Hematocrit is the percentage of your blood that is made up of red blood cells. If your test result says 42, that means 42% of your blood volume is red blood cells, with the remaining 58% being plasma (the liquid part), white blood cells, and platelets. It’s one of the most common values on a routine blood test called a complete blood count, or CBC.
Normal Hematocrit Ranges
Normal levels differ by sex. For adult males, the typical range is 41% to 50%. For adult females, it’s 36% to 44%. These ranges can vary slightly between labs, so your results will usually include a reference range specific to the lab that processed your blood.
Children have different normals depending on age. Newborns run surprisingly high, between 42% and 64%, because they carry extra red blood cells from the womb. That number drops during the first few months of life, settling into the 31% to 41% range by six months to a year. By the teenage years, boys and girls begin to diverge: boys trend toward 37% to 48%, while girls stay closer to 34% to 44%, a pattern that continues into adulthood.
During pregnancy, hematocrit naturally drops. The body increases its blood plasma volume significantly to support the growing fetus, but red blood cell production doesn’t keep pace. This is sometimes called dilutional anemia of pregnancy and is considered normal, not a sign that something is wrong.
What Low Hematocrit Means
A hematocrit below the normal range usually signals anemia, meaning your blood isn’t carrying as much oxygen as it should. The most common symptoms are fatigue, weakness, pale skin, and feeling short of breath during activities that didn’t used to wind you. Some people notice cold hands and feet, dizziness, or headaches.
The most frequent cause is iron deficiency. Your body needs iron to build the oxygen-carrying protein inside red blood cells. Without enough iron, it produces fewer red blood cells and smaller ones. Women with heavy periods are especially prone to this because monthly blood loss depletes iron stores over time. Pregnant women who don’t supplement with iron and folic acid also face a higher risk.
Other causes include deficiencies in vitamin B-12 or folate, chronic blood loss from something like a stomach ulcer, and chronic diseases such as kidney failure, diabetes, or cancer that interfere with red blood cell production. In some conditions, the body actively destroys its own red blood cells faster than it can replace them.
What High Hematocrit Means
A hematocrit above normal means there’s an unusually high concentration of red blood cells in your blood. This thickens the blood, which can slow circulation and raise the risk of clots. Symptoms often include headaches, dizziness, blurred vision, and skin that looks flushed or reddish.
The most benign explanation is dehydration. When you lose fluid (from illness, exercise, or simply not drinking enough), the liquid portion of your blood shrinks while the number of red blood cells stays the same. That makes hematocrit look artificially high. Rehydrating brings it back to normal. This is sometimes called relative erythrocytosis, because the red blood cell count itself isn’t actually elevated.
Living at high altitude also raises hematocrit. Thinner air contains less oxygen, so the body responds by producing more red blood cells to compensate. This is a normal adaptation, not a disease. Smoking triggers a similar response by reducing how much oxygen your lungs deliver to the blood.
More serious causes include polycythemia vera, a blood disorder where a genetic mutation causes bone marrow to overproduce red blood cells. Unlike dehydration or altitude, polycythemia vera requires ongoing medical management. Some inherited gene mutations can cause the same overproduction from birth. Chronic lung disease and certain heart conditions can also drive hematocrit up by keeping oxygen levels persistently low.
How Hematocrit Is Measured
In most modern labs, hematocrit is calculated automatically by a machine that counts and sizes individual blood cells from a standard blood draw. This automated method is precise and highly reproducible. An older technique, still used in some settings, involves spinning a thin tube of blood in a centrifuge until the red blood cells pack tightly at the bottom. A technician then reads the percentage visually. The centrifuge method tends to read 1% to 3% higher than automated results because a small amount of plasma gets trapped between the packed cells.
Either way, the test requires only a routine blood sample and results are typically available the same day.
Factors That Shift Your Number
Beyond disease, several everyday factors influence hematocrit. Age and sex are the biggest. Testosterone stimulates red blood cell production, which is why adult males run higher than females. Hydration status can swing your result by several percentage points in either direction. Even the time of day and body position during the blood draw can cause minor variation.
Altitude matters more than most people realize. If you’ve recently moved from sea level to a mountain town, your hematocrit will gradually climb over weeks as your body adapts to lower oxygen. This is why labs in high-altitude cities may use slightly different reference ranges.
Because so many variables affect the number, a single abnormal result doesn’t necessarily point to a problem. Doctors typically look at hematocrit alongside other values on the CBC, your symptoms, and your medical history before drawing conclusions. If a result is significantly outside normal range or trending in one direction over time, further testing can pinpoint whether the cause is something simple like dehydration or a condition that needs treatment.