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.

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 Iclass IIclass IIIclass IV
blood loss (mL)up to 750750 to 1,0001,500 to 2,000greater than 2,000
blood loss (% of blood volume)up to 1515 to 3030 to 40greater than 40
pulse rate (per minute)less than 100100 to 120120 to 140greater than 140
blood pressurenormal or increaseddecreaseddecreaseddecreased
respiratory rate (per minute)14 to 2020 to 3030 to 40greater than 35
urine output (mL per hour)greater than 3020 to 305 to 15negligible
mental statusslightly anxiousmildly anxiousanxious, confusedconfused, 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? »

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 »