A red blood cell is the cell in blood that carries oxygen around the body. It is a flattened disc about 7–8 micrometers across. A micrometer is a millionth of a meter, or a thousandth of a millimeter. Red blood cells are the most numerous of the human body's own cells.

Size

The size of a red blood cell usually means its diameter, the width of the disc. A 2010 review of the cell's shape and mechanics gives the diameter of healthy human red blood cells as 7.5–8.7 micrometers and their thickness as 1.7–2.2 micrometers. So a red blood cell is three to five times as wide as it is thick.

In a blood smear, a drop of blood spread into a thin layer on a microscope slide and stained, red blood cells measure 7–8 micrometers across. The explorer shows 7 micrometers, at the small end of that range.

The cell's shape is called a biconcave disc: both faces curve inward, so the middle is thinner than the rim. A red blood cell has no nucleus. Its inside is packed with hemoglobin, the protein that carries oxygen.

Size comparisons

• About 11–13 red blood cells, lined up edge to edge, would span the width of a human hair, which is 80–100 micrometers wide.
• A neutrophil, the most common kind of white blood cell, is 12–15 micrometers across on a blood smear, about 1.6–2 times the width of a red blood cell.
• Red blood cells can pass through blood vessels as narrow as 2–3 micrometers, less than half their own width.
• If a red blood cell were enlarged to the width of a basketball, about 24 centimeters (9.4 inches), a person 1.7 meters (5 feet 7 inches) tall would stand about 54 kilometers (34 miles) high.
• A typical adult man's 25 trillion red blood cells, laid edge to edge, would stretch about 190,000 kilometers, almost half the distance from Earth to the Moon.

How the cell bends

To pass through the narrowest blood vessels, a red blood cell has to fold and stretch. Its outer membrane is a thin layer of fat-like molecules resting on a net of a protein called spectrin. Together they make the membrane elastic, so the cell can stretch and fold again and again.

The spleen filters red blood cells out of the blood. A healthy one circulates for about 120 days before it is removed. In hereditary elliptocytosis, an inherited disorder, the cells are oval and less flexible, and the spleen removes them sooner.

How many there are

A 2016 study estimated that a typical adult man has about 30 trillion human cells, and that about 25 trillion of them, or 84 percent, are red blood cells. Because each one is so small, they make up only a small part of the body's weight. Most of the weight of the body's cells is in much larger fat and muscle cells.

A microliter of blood, a millionth of a liter, normally holds about 4–6 million red blood cells. The usual range is 4.2–5.7 million for men and 3.8–5.1 million for women. A blood test called an RBC count measures this number.

Sources

• Shape and Biomechanical Characteristics of Human Red Blood Cells in Health and Disease. Diez-Silva et al., MRS Bulletin, 2010.
• MH 033hr Blood Smear. Histology Guide, University of Minnesota.
• Peripheral Blood. Histology Guide, University of Minnesota.
• Revised Estimates for the Number of Human and Bacteria Cells in the Body. Sender et al., PLoS Biology, 2016.
• RBC count. MedlinePlus, National Library of Medicine, 2026.
• The International System of Units (SI), 9th edition. Bureau International des Poids et Mesures, 2026.
• Just How Small Is "Nano"?. National Nanotechnology Initiative.
• Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.
• Moon Fact Sheet. NASA, 2024.