The average adult human body contains about 4,500 to 5,500 cc (cubic centimeters) of blood. Since one cc equals one milliliter, this is the same as 4.5 to 5.5 liters. The exact amount depends primarily on your body weight, sex, and age.
How Blood Volume Is Calculated
The standard medical formula multiplies your body weight in kilograms by a fixed number. For adult men, that number is 75 cc per kilogram. For adult women, it’s 65 cc per kilogram. So a 180-pound (82 kg) man carries roughly 6,150 cc of blood, while a 140-pound (64 kg) woman has closer to 4,160 cc.
A simpler rule of thumb puts blood at about 7% of total body weight. For a 70 kg person, that works out to roughly 4,900 cc. This quick estimate is useful but less precise than the weight-based formulas, especially for people who are significantly overweight or underweight. Newer research has found that using ideal body weight rather than actual body weight, with an adjustment for age (dropping from 70 cc/kg to 60 cc/kg for adults 65 and older), produces more accurate results in clinical settings.
Blood Volume in Children and Infants
Children carry proportionally more blood per kilogram than adults. Newborns have roughly 80 to 100 cc of blood per kilogram of body weight, meaning a 3.5 kg newborn has only about 280 to 350 cc total. That small volume is why even routine blood draws in neonatal care are carefully limited. Hospital guidelines typically cap a single blood draw at 2 to 4 cc per kilogram for pediatric patients to stay well within safe margins.
As children grow, their blood volume per kilogram gradually decreases toward adult levels. By the time a child reaches school age, the ratio is closer to 70 to 75 cc per kilogram, essentially the same as an adult.
What Makes Up Those CCs
Not all of that volume is the same substance. About 54% is plasma, the straw-colored liquid that carries nutrients, hormones, and waste products. Red blood cells make up roughly 45%, and white blood cells and platelets account for less than 1%. The proportion of red blood cells, called hematocrit, normally ranges from 35 to 54% in healthy adults, running higher in men than in women. This is the main reason men tend to have a higher total blood volume per kilogram.
When Blood Volume Changes
Your blood volume isn’t fixed. It shifts in response to several normal physiological conditions.
Pregnancy causes one of the most dramatic changes. A pregnant woman’s blood volume increases by about 45%, adding 1,200 to 1,600 cc above her baseline. Most of this expansion is plasma, which rises by 30 to 50%, with the majority of the increase happening by week 34. This extra volume supports blood flow to the placenta and helps buffer the blood loss that occurs during delivery.
Endurance training also expands blood volume significantly. Runners, cyclists, and other endurance athletes develop a measurable increase in total blood volume compared to sedentary individuals. In the first two to four weeks of training, the expansion comes almost entirely from extra plasma. After that, red blood cell volume catches up. This larger blood supply gives trained athletes better heat dissipation, greater cardiac output per heartbeat, and lower resting heart rates.
On the other end, dehydration, significant bleeding, and certain chronic illnesses all reduce circulating blood volume, sometimes quickly enough to become dangerous.
How Much Blood You Can Safely Lose
Trauma medicine classifies blood loss into four stages. Losing up to 15% of your blood volume (roughly 750 cc in an average adult) is Class I hemorrhage. Your body compensates well at this level, and you may not even notice symptoms beyond mild anxiety. A standard blood donation, which takes 350 to 450 cc (about 8 to 12% of your total volume), falls comfortably in this range.
Class II hemorrhage, 15 to 30% loss, triggers a faster heart rate and pale, cool skin as your blood vessels constrict to maintain pressure. At Class III, 30 to 40% loss, blood pressure drops rapidly and shock sets in. Blood transfusion becomes necessary. Class IV hemorrhage, above 40%, is immediately life-threatening and can cause loss of consciousness.
For context, 40% of 5,000 cc is 2,000 cc, roughly four times what’s collected during a blood donation. The body replaces donated plasma within about 24 to 48 hours, though rebuilding the full red blood cell count takes several weeks.