The average adult has about 10 units of blood in their body. One unit equals roughly 450 mL (just under a pint), so total blood volume for a typical adult weighing 150 to 180 pounds comes out to around 4.5 to 5.5 liters, or 1.2 to 1.5 gallons.
That number isn’t fixed, though. Your blood volume depends on your size, sex, age, and whether you’re pregnant. Here’s how it breaks down.
How Blood Volume Varies by Size and Sex
The most reliable way to estimate blood volume is by body weight. Adult men carry about 75 mL of blood per kilogram of body weight, while adult women carry about 65 mL per kilogram. In practical terms, a 200-pound man who stands about 6 feet tall has roughly 5.7 liters (around 12 pints, or 12 units) of blood. A 165-pound woman at about 5 feet 5 inches has roughly 4.3 liters (about 9 pints, or 9 to 10 units).
The difference between men and women comes down to body composition. Men typically have more lean muscle mass, which requires a richer blood supply. Women generally carry more fat tissue, which is less metabolically active and needs less blood flow. So even at the same weight, a man will usually have a higher blood volume than a woman.
Blood Volume in Babies and Children
Babies have far less total blood, but proportionally more of it relative to their body weight. A premature newborn carries 90 to 100 mL per kilogram, and a full-term newborn has 80 to 90 mL per kilogram. That means a 7-pound newborn has only about 270 to 290 mL of blood total, less than a single donation unit.
As children grow, their ratio of blood to body weight gradually decreases. Infants settle to about 70 to 80 mL per kilogram, older children to 70 to 75 mL per kilogram, and adults level off at 65 to 75 mL per kilogram. This higher proportion in newborns reflects their rapid growth and the demands of developing organs.
How Pregnancy Changes the Numbers
Pregnancy triggers one of the most dramatic shifts in blood volume outside of illness. Blood volume starts rising as early as 6 to 8 weeks of gestation and increases progressively until about 28 to 30 weeks. By that point, most pregnant women have about 45% more blood than before pregnancy, though the increase can range anywhere from 20% to 100%.
For a woman who started with roughly 4.3 liters, a 45% increase adds nearly 2 extra liters, bringing the total close to 6.2 liters. This expansion supports the placenta, cushions against blood loss during delivery, and meets the oxygen demands of a growing fetus.
What Your Blood Is Made Of
Blood is roughly 55% plasma and 45% cells. Plasma is the liquid portion: mostly water, plus proteins, salts, and nutrients that keep your tissues functioning. The cellular portion is dominated by red blood cells, which carry oxygen and account for 40% to 45% of total blood volume. That percentage is called hematocrit, and it’s one of the most common measures in a routine blood test. White blood cells and platelets make up a much smaller slice.
How Much Blood You Can Safely Lose
When you donate blood, you give one unit, about 450 mL, which is roughly 8% to 10% of your total supply. Your body handles this without trouble. Losing up to 15% of your blood volume (Class I hemorrhage) typically causes minimal symptoms, maybe a slightly faster heartbeat.
Beyond that, the picture changes quickly. Losing 15% to 30% (Class II) causes noticeable anxiety, faster breathing, and a drop in blood pressure when standing. At 30% to 40% (Class III), confusion and a significant blood pressure drop set in. Loss greater than 40% (Class IV) is immediately life-threatening. For a person with 5 liters of blood, that 40% threshold is about 2 liters, or roughly 4 units.
Hospitals activate emergency protocols when estimated blood loss exceeds 1 liter, which is just over 2 units.
How Quickly Your Body Replaces Blood
After donating a single unit, your body begins replenishing almost immediately, but different components come back on different timelines. Plasma volume recovers within 24 to 48 hours, which is why staying hydrated after a donation matters. Your body produces about 2 million new red blood cells every second under normal conditions, but building back the full supply of red cells takes longer. Hemoglobin levels, the measure of oxygen-carrying capacity, typically return to normal within 6 to 12 weeks.
This is why blood donation centers space whole blood donations at least 8 weeks apart. Your plasma bounces back fast, but your red blood cells need time to catch up.