Red Blood Cell Count
The number of red blood cells in a sample of your blood. These cells carry oxygen from your lungs to the rest of your body.
AI-verified against cited sources. Awaiting expert reviewAlso called RBC, RCCLOINC 789-8
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The usual range for RBC depends on sex. Choose female or male above to see where you stand.
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What it is
Red blood cells (erythrocytes) are made in your bone marrow and live about 120 days. They contain hemoglobin, an iron-rich protein that carries oxygen. The RBC count is how many red cells are in a set amount of blood. It is a routine part of a complete blood count (CBC), where it is read together with hemoglobin, hematocrit and the red cell indices (MCV, MCH, MCHC and RDW).
Why it matters
Every cell in your body needs oxygen, and red blood cells deliver it. A low count can come with tiredness, shortness of breath or pale skin. A high count can come with headaches or dizziness. The count alone can't say why it is off, so clinicians read it with hemoglobin and the size of your red cells. Many people have a count slightly outside the range for harmless reasons.
If it’s low
A low count means fewer red cells than usual, which can leave you tired, short of breath or pale. It often goes along with anemia, but the count alone doesn't confirm that, so clinicians check hemoglobin and red cell size too. Common causes include low iron, low vitamin B12 or folate, blood loss and long-term illness. Pregnancy can also lower it, because the extra fluid in your blood dilutes the count.
Common causes
- Low iron
- Low vitamin B12 or folate
- Blood loss, such as from heavy periods or the digestive tract
- Chronic kidney disease
- Long-term illness or inflammation
- Pregnancy
- Poor nutrition or heavy alcohol use
- Breakdown of red cells (hemolysis)
- Bone marrow conditions, including some blood cancers
What can help
- Ask your clinician which tests, such as iron studies, B12, folate or a reticulocyte count, could help find the cause
- Eat a balanced diet that includes iron, B12 and folate foods, and talk with your clinician before starting supplements
- Mention heavy periods, blood in your stool or other signs of blood loss
If it’s high
A high count means more red cells than usual in a set amount of blood. Common reasons include dehydration, living at high altitude and smoking. A high count with small red cells can also be how an inherited trait such as thalassemia trait looks. Less often, it points to a lung, heart or kidney condition, or a bone marrow condition that makes too many red cells.
Common causes
- Dehydration, which makes the blood more concentrated
- Living or training at high altitude
- Smoking or lung disease
- Sleep apnea
- Heart disease
- Some kidney conditions
- Anabolic steroids or testosterone products
- Inherited traits such as thalassemia trait, where red cells are numerous but small
- Less often, a bone marrow condition such as polycythemia vera
What can help
- Ask your clinician whether hydration, a recent illness or altitude could explain the result
- Ask whether hemoglobin, hematocrit and red cell size (MCV) show a pattern worth following up
- If you smoke, or snore loudly with daytime sleepiness, mention it to your clinician
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
The usual range depends on sex. Choose women or men to see the one that applies.
- Men, 18+4.3–5.7 ×10⁶/µL
Sources: American Board of Pathology, Labcorp, Te Whatu Ora Health New Zealand, Waikato, Oregon Health & Science University, University of Iowa Hospitals & Clinics
- Women, 18+3.9–5 ×10⁶/µL
Sources: American Board of Pathology, Labcorp, Te Whatu Ora Health New Zealand, Waikato, Oregon Health & Science University, University of Iowa Hospitals & Clinics
See RBC by age and sex, with the U.S. survey numbers where they exist.
How common is your level?
NHANES describes the US civilian, non-institutionalized population, not a 'healthy' reference group. People with disease, on medication, fasting or not (except fasting-subsample markers), smokers and so on are all included. A percentile says where a value sits among Americans of that sex and age, not whether it is normal.
Use the left and right arrow keys to read each age. Shift moves five years.
Reference data derived from CDC/NCHS National Health and Nutrition Examination Survey (NHANES), cycles 2015-2016, 2017-March 2020 (pre-pandemic), August 2021-August 2023. Analysis by bloodtesttracker.com; not endorsed by CDC, NCHS or HHS.
Explore RBC by age and sex: every percentile as a table, with a download.
What can affect your result
- Sex: men usually have higher counts than women
- Age
- Altitude: counts are higher at high altitude
- Pregnancy: counts drop as blood volume increases
- Hydration: dehydration raises the count
- Smoking raises the count
- Recent blood loss, blood donation or transfusion
- Cold agglutinins, a rare cause of an undercount
- Cold agglutinins (antibodies that clump red cells in the cold) can make the machine undercount red cells. Labs can often fix this by warming the sample.
- Very high white cell counts, red cell fragments and very large platelets can be miscounted by automated analyzers.
- A sample diluted with IV fluid or drawn into the wrong tube can read falsely low, and labs reject these.
- Dehydration or overhydration changes how concentrated your blood is, and a recent transfusion or blood loss changes the count.
Red cells live about 120 days, so the count changes over weeks to months after a change in diet, blood loss or treatment.
When to talk to a clinician
Labs usually set urgent-call limits on hemoglobin and hematocrit rather than on the red cell count. Contact a clinician soon if you have a low count with shortness of breath, a racing heart, dizziness or pale skin, or a high count with a headache, vision changes or dizziness. Call 911 or your local emergency number for chest pain, severe trouble breathing, confusion or fainting.
Questions you could ask
- How has my red cell count changed over time?
- Could dehydration, altitude, smoking or sleep problems explain my result?
- Should we look at hemoglobin, hematocrit and red cell size together?
- Should we check iron, B12 or folate?
- Could blood loss or a medicine be part of this?
Questions people ask
Is the RBC count the same as hemoglobin?
No. The RBC count is how many red cells there are. Hemoglobin is the oxygen-carrying protein inside them. The two usually move together, but not always. Small red cells can be numerous yet hold little hemoglobin, so clinicians look at both.
Why is my RBC count in the usual range or high when my hemoglobin is low?
That pattern, with small red cells (a low MCV), is typical of inherited traits such as thalassemia trait. Iron deficiency usually lowers the red cell count instead. Your clinician can use red cell size and other tests to sort out which applies.
Why do men and women have different ranges?
Men usually have more red blood cells than women. Testosterone helps the body make red cells, and women lose some blood each month during periods. That is why labs list separate ranges.
How do I convert red blood cell units?
×10⁶/µL, M/µL and ×10¹²/L are the same number, so 4.8 in one is 4.8 in the others. Some older reports print cells per cubic millimeter, for example 4,800,000 /cumm. Divide that by 1,000,000 to get ×10⁶/µL.
Show the science and sources
Units
- 1 ×10⁶/µL = 1 ×10⁶/µL
- 1 ×10⁶/µL = 1 ×10¹²/L
- 1 ×10⁶/µL = 1000000 cells/µL
Is a change real?
RBC naturally varies about 2.8% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 9% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Systematic review and meta-analysis of within-subject and between-subject biological variation estimates of 20 haematological parameters (Coskun A et al., EFLM WG-BV).
For clinicians and researchers
Analyzers count red cells by electrical impedance (the Coulter principle) or optical methods. Hematocrit is either measured directly or calculated as RBC × MCV, and MCH and MCHC are calculated from hemoglobin, RBC and hematocrit. Reference limits differ by sex (androgen-driven erythropoiesis in men, menstrual loss in women), altitude, smoking and pregnancy. Interferences include cold agglutinins (low RBC, high MCV and MCHC), very high white cell counts, red cell fragments and giant platelets. In iron deficiency the RBC count is usually low or low-normal with a low MCV; in thalassemia trait it is often normal or high with a low MCV, and the Mentzer index (MCV divided by RBC) below about 13 is a screening hint. Diagnostic criteria for polycythemia vera (WHO) use hemoglobin, hematocrit or red cell mass, not the RBC count. Across the sources used, lower limits differ by about 0.4 and upper limits by about 0.5–0.6 ×10⁶/µL.
LOINC 789-8 (primary), 26453-1, 790-6. Specimen: whole-blood.
Sources
- Hematopathology reference ranges, American Board of Pathology
- CBC With Differential/Platelet (test 005009), sample report, Labcorp (2024)
- CBC, Platelet, No Differential (test 028142), sample report, Labcorp (2020)
- CBC reference intervals (HEM-2597.01), Te Whatu Ora Health New Zealand, Waikato (2024)
- Heme Reference Ranges (updated 6/5/18), Oregon Health & Science University (2018)
- CBC (Complete Blood Count), Pathology Handbook, University of Iowa Hospitals & Clinics
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.