What percentage of body weight is blood?

Blood accounts for about 7–8% of an average adult’s body weight. This proportion provides a quick way to estimate total blood volume. For instance, if a person weighs 60 kg, they carry around 4.2–4.8 liters of blood. 

The percentage can vary slightly based on age, gender, and physical fitness. Men tend to have a slightly higher blood volume because they typically have greater muscle mass, while women may have slightly less due to higher fat content, which is less vascularized. Newborns, however, have proportionally more blood — around 10% of body weight, supporting rapid growth and metabolism.

Hydration levels also influence blood volume. Dehydration lowers plasma volume, making blood thicker, whereas overhydration dilutes it slightly. Medical procedures like transfusions or dialysis consider this percentage to maintain circulation stability.

Overall, the body maintains blood within tight limits using hormones like aldosterone and antidiuretic hormone (ADH) to regulate plasma levels. So, even though blood is only a small portion of body weight, it’s a vital percentage that keeps all tissues alive and functioning efficiently.