How Much Blood Can You Lose Before It’s Fatal?

Losing blood for any reason can be frightening, but how much blood can you lose before you die? The answer to this question is usually expressed as a percentage rather than as an absolute figure because an adult male, for example, generally has a higher blood volume than a woman, child, or infant. It's important to keep in mind that the rate of blood loss—as well as the amount of blood lost—will ultimately determine the effects and outcomes experienced by the patient. 

How Much Blood You Can Lose Before You Die

For a typical, healthy adult:

• Losing 40% or more of your blood volume will usually lead to death without immediate and aggressive life-saving measures. 

• Losing 30% of your blood volume can lead to death without fluid replenishment with saline solution, Lactated Ringer's solution, or a blood transfusion. 

• Losing between 15% and 30% of your blood volume won’t usually cause death but will definitely affect your vital signs. 

• Losing up to 14% of your blood volume may make you feel weak and lightheaded or dizzy but won't typically have any effect on vital signs like blood pressure or heart rate. 

The typical adult has 10-12 pints or 5 liters of blood circulating at any given time, so you could technically lose up to 5-6 pints or 2 liters of blood before you would die.

The Rate of Blood Loss Matters

When your blood supply drops, your body works hard to compensate for this loss in several ways. The slower blood loss occurs, the more time your body has to compensate and the less likely you are to die.

When you start to lose blood:

• Smaller blood vessels constrict to allow less blood to flow through.
• Your heart rate increases as your heart works to maintain sustainable blood pressure and oxygen delivery to the vital organs, including the brain and the heart.
• Your extremities turn pale, cold, and sweaty as the body pulls blood from non-essential body parts and concentrates circulation around the vital organs.
• Your urine output decreases to conserve the volume of bodily fluids.
• You feel weak and tired to prevent exertion, which would put additional strain on your vital organs.

When blood loss occurs slowly, your body will work hard to compensate for the loss. While you might feel weak, you could still be walking around after a blood volume loss of 30% if it occurs over a period of weeks. In contrast, a blood volume loss of 15% that occurs all at once could become dangerous. That's why it's so crucial to stop severe bleeding with a bleeding control kit as soon as you see it occur.

The American College of Surgeons’ Four Classes of Hemorrhage

The American College of Surgeons Advanced Trauma Life Support (ATLS) division has defined four levels of hemorrhage according to severity. According to Oxford Languages, hemorrhage means “an escape of blood from a ruptured blood vessel”. This condition ranges from mild to life-threatening.

Class I Hemorrhage

Class I hemorrhage is defined as a loss of up to 15% of blood volume. If this happens quickly, the patient might feel lightheaded or dizzy. When it occurs slowly, the patient might have no symptoms at all. 

You might lose this much blood:

• When you donate blood (1 pint, 10% of total blood volume)
• In vaginal childbirth (500 mL or half a quart, 10% of total blood volume)
• During a menstrual period (2 to 4 tablespoons or 1% of total blood volume*)
• From a mild cut
• From very slow internal bleeding

Besides replacing fluids by drinking water or fruit juice, nothing else usually needs to be done for a class I hemorrhage. 

*Note: In a menstrual period, women lose around 2 to 4 tablespoons of blood (30-60 mL). As no blood vessels are ruptured, it isn’t technically a hemorrhage. However, we’re including it here for the sake of comparison.

Class II Hemorrhage

A class II hemorrhage is defined as losing 15-30% of total blood volume. At this point, the patient would have a rapid heartbeat (tachycardia) as the blood vessels constrict; he or she might feel nauseous or anxious; their skin may become cool, pale, and sweaty (shock); their urine output would decrease; and their breathing would become faster.

You might lose this much blood:

• During surgery (500-1500 mL or 0.5 to 1.5 quarts, 10-30% of total blood volume)
• During a cesarean delivery (1000 mL or one quart, 20% of total blood volume)
• After a traumatic external or internal injury

After losing 15-30% of blood volume, patients typically require volume resuscitation with saline solution or Lactated Ringer's solution. They don't usually need a blood transfusion because the remaining blood contains enough red blood cells to continue to transport oxygen to the body.

Class III Hemorrhage

A class III hemorrhage involves a 30-40% blood volume loss. As more blood is lost, the patient's blood pressure drops further, their heart rate and breathing rate increase further, the patient may become confused and disoriented, and the body typically goes into hypovolemic shock. As the body struggles to maintain adequate blood pressure, the patient might pass out or slip into a coma.

You might lose this much blood:

• During heart surgery
• After a traumatic injury affecting a major blood vessel

After a class III hemorrhage, patients usually require one or more blood transfusions to prevent death.

Class IV Hemorrhage

A class IV hemorrhage occurs when more than 40% of a patient's circulating blood volume is lost. At this point, there is not enough circulating fluid to maintain blood pressure and the patient may become unconscious. 

You might lose this much blood:

• During certain kinds of heart surgery
• After a traumatic injury to any of the major blood vessels

A class IV hemorrhage requires immediate and aggressive life-saving measures to prevent death. Otherwise, the patient will usually die.

When Might Life-Threatening Bleeding Occur?

Deaths from traumatic bleeding can occur:

1. During surgery, if a major artery is (accidentally) cut
2. After a gunshot wound or blunt trauma that affects a major artery
3. Due to an uncontrolled postpartum hemorrhage

Knowing the signs of life-threatening bleeding can help you identify a bleed that could potentially cause death. Life-threatening bleeding typically looks like blood squirting or flowing out of a wound rather than coming out as a slow trickle or drip. Signs of severe internal bleeding may include some combination of the following:

• Blood coming out of the mouth, eyes, ears, nose, or genital openings
• Swelling and bruising under the skin
• Rapid breathing
• An accelerated heart rate
• Symptoms of shock (pale, cool, sweaty skin, confusion or disorientation)

What Should You Do if You See Someone Who's Bleeding?

If you see someone bleeding or you are bleeding, put on sterile gloves if available, and carefully uncover the wound. If the blood is squirting or coming out fast (more than a trickle or slow ooze), have someone call 911 while you locate a trauma kit, apply direct pressure with a sterile gauze or clean cloth, and elevate the wound.

If direct pressure doesn't stop the bleeding after a minute or two and the wound is on an arm or a leg, apply a tourniquet (in extreme life-or-death emergencies, an improvised tourniquet may be used in the absence of a commercial tourniquet). Then, write the time the tourniquet was applied on the tourniquet itself or on the patient's forehead: T = TIME. This will help the emergency services personnel plan their response so that the tourniquet isn't left on for longer than is safe.

Life-Threatening Bleeding Can Be Stopped If You Act Quickly

The most important thing to recognize is that no one needs to die from blood loss. In the first couple of minutes of even the most severe bleeding, applying immediate measures to stop the bleeding—like a tourniquet—can limit the blood loss to an amount that can be treated with fluid resuscitation or a blood transfusion, if needed. 

To prevent yourself or someone else from dying from blood loss, the best thing you can do right now is take a bleeding control course, have a STOP THE BLEED® Kit on hand (these contain commercial tourniquets and pressure bandages), and practice your tourniquet skills every three months or so. Then, when the moment comes, you’ll be ready to act fast and stop the bleeding in time.

Article written by

Brian Graddon

Brian is a former Firefighter Paramedic who also worked as a SWAT Medic, Engineer, and Captain over a 15-year career. Brian is devoted to providing life-saving information based on his first hand experience in life-saving application of tourniquets, hemostatic gauze, chest seals and other bleeding control products.