Red cell distribution width (RDW) is a red blood cell parameter that measures variability of red cell volume/size (anisocytosis). Depending on the types of hematology analyzer instruments, RDW can be reported statistically as coefficient of variation (CV) and/or standard deviation (SD), RDW-CV and/or RDW-SD, respectively.
RDW-SD takes measurements in "fL" and basically measures the width of red cells size distribution histogram – it calculates the width at the 20% height level of the histogram. The average RBC size therefore has no effect on this parameter and you get MCV or mean corpuscular volume.
RDW-CV is expressed in percentage and is calculated from MCV and standard deviation as follows:
RDW-CV (in percentage) = 1 SD of RBC volume/MCV x 100%