Hematocrit is a measure of how much space (volume) the red blood cells take up in the blood. A hematocrit check is often used to test for anemia, which is a decrease in the amount of oxygen-carrying substance (hemoglobin) found in red blood cells.

The hematocrit value may be given as a percentage of red blood cells in a volume of blood. For example, a hematocrit of 33 means that 33% of the blood's volume is composed of red cells. It may also be measured in milliliters per deciliter (mL/dL) of blood.