The average adult has about 4.5 to 5.5 liters of blood, or roughly 1.2 to 1.5 gallons. The exact amount depends primarily on your body size and sex, since blood volume scales with weight rather than being a fixed number for everyone.
Blood Volume by Body Size and Sex
A practical way to estimate your own blood volume is by body weight. Adult men carry approximately 75 milliliters of blood per kilogram of body weight, while adult women carry about 65 milliliters per kilogram. That means a 180-pound (82 kg) man has around 6.1 liters of blood, while a 140-pound (64 kg) woman has closer to 4.2 liters. The American Red Cross puts it simply: blood makes up roughly 10% of an adult’s weight.
The difference between men and women isn’t just about average body size. Men typically have a higher proportion of red blood cells relative to plasma, which accounts for some of the gap even when body weight is similar.
How Much Blood Children Have
Children actually have more blood per kilogram than adults. A child’s blood volume runs about 75 to 80 milliliters per kilogram, compared to the 65 to 70 mL/kg range in adults. Newborns have an even higher ratio. In absolute terms, though, a child’s total blood volume is much smaller because they weigh so much less. A 30-pound toddler, for instance, has only about 1 liter of blood total, which is why even small amounts of blood loss can be serious in young children.
What Makes Up That Volume
Roughly 55% of your blood is plasma, the pale yellow liquid that carries everything else. Plasma is mostly water, along with dissolved proteins, salts, sugars, and hormones. The remaining 45% is almost entirely red blood cells, the oxygen-carrying cells that give blood its color. White blood cells and platelets, despite their critical roles in fighting infection and stopping bleeding, together account for less than 1% of total blood volume.
This ratio between red blood cells and plasma isn’t fixed. It shifts with hydration, altitude, illness, and other factors. When you’re dehydrated, plasma volume drops while your red blood cell count stays the same, making your blood thicker and more concentrated.
When Blood Volume Changes
Your blood volume isn’t a constant. Several normal and abnormal situations can shift it significantly.
Pregnancy causes the most dramatic natural increase. Starting around 6 to 8 weeks of gestation, a pregnant person’s blood volume begins climbing steadily, peaking between weeks 28 and 34 at roughly 50% above pre-pregnancy levels. That’s an extra 1.5 liters of blood. This expansion supports the growing placenta and protects against blood loss during delivery.
High altitude triggers the opposite response. When you travel to elevation, your body initially loses plasma volume as it adjusts to lower oxygen levels. Over several weeks, red blood cell production ramps up to compensate, but the plasma loss typically outpaces the new red cells. The net result is that total blood volume actually decreases at altitude, at least during the first weeks of acclimatization, even though your blood becomes more concentrated with oxygen-carrying cells.
Exercise, particularly endurance training over weeks and months, expands blood volume as the body adapts to higher cardiovascular demands. Severe dehydration, burns, and certain kidney or heart conditions can reduce it.
How Much You Can Safely Lose
Your body tolerates modest blood loss remarkably well. Losing up to 15% of your blood volume (roughly 750 mL in an average adult) produces minimal symptoms. This is why blood donation is safe: a standard donation draws 350 to 450 mL, which is well under that 15% threshold. Most people’s bodies replace the plasma portion within 24 to 48 hours, though rebuilding the full red blood cell count takes several weeks.
Beyond that safe margin, the picture changes quickly. Losing 15 to 30% of blood volume (750 to 1,500 mL) causes a faster heart rate, anxiety, and pale skin as the body scrambles to maintain blood pressure. At 30 to 40% loss (1,500 to 2,000 mL), confusion sets in, blood pressure drops significantly, and the situation becomes life-threatening without intervention. Losing more than 40%, or roughly 2 liters, is classified as the most severe category of hemorrhage and can be fatal without immediate treatment. These thresholds are based on a 70 kg adult, so they scale up or down with body size.
Why Blood Volume Matters
Blood volume isn’t just a trivia question. It directly determines how efficiently your heart can deliver oxygen to tissues. Too little blood volume, whether from dehydration, blood loss, or illness, forces the heart to beat faster to maintain the same delivery. Too much, as can happen with certain heart or kidney conditions, raises blood pressure and strains the cardiovascular system. The body continuously fine-tunes blood volume through thirst signals, kidney function, and hormone regulation, keeping it within a narrow range that supports steady circulation without overloading the system.