Calculate Red Cell Distribution Width (RDW-CV), Mean Corpuscular Volume (MCV), or red blood cell size standard deviation using one simple tool. Ideal for quick CBC math checks, study use, and clinical education.
Enter any two values to calculate the third. Units: RDW-CV in %, MCV in fL, RBC SD in fL.
RDW stands for Red Cell Distribution Width. It is a numerical measurement in the complete blood count (CBC) that reflects how much variation exists in the size of your red blood cells. If red blood cells are mostly uniform in size, RDW is lower. If there is a wide mix of smaller and larger cells, RDW increases. This size variability is commonly called anisocytosis.
In practical terms, RDW is not usually interpreted as a standalone diagnosis. It works best when paired with other CBC indices such as hemoglobin, hematocrit, MCV, MCH, and MCHC, plus iron studies and clinical findings. Even so, RDW is highly useful because it often shifts early in nutrient deficiency states and may support pattern recognition in different anemia types.
Many clinical labs report RDW as RDW-CV (coefficient of variation, expressed as a percentage). Some analyzers also report RDW-SD (standard deviation, expressed in femtoliters). Because labs may differ in analytic methods and reference intervals, always compare your value with the specific lab report range.
The classic RDW-CV equation links three variables: red blood cell volume standard deviation (SD), MCV, and RDW percentage.
This RDW calculator lets you solve for whichever value is missing when the other two are available. It can be useful for educational review, checking arithmetic, or understanding how shifts in MCV and SD change RDW-CV.
If MCV is 90 fL and RBC size SD is 13 fL, then:
RDW-CV = (13 / 90) × 100 = 14.44%
That result may sit near the upper end of many typical reference intervals, depending on the specific laboratory range.
RDW interpretation is contextual. A normal RDW does not always rule out disease, and an elevated RDW does not by itself prove a specific diagnosis. Broadly, higher RDW suggests greater heterogeneity in red cell size, which can occur when bone marrow is releasing a mixed population of cells or when nutritional and hematologic factors alter erythropoiesis.
A commonly quoted adult reference interval for RDW-CV is around 11.5% to 14.5%, but your report may differ. The analyzer, methodology, local population, and calibration all influence reference limits. For that reason, clinical decisions should follow the reporting lab’s own range and physician interpretation.
| RDW Pattern | Possible Meaning | Common Next Steps |
|---|---|---|
| Low/normal RDW | RBC size relatively uniform; may be normal or seen in some chronic patterns | Review with hemoglobin, MCV, reticulocyte count, iron studies as needed |
| High RDW | Greater cell size variability; can appear in iron deficiency, mixed deficiency states, recovery from blood loss, and other conditions | Correlate with ferritin, transferrin saturation, B12, folate, inflammation markers, peripheral smear |
RDW can help narrow differential thinking when interpreted with MCV. The combinations below are educational patterns rather than diagnostic rules:
This pattern often appears in iron deficiency anemia. Early iron depletion can increase size variability before all cells become uniformly microcytic.
This may be seen in thalassemia trait where red cells can be consistently small, producing a narrower distribution. Confirmatory testing is required for diagnosis.
Can occur in vitamin B12 or folate deficiency, mixed deficiency states, alcohol-related marrow effects, or during recovery phases with reticulocytosis.
Sometimes indicates a mixed population where microcytic and macrocytic cells average into a “normal” MCV, masking abnormalities that RDW reveals.
RDW-CV is a relative measure expressed as a percentage and depends on MCV. RDW-SD is an absolute width measure in fL based on the red cell volume histogram. Because RDW-CV incorporates MCV, two samples with similar SD but different MCV can have different RDW-CV values. Understanding this distinction is helpful when comparing results across reports or analyzers.
RDW is informative but nonspecific. Inflammation, recent transfusion, marrow disorders, liver disease, renal disease, nutritional deficiencies, and treatment response can all influence values. Timing also matters: dynamic changes in erythropoiesis may produce temporary shifts. A single result should not be overinterpreted without trend data and clinical assessment.
Pre-analytical and analytical factors may also affect CBC indices. Sample handling delays, instrument calibration differences, and methodology variability can influence reported RDW. Always interpret in relation to the same laboratory’s standards and, ideally, serial measurements over time.
When you use this calculator, keep units consistent: MCV and SD should both be in femtoliters (fL), and RDW is a percentage. If you are converting from a report format that lists RDW-SD separately, ensure you enter the correct value into the SD field. Small input errors can produce noticeably different outputs, especially when MCV is near the lower end of normal.
If your calculated RDW does not match a lab report exactly, rounding and analyzer-specific methods can explain small differences. Clinical laboratories may round at different decimal places or use slightly different computational pathways internally.
No. High RDW is a signal, not a diagnosis. It indicates greater red cell size variation and should be interpreted with full CBC data, symptoms, history, and additional tests.
Yes. Some anemia types may present with a normal RDW, especially at certain stages. That is why hemoglobin, MCV, reticulocytes, and iron/B12/folate studies are important.
Many labs use a range around 11.5% to 14.5% for RDW-CV, but reference intervals vary by lab and analyzer. Use your report’s listed range.
No. It performs mathematical calculations only. Diagnosis requires clinical evaluation and laboratory interpretation by a qualified professional.
An RDW calculator is a practical way to compute or verify CBC relationships quickly. RDW itself is a valuable clue about red blood cell size variability, especially when integrated with MCV and broader hematologic data. Use this tool for fast calculations, but rely on full medical context for any health decisions.