The amount of blood in the human body is generally equivalent to 7 percent of body weight. The average amount of blood in your body is an estimate because it can depend on how much you weigh, your sex, and even where you live.
- Babies: Babies born full-term have about 75 milliliters (mL) of blood per kilogram of their body weight. If a baby weighs about 8 pounds, they’ll have about 270 mL of blood in their body, or 0.07 gallons.
- Children: The average 80-pound child will have about 2,650 mL of blood in their body, or 0.7 gallons.
- Adults: The average adult weighing 150 to 180 pounds should have about 1.2 to 1.5 gallons of blood in their body. This is about 4,500 to 5,700 mL.
- Pregnant women: To support their growing babies, pregnant women usually have anywhere from 30 to 50 percent more blood volume than women who are not pregnant. This is about 0.3 to 0.4 additional gallons of blood.
Sometimes the amount of blood in the human body can differ based on where you live. For example, people who live at high altitudes have more blood because there isn’t as much oxygen at higher altitudes.
How much blood can you lose?
If you lose too much blood, your brain doesn’t get enough oxygen to support life. People who experience major injury and trauma, such as a car accident, may lose blood very quickly. Losing an excessive amount of blood is known as hemorrhagic shock. Doctors categorize hemorrhagic shock into four classes based on how much blood is lost. In class IV, the amount of blood loss can be fatal.
Here are the classes of hemorrhagic shock:
|class I||class II||class III||class IV|
|blood loss (mL)||up to 750||750 to 1,000||1,500 to 2,000||greater than 2,000|
|blood loss (% of blood volume)||up to 15||15 to 30||30 to 40||greater than 40|
|pulse rate (per minute)||less than 100||100 to 120||120 to 140||greater than 140|
|blood pressure||normal or increased||decreased||decreased||decreased|
|respiratory rate (per minute)||14 to 20||20 to 30||30 to 40||greater than 35|
|urine output (mL per hour)||greater than 30||20 to 30||5 to 15||negligible|
|mental status||slightly anxious||mildly anxious||anxious, confused||confused, lethargic|
Your blood pressure and heart rate will stay close to normal as you lose up to 30 percent of your blood, or up to 1,500 mL of blood (0.4 gallons). After losing this amount of blood, you will experience the following:
- You’ll start to have a rapid heart rate higher than 120 beats per minute.
- Your blood pressure will drop.
- Your breathing rate will go up.
If you lose more than 40 percent of your blood, you will die. This is about 2,000 mL, or 0.53 gallons of blood in the average adult.
It’s important to get to a hospital to start receiving blood transfusions to prevent this.
Why and how would your doctor measure
your blood volume?
Your doctor won’t usually directly measure the amount of blood that you have because they can estimate it based on other factors and tests. For example, a blood test known as a hemoglobin and hematocrit test can estimate how much blood is in your body compared with the amount of fluid in your body. Then, your doctor can consider your weight and how hydrated you likely are. All of these factors can indirectly measure how much blood volume you have.
If you experience a major trauma that causes blood loss, doctors will usually use your weight as the starting point to guess how much blood you have. They will then use factors like your heart rate, blood pressure, and breathing rate to estimate how much blood may have been lost. They’ll also try to keep track of any additional blood loss so they can quickly replace it with a blood transfusion.