The average adult has about 4.7 to 5.5 quarts (1.2 to 1.5 gallons) of blood circulating through their body at any given moment. That works out to roughly 4.5 to 5.5 liters. The exact amount depends primarily on your body size and sex, with smaller but meaningful differences based on age, fitness level, and whether you’re pregnant.
How Blood Volume Varies by Size and Sex
Blood volume scales with body weight, not height. The standard clinical estimate is 75 milliliters per kilogram of body weight for adult men and 65 milliliters per kilogram for adult women. So a 180-pound man carries roughly 6.1 liters (about 6.5 quarts) of blood, while a 140-pound woman carries about 4.1 liters (roughly 4.3 quarts). The difference between sexes comes down to body composition: muscle tissue holds more blood vessels per pound than fat tissue, and men on average carry more muscle mass.
A quick rule of thumb: blood makes up about 7 to 8 percent of your total body weight. For a 150-pound person, that’s roughly 10.5 to 12 pounds of blood, which translates to about 5 quarts.
What That Blood Is Made Of
Not all of those quarts are the same substance. About 54% of blood by volume is plasma, a pale yellow liquid that’s mostly water plus dissolved proteins, salts, and hormones. Red blood cells make up about 45%, and the remaining fraction (less than 1%) is white blood cells and platelets. In practical terms, the average person carries 2.7 to 3.0 liters of plasma and about 2 liters of packed red blood cells.
The ratio of red blood cells to total blood volume is what doctors call hematocrit. When you get blood work done and see a hematocrit number, it’s telling you what percentage of your blood is red cells versus liquid. A typical healthy range is 36% to 44% for women and 40% to 54% for men.
Blood Volume in Children and Newborns
Children carry less total blood than adults, but proportionally more relative to their weight. A full-term newborn has about 80 to 85 milliliters of blood per kilogram, which means a 7.5-pound baby has roughly 0.3 quarts (about 10 ounces) of total blood in their body. Premature infants carry even more per kilogram, around 90 to 100 milliliters per kilogram, because their bodies retain a higher proportion of fluid.
As children grow, the ratio gradually drops. Infants have about 75 to 80 milliliters per kilogram, and by the time a child reaches school age, the ratio settles to about 70 to 75 milliliters per kilogram, closer to adult proportions. A 60-pound child has roughly 2 quarts of blood.
How Pregnancy Changes Blood Volume
Pregnancy causes one of the most dramatic natural shifts in blood volume. Starting around 10 to 12 weeks, a pregnant person’s blood volume begins increasing significantly and nearly doubles by the third trimester. Most of that increase is plasma, which is why pregnant women often have slightly lower hematocrit levels even when they’re perfectly healthy. The extra blood supports the placenta, cushions against blood loss during delivery, and keeps the growing fetus supplied with oxygen and nutrients.
How Much Blood You Can Lose
Understanding your total blood volume puts blood loss in perspective. Donating blood removes about 1 pint (half a quart), which is roughly 10% of the average adult’s supply. Your body replaces the plasma within 24 to 48 hours, though rebuilding red blood cells takes four to six weeks.
Losing more than that gets progressively serious. The stages break down like this:
- Up to 15% lost (about 750 mL, or 0.8 quarts): Blood pressure and heart rate often remain normal. You may not feel much beyond mild lightheadedness.
- 15% to 30% lost (750 to 1,500 mL): Heart rate climbs and breathing speeds up as your body compensates for the reduced volume.
- 30% to 40% lost (1,500 to 2,000 mL): Blood pressure drops noticeably. Organs start getting less oxygen, and urine output drops sharply.
- More than 40% lost (over 2,000 mL, or about 2 quarts): This is life-threatening. Blood pressure is dangerously low, the heart races, and organs begin to fail without emergency intervention.
For the average adult with 5 quarts of blood, losing 2 quarts puts you in the most dangerous category. This is why severe trauma and surgical blood loss are treated so aggressively.
What Affects Your Blood Volume Day to Day
Your blood volume isn’t perfectly constant. Dehydration can reduce plasma volume by several hundred milliliters, which is one reason you feel dizzy and your heart races when you’re severely dehydrated. Drinking fluids restores it relatively quickly. Altitude also triggers changes: living at high elevation for several weeks causes your body to produce more red blood cells, gradually increasing total blood volume. Endurance athletes experience a similar adaptation, sometimes carrying 20% to 30% more blood than sedentary people of the same size.
Conditions like heart failure and kidney disease can cause the body to retain too much fluid, inflating blood volume beyond what the heart can efficiently pump. On the other end, chronic bleeding from an ulcer or heavy menstrual periods can slowly reduce blood volume over weeks, sometimes without obvious symptoms until anemia sets in.