The body can give up a surprising amount of blood without trouble, which is why millions of people safely donate a unit every year. A healthy adult can lose about 10 to 15 percent of their blood, roughly the half-liter taken in a donation, with little or no harm. Losing more than 15 to 20 percent can start to cause shock, and around 40 percent is often fatal without treatment. The safe amount is a share of your own blood supply rather than a fixed volume, so a small person reaches the danger zone sooner than a large one. Speed matters as much as amount, because the body adjusts to a slow loss far better than to a sudden one.
How Much Blood Your Body Holds
An average adult’s blood makes up about 7 percent of body weight, or about 70 mL for each kilogram.1PubMed Central. Clinical review: Hemorrhagic shock A 70-kilogram (154-pound) adult has about 5 liters of blood, a little over a gallon.2PubMed Central. Clinical review: Hemorrhagic shock A 50-kilogram (110-pound) person has closer to 3.5 liters. Children carry 8 to 9 percent of their weight as blood, and infants up to 9 to 10 percent.3PubMed Central. Clinical review: Hemorrhagic shock
To work out your own rough volume, divide your weight in pounds by 2.2 to get kilograms, then multiply by 70 to get milliliters of blood. Women generally have slightly less blood per kilogram than men, so treat the result as a ballpark figure rather than a measurement.
Weight alone is a rough guide. Blood volume is not a fixed fraction of weight for everyone, so the 7 percent rule can be off for very heavy or very lean people.4PubMed. Prediction of the normal blood volume. Relation of blood volume to body habitus Fat tissue holds less blood than muscle, so people well above a healthy weight have less blood per kilogram than the rule suggests. Estimates that use sex and height along with weight come much closer to the real number.5Surgery. Prediction of normal blood volume with relation to body weight, body surface area, and height in the inches cubed to body mass index Pregnancy is a well-known exception too: blood volume rises substantially, so a pregnant woman’s total is higher than her weight alone would predict.
Doctors judge blood loss as a share of a person’s total, not in cups or liters. The same 500 mL is about 10 percent for a large man but 15 percent for a small woman.
Safe Losses Like Blood Donation
A standard blood donation takes about 500 mL, roughly a pint. For a 5-liter adult that is about 10 percent of their blood. US blood bank rules cap a donation at 10.5 mL per kilogram of body weight.6PubMed Central. The potential impact of selective donor deferrals based on estimated blood volume on vasovagal reactions and donor deferral rates Even so, someone with less than 3.5 liters of blood can lose more than 15 percent of it in one standard donation.7PubMed Central. The potential impact of selective donor deferrals based on estimated blood volume on vasovagal reactions and donor deferral rates
The body treats a donation as a controlled, modest loss. The fluid portion of blood is pulled back from tissues quickly, which keeps blood pressure steady while the blood cells are slowly rebuilt. Before you donate, staff check weight, pulse, blood pressure and hemoglobin so people with a smaller margin are screened out or deferred. They also take only a set amount, so you are not losing an open-ended volume the way you would in an injury.
Feeling lightheaded or fainting is the most common reaction to giving blood. After blood banks began turning away young donors with low estimated blood volume, lightheadedness among 16-year-old donors fell from about 10.5 percent to 7.3 percent.8PubMed. Improved safety for young whole blood donors with new selection criteria for total estimated blood volume
Drink several glasses of water or juice over the next few hours, eat something, and skip heavy exercise or heat for the rest of the day. If you feel dizzy, sit or lie down right away and raise your legs. If you have fainted before, tell the staff so they can take precautions.
Everyday bleeding usually adds up to far less than a donation. Blood spreads widely on tissue, a sink or clothing and looks like much more than it is, so a dramatic-looking mess is often a small amount. A typical nosebleed or shaving cut loses a small fraction of what a donation takes, so these almost never come near a dangerous level. With a heavy period, soaking through a pad or tampon every hour for several hours in a row is a reason to call a doctor, since the loss can add up over a day. A puddle that keeps growing despite pressure is the sign to take seriously and call for emergency help.
When Blood Loss Turns Dangerous
Losing more than 15 to 20 percent of your blood can cause shock, meaning the body no longer has enough blood to supply its organs.9MedlinePlus. Hypovolemic shock: MedlinePlus Medical Encyclopedia For an average adult that is roughly 750 mL to 1 liter, about two donations’ worth. Death from shock from blood loss is estimated to happen once a person loses about 40 percent of their blood,10PubMed Central. Haemorrhage and Survival Times: Medical–Legal Evaluation of the Time of Death and Relative Evidence which for an average adult is about 2 liters.
Early in bleeding, pulse and blood pressure can stay normal because the body squeezes blood vessels and speeds the heart to make up the difference.11PubMed Central. Integrated Compensatory Responses in a Human Model of Hemorrhage It can do this because it has a backup capacity for coping with lost blood.12PubMed Central. Integrated Compensatory Responses in a Human Model of Hemorrhage Young, fit people compensate especially well, which can make them look fine right up until they collapse. If shock goes on too long, the body can fail even after all the lost blood is replaced by transfusion.13CV Physiology. Hemorrhagic Shock
Speed matters too. Losing all of your blood volume over 24 hours, or half of it within 3 hours, counts as massive bleeding.14PubMed Central. Clinical review: Hemorrhagic shock A steady bleed that seems slow can still reach a dangerous total within hours.
The warning signs follow the body’s effort to cope. Early on, expect a racing pulse, fast breathing and dizziness when you stand up. Pale, cold, clammy skin suggests blood is shifting away from the skin toward the vital organs. Confusion, anxiety or unusual drowsiness suggest the brain isn’t getting enough oxygen, and they deserve attention even if the person says they are fine. Fainting and passing little or no urine are late, serious signs that the body’s backup capacity is running out.
Who Has Less Room to Spare
Babies and toddlers have the least room to spare: although they carry more blood per kilogram than adults, their total supply is so small that heavy bleeding needs urgent care even if the volume looks small.
People who start with low blood counts tolerate less additional loss, at least during surgery.15PubMed. The influence of baseline hemoglobin concentration on tolerance of anemia in cardiac surgery Anemia is the common example: their blood already carries less oxygen, so they often feel tired to begin with and can tip into breathlessness and dizziness faster than others. Older adults with heart disease also handle blood loss less well, because the heart has less ability to speed up and make up the difference.
Blood thinners such as warfarin and daily aspirin make clots form less effectively, so bleeding lasts longer and is harder to control. Among injured older adults on such medicines, those on aspirin had the highest rate of bleeding inside the skull, about 35 percent.16PubMed. Novel oral anticoagulants and trauma: The results of a prospective American Association for the Surgery of Trauma Multi-Institutional Trial That bleeding can follow a minor head bump, sometimes without obvious outward signs. If you take a blood thinner, get any head bump or heavy bleed checked promptly. Tell every doctor about every blood thinner you take, including over-the-counter aspirin, since it changes how they judge and treat bleeding.
How Long It Takes to Replace Lost Blood
After a 500-mL donation, the lost fluid volume is replaced quickly, within hours.17University of Western Ontario Medical Journal. Blood Loss and Its Replacement Bringing red blood cells back to their previous level takes about seven weeks on average.18University of Western Ontario Medical Journal. Blood Loss and Its Replacement The body pulls water in from tissues and from what you drink, but red cells must be built in the bone marrow. That’s why you may feel better within a day yet still be run-down for a while, and why blood banks require about eight weeks between whole-blood donations.
Iron-rich foods like red meat, beans and leafy greens support new red cells. Frequent donors and people with heavy periods may need iron supplements. Tiredness and shortness of breath weeks later can mean low iron, so ask your doctor for a blood count if fatigue lingers.
Hospitals commonly give a transfusion when hemoglobin falls below about 7 to 8 g/dL.19PubMed Central. Hemoglobin-based transfusion strategies for cardiovascular and other diseases: restrictive, liberal, or neither? Transfusion is saved for larger deficits or symptoms because it carries its own risks and many people recover without it. Patients with heart disease may do better with transfusion at a higher level, below 10 g/dL,20PubMed Central. Hemoglobin-based transfusion strategies for cardiovascular and other diseases: restrictive, liberal, or neither? since their hearts cope less well with reduced oxygen delivery.
What to Do When Someone Is Bleeding Heavily
Call emergency services first, or while you start, because help takes time to arrive. Firm, steady pressure directly on the wound is the first step. Press a clean cloth on it with both hands, pushing hard, and add more layers on top if blood soaks through rather than lifting the first one. Lifting the cloth to check can tear away clots that have started to form.
For severe bleeding from an arm or leg, a tourniquet, a tight band above the wound, can stop the flow. Tourniquet use by paramedics in the US is linked to more patients surviving to reach the hospital.21PubMed. Characteristics and Outcomes of Prehospital Tourniquet Use for Trauma in the United States Tightening a tourniquet early rather than waiting is reasonable for life-threatening limb bleeding. A loose one can make bleeding worse by blocking blood returning to the heart while still letting blood flow in, so it must be tight enough to stop the bleeding. Note the time it went on and tell the responders.
Lay the person down and cover them with a coat or blanket. Getting cold and losing clotting ability make bleeding worse, so keeping an injured person warm matters.22PubMed. Clinical and cellular effects of hypothermia, acidosis and coagulopathy in major injury Don’t give them anything to drink; it doesn’t help and may delay surgery if they need an anesthetic.