Comparison Complete Blood Count (CBC) Updated May 1, 2026

Mean Corpuscular Volume vs Red Cell Distribution Width

Mean Corpuscular Volume (MCV) and Red Cell Distribution Width (RDW) are two CBC values that describe different features of red blood cells on a lab report. Both usually appear on the same complete blood count (CBC) panel, where MCV shows average red cell size and RDW shows how varied those sizes are. The main difference between MCV and RDW is that one reflects typical cell size in femtoliters (fL), while the other reflects size spread as a percentage (%).

Mean Corpuscular Volume

MCV

MCV is the average size of your red blood cells. Small cells lean toward iron trouble, large cells toward B12 or folate, and a crowd of both can average out to a number that looks fine.

Adult fL
80
100

Red Cell Distribution Width

RDW

RDW measures how much your red blood cells vary in size. It often climbs before hemoglobin or MCV drift out of range, and it splits two anemias that otherwise look identical.

Adult Male %
11.5
14.5

Mean Corpuscular Volume (MCV) and Red Cell Distribution Width (RDW) are two lab values that both appear on the CBC panel. MCV describes the average size of red blood cells, while RDW describes how much those cell sizes vary. On MCV vs RDW reports, both numbers help show related features of blood composition. They are often reviewed together on MCV and RDW on a blood test because they describe different parts of the same red cell picture.

Average Cell Size and the Spread Around That Average

MCV measures average red cell size in femtoliters (fL), while RDW measures the spread of red cell sizes as a percentage. Because both MCV and RDW come from the same red cell sample, changes in one can appear alongside changes in the other. A higher RDW with a normal MCV can point to mixed cell sizes, while a higher MCV with a normal RDW can mean the cells are larger but fairly uniform. In some reports, MCV shifts before RDW does, since MCV is a single average and RDW tracks variation. The difference between MCV and RDW is therefore about size versus size spread, not two versions of the same measure.

fL Volume vs Percentage Variation

Aspect Mean Corpuscular Volume Red Cell Distribution Width
What it measures Average cell size Size variation
Units fL %
Typical adult range 80–100 fL 11.5–14.5%
Reported as Mean value Percent spread
Directly reflects Cell volume Cell size range
How it's calculated Average volume Width of distribution
Common pairing CBC red-cell index CBC red-cell index
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Reading Cell Size Together With Size Uniformity

Reading MCV and RDW together gives a more complete view of red cell size patterns. If MCV is high and RDW is also high, the report shows larger cells with a wide range of sizes. If MCV is low and RDW is high, the report shows smaller cells with uneven sizing. If MCV is near the middle range and RDW is normal, the red cells are usually fairly uniform in size. In MCV vs RDW review, the combination matters more than either number alone.

CBC Red-Cell Indices That Group Both Values

MCV and RDW are usually reported on a CBC, not on panels like a CMP, lipid panel, or thyroid panel. They often appear together on routine blood work, follow-up CBCs, and pre-procedure lab reports. Because both are part of the same CBC red cell section, they are commonly listed side by side. On a lab report, MCV and RDW usually help compare red cell size and size variation in the same sample.

MCV vs RDW - Reader Questions

What is the difference between MCV and RDW?
MCV shows the average size of red blood cells, while RDW shows how much those sizes vary. In an MCV vs RDW comparison, one is a size average in fL and the other is a spread measure in %. That is the core difference between MCV and RDW on a lab report.
Which is more accurate, MCV or RDW?
MCV and RDW are not competing measures, so one is not more accurate than the other. MCV is better for describing average cell size, while RDW is better for describing size variation. Used together, MCV and RDW give a fuller CBC picture.
Why are MCV and RDW tested together?
MCV and RDW are tested together because both come from the same red cell sample and describe related features. On a CBC, MCV shows average size and RDW shows size spread. That makes the pair useful for reading blood composition patterns on the same report.
Can MCV be high while RDW is low?
Yes, that pattern can appear on a CBC. A high MCV with a low or normal RDW means the red cells are larger than expected but still fairly similar to one another. In MCV vs RDW review, that pattern is different from a high MCV with a high RDW.
How are MCV and RDW related mathematically?
MCV and RDW are related by red cell size data, but they are not a single ratio. MCV is an average volume value, while RDW is a width measure based on size variation. RDW is often reported as a percentage, while MCV is reported in fL.
What units are MCV and RDW measured in?
MCV is measured in femtoliters, written as fL. RDW is usually measured as a percentage, written as %. Those units show that MCV and RDW describe different parts of the CBC red cell data.
Are MCV and RDW part of the same panel?
Yes, both MCV and RDW are usually part of the CBC. They commonly appear together on the same lab report in the red cell index section. That is why MCV and RDW are often compared side by side.
What does high MCV with high RDW usually show?
High MCV with high RDW usually shows larger red cells with a wide range of sizes. In MCV and RDW data, that means both the average size and the size spread are above typical adult ranges. It is a pattern description, not a standalone conclusion.
What does normal MCV with high RDW mean?
Normal MCV with high RDW means the average red cell size is in range, but the cell sizes vary more than expected. On a CBC, that combination can suggest a mixed-size pattern rather than a uniform one. The difference between MCV and RDW is especially clear in this combination.

Disclaimer

This content is for informational and educational purposes only. It is not intended as medical advice, diagnosis, or treatment recommendation. Reference ranges may vary by laboratory. Always discuss your results with a qualified healthcare professional.