Red Blood Cell Distribution Width (RDW) Calculator

Estimates the distribution width of RBC based on the MCV and its standard deviation.

In the text below the form there is more information about the calculation method and about the involvement of RDW in the determination of anemia causes.

The RDW calculator determines the distribution width of red blood cells based on the mean corpuscular volume and its standard deviation.

RDW can offer information about the cause of the anemia. It is important to note that not only RDW elevations are consistent with disease, e.g. anemia and that even normal values can accompany conditions like hereditary spherocytosis or hemoglobinopathy.

The formula for red blood cell distribution width is:

RDW-SD = (Std Dev of MCV x 100 / MCV)

The standard size of red blood cells is between 6 – 8 microns. However, their normal reference range for volume is between 11 – 15%, therefore the RDW-SD is between 39 and 46 fL.

Standard dev. of MCV
Mean Corpuscular Volume
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Steps on how to print your input & results:

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.


Variables and formula

The above calculator uses the values of MCV and its standard deviation to estimate the red blood cell distribution width (RDW). The formula used is the following:

RDW-SD = (Std Dev of MCV x 100 / MCV)

Red blood cells have a standard size of approximately 6 to 8 microns. Their normal reference for volume is between 11 and 15%. The RDW-SD normal value, which is the width of the volume curve, is between 39 and 46 fL.

There is also a different, less used way of expressing RDW, which is the RDW-CV, also known as the reporting of RDW as coefficient of variation:

RDW-CV = 1 SD of RBC volume / MCV x 100%

Several hematologic conditions impact on the size of the red blood cells and these may or may not be associated with low, normal or elevated MCV:

■ High RDW & high MCV indicate liver disease with or without hemolytic anemia;

■ High RDW & low MCV indicate iron deficiency which can be associated with a decrease in haemoglobin.

High RDW values are associated with increased risk of adverse outcome, especially when the patient suffers from a concurrent cardiovascular disease (e.g. heart failure) or has risk factors associated with raised cardiac risk.

In patients diagnosed with hypertension, it was observed a correlation between abnormal RDW values and risk of carotid artery atherosclerosis.


RDW and MCV in anemia

The two determinations are used in the diagnosis of anemia, the mean corpuscular volume offering information on the dimension variation of the RBCs.

The complete blood count, is a laboratory test that describes the blood parameters in terms of size, composition and protein concentration.

The procedure involves the drawing of a sample of venous blood, mixing it with anticoagulant, usually EDTA and then analysing it part under microscope and part in an automated analyser. Through centrifugation, the erythrocytes sediment and the hematocrit, component of the MCV, is measured.

Low MCV values are suggestive of microcytic anemia whilst elevated values may indicate macrocytic anemia. Even when MCV is in the normal range, which is 80-96 fL, normocytic anemia may be present. The most common cause of this type of anemia is iron deficiency.

Elevated MCV is also associated with alcoholism and vitamin B12 deficiency.

On the other hand, anemia is not present in all cases with elevated RDW and even normal values can accompany chronic disease, spherocytosis or hemoglobinopathies of genetic cause.

However, not all RDW elevations are consistent with anemia, even normal levels can underpoint chronic disease, hereditary spherocytosis or certain hereditary hemoglobinopathies.



1. Red Cell Distribution Width in Family Practice Notebook [available online].

2. Wen Y. High red blood cell distribution width is closely associated with risk of carotid artery atherosclerosis in patients with hypertension. Exp Clin Cardiol. 2010; 15(3): 37–40.

3. Buttarello M, Plebani M. Automated blood cell counts: state of the art. Am J Clin Pathol. 2008; 130(1):104-16.

4. Balta S, Demirkol S, Aparci M, Yildirim AO, Ozturk C, Celik T. Red blood cell distribution width: Just one of many things to consider. Int J Cardiol. 2015; 203:438-439.

Specialty: Hematology

System: Cardiovascular

Objective: Determination

Type: Calculator

No. Of Variables: 2

Article By: Denise Nedea

Published On: June 23, 2017

Last Checked: June 23, 2017

Next Review: June 23, 2023