How many mL is a unit of blood transfusion?

Blood transfusions are a critical medical procedure that saves lives every day. During a transfusion, patients receive blood from a donor to replace lost blood or to treat various medical conditions. But how much blood is actually transfused during this process? Let’s take a closer look at the volumes involved in a single unit of blood transfusion.

What is a Blood Transfusion?

A blood transfusion is a medical procedure where blood or blood components from a donor are transferred into the circulatory system of a recipient. There are a few different reasons why a doctor may order a transfusion:

  • Replace significant blood loss due to trauma, surgery, childbirth, or gastrointestinal bleeding
  • Improve oxygen delivery in patients with anemia or other blood disorders
  • Replace clotting factors in patients with clotting disorders like hemophilia
  • Improve platelet counts in patients receiving cancer chemotherapy

During the procedure, blood is delivered from an IV bag into the patient’s vein. Careful monitoring ensures the transfusion goes smoothly and the patient does not have an adverse reaction.

What is Considered a Unit of Blood?

When referring to transfusions, doctors specify the amount of blood in terms of “units.” But what exactly constitutes a unit?

One unit of blood is defined as the amount of blood collected from a single donor into an empty blood bag. This is approximately 350-450 mL of blood depending on the weight of the donor. The standard blood bag holds around 500 mL.

There are actually several different components that can be transfused, each with specific therapeutic purposes:

  • Red blood cells – The most common type of transfusion which carries oxygen. One unit is roughly 200-250 mL.
  • Platelets – Helps blood clot. A platelet concentrate from 1 donor is typically 200-300 mL.
  • Plasma – The liquid portion of blood containing clotting factors. One unit of plasma is 200-300 mL.
  • Cryoprecipitate – Concentrate of clotting factors from plasma. One unit is 10-20 mL.

However, when doctors talk about “1 unit” for transfusion, they are generally referring to a standard unit of red blood cells.

Volume of Red Blood Cell Transfusion

As mentioned, a single unit of packed red blood cells (PRBCs) contains a volume of around 200-250 mL. This can vary slightly depending on the hematocrit level of the donated blood, which is a measure of the percentage of red blood cells compared to plasma.

To put this number into perspective, an average adult human body contains 4-5 liters of blood. Therefore, a single unit transfusion would increase total blood volume by 4-6%.

Typical Transfusion Volumes

The number of units transfused depends on the clinical situation and degree of blood loss or anemia. Some general transfusion volumes include:

  • 1-2 units for moderate blood loss or anemia
  • 2-4 units to treat major bleeding or severe trauma
  • 4-8 units or more for massive hemorrhage or acute crisis

For perspective, during childbirth an average of 500 mL or 2 units may be transfused if there is excessive bleeding. For major surgery like a coronary bypass, 6 units is fairly standard.

Rate of Administration

In addition to the number of units, doctors also calculate the rate at which the transfusion is administered. This is measured in mL per hour or units per hour.

Typical transfusion rates are:

  • 1-2 hours per unit for routine situations
  • 2-4 hours per unit when transfusing patients with cardiac or respiratory issues
  • Up to 1 unit per 15 minutes for severe hemorrhage and emergency settings

Faster infusion rates allow more blood to be transfused quickly during crisis situations. However, too rapid administration can lead to circulatory overload.

Pediatric Volumes

Transfusion volumes are significantly reduced when giving blood to infants and children. This is because they have much lower overall blood volumes compared to adults.

Typical unit volumes for pediatric patients include:

  • 10-20 mL/kg for neonates
  • 10-15 mL/kg for infants
  • 5-10 mL/kg for children

To avoid over-transfusion, the doctor carefully calculates the dose based on the child’s weight. The rate of infusion is also reduced to 1-2 mL/kg/hour for routine transfusions.

Special Transfusion Circumstances

There are some unique clinical situations where blood transfusion volumes differ from the standard usage:

Exchange Transfusion

Exchange transfusion is when a significant portion of the patient’s blood is removed while simultaneously replaced with donor blood. It may be used to rapidly reduce hemoglobin S levels in sickle cell patients or bilirubin levels in severe neonatal jaundice.

During an exchange transfusion, the volume exchanged is typically 1-1.5x the patient’s total blood volume. This often equals 8-12 units for an adult.

Intraoperative Blood Salvage

Intraoperative blood salvage is a technique that collects blood lost during surgery, filters it, and returns it to the patient. This allows patients to receive autologous (their own) blood rather than allogeneic (donor) blood.

The amount returned can be substantial – usually around 500-1200 mL. This blood is given back to the patient in addition to any donor blood units transfused.


Plasmapheresis is a procedure that separates plasma from the cellular components of blood. The plasma is removed while the cells are returned to the patient.

A typical plasmapheresis treatment removes 800-1500 mL of plasma per session. Multiple sessions may be given depending on the condition being treated.

Key Takeaways

  • One unit of packed red blood cells contains around 200-250 mL of volume.
  • An average transfusion is 1-2 units, but can be much higher in emergency situations.
  • The rate of infusion is calculated in units transfused per hour.
  • Smaller transfusion volumes are given to infants and children based on weight.
  • Exchange transfusions, blood salvage, and plasmapheresis involve unique volumes.


Determining transfusion volumes requires careful calculations by skilled doctors. A “unit” of blood is not a standardized amount. Rather, it refers to the contents of one blood bag collected from an individual donor. Monitoring the rate and total quantity transfused ensures patients receive the benefits of blood products safely.

While approximately 250 mL is a reasonable rule of thumb, the volume transfused should be personalized for each patient’s situation. This takes into account factors like their total blood volume, severity of illness, and ability to tolerate infusion. With careful dosing, blood transfusions can truly be life-saving therapies.

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