellular material packed tightly enough to give blood its density of about 1,060 kg per cubic meter, slightly heavier than water.
How Pregnancy Changes the Equation
During pregnancy, blood volume increases by 30 to 50 percent. By the third trimester, a pregnant woman has about 1.5 liters more blood circulating than she did before conception. This increase means blood temporarily accounts for a larger share of total body weight, pushing the ratio above the usual 7 to 8 percent even before accounting for pregnancy-related weight gain.
Most of this extra volume is plasma rather than red blood cells, which is why pregnant women often show lower hematocrit readings on blood tests. It’s not that they have fewer red blood cells; the cells are simply diluted in a larger volume of fluid. The body builds this extra supply to support the placenta, cushion against blood loss during delivery, and meet the increased metabolic demands of growing a fetus.
How Much Blood You Can Lose
Understanding your total blood volume puts blood loss in perspective. Trauma guidelines divide hemorrhage into four classes based on what percentage of total blood volume is lost:
- Less than 15 percent: Your body compensates well. Heart rate barely changes, and most people feel fine. This is comparable to donating blood, which removes about 10 percent of your volume.
- 15 to 30 percent: Heart rate rises, blood pressure may start to drop, and you’ll feel anxious or restless. For a 70 kg person, this is roughly 0.8 to 1.6 liters.
- 30 to 40 percent: Blood pressure drops significantly, mental status deteriorates, and the body is struggling to deliver enough oxygen to organs.
- More than 40 percent: Life-threatening. Without immediate intervention, this level of loss is often fatal.
For a person with 5.5 liters of blood, crossing into the dangerous 30 percent threshold means losing about 1.65 liters. That context helps explain why major surgery and severe trauma require such careful blood management, and why your body invests so heavily in clotting mechanisms to prevent runaway bleeding from even minor injuries.
Putting the Numbers in Perspective
At 7 to 8 percent of body weight, blood is one of the lighter organ systems relative to its outsized importance. Your skeleton accounts for about 15 percent of body weight, muscle around 40 percent, and skin roughly 16 percent. Blood ranks closer to the weight of your brain (about 2 percent) in terms of its share, yet it touches every tissue in the body, cycling through the full circuit roughly once per minute at rest.
If you’re trying to estimate your own blood volume quickly, multiply your weight in kilograms by 70. That gives a reasonable middle-ground estimate in milliliters for most adults. A 75 kg person, for instance, would have approximately 5,250 milliliters, or about 5.25 liters. For a more precise figure, use 75 mL/kg for men and 65 mL/kg for women.