The amount of blood in your body is generally equivalent to 7% of your body weight. But this is an estimate because it can depend on your sex, where you live, and other factors.

The estimates for blood volume are as follows:

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 people: To support their growing babies, pregnant people 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.

Learn more: How long does a blood transfusion last? »

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.

Read more: How to lower your heart rate »