🩸 Hematocrit / Hemoglobin Converter

Hematocrit and hemoglobin track each other by the rule of three: Hct 42% converts to hemoglobin ≈ 14.0 g/dL, and 14 g/dL converts back to 42%. Select the direction and enter your value.

How to Use This Converter

Labs report one or both values; the conversion fills in whichever is missing and sanity-checks whichever is present. It assumes normocytic red cells.

The Rule of Three

The relationship holds because hematocrit is the volume fraction of red cells and hemoglobin their average concentration: with a normal mean corpuscular hemoglobin concentration (~33 g/dL), the arithmetic collapses to division by three. The full bedside rule adds RBC ≈ Hgb/3 (e.g. Hgb 15, Hct 45, RBC 5.0: "15/45/5"). It breaks when red cells are abnormal: microcytic cells (iron deficiency, thalassemia) pack less hemoglobin per volume, so Hgb runs lower than Hct/3, while macrocytic cells skew the other way. A consistent mismatch is itself diagnostic information, not calculator error.

Worked Example

Hct 42% → Hgb 14.0 g/dL: normal adult male range. Hgb 9.0 → Hct 27%: moderate anemia. Now the mismatch case: measured Hct 38% with measured Hgb 10.5 (expected 12.7): the 17% shortfall suggests microcytosis, pointing toward iron studies or thalassemia screening rather than accepting the numbers. I verified the converter's arithmetic. Pregnancy dilutes both (plasma volume expands more than red cell mass), so Hgb 11 in the second trimester can be physiologic while 11 in a nonpregnant adult is anemia: context decides, rather than the number alone.

Reference Ranges

Ranges are lab-specific: always read against your lab's printed range. For anemia workup context, our GFR calculator matters because CKD causes anemia via low erythropoietin, iron studies (ferritin, transferrin saturation) come next when microcytosis appears, and our blood pressure checker follows the hypertension that often accompanies CKD anemia.

When the Numbers Disagree

Measured Hct and Hgb come from different instruments (centrifuged or calculated volume vs photometric hemoglobin), so small differences are noise. A persistent gap beyond ~10% deserves a look at the MCV: low MCV explains Hgb < Hct/3 (iron deficiency, thalassemia trait, anemia of chronic disease), high MCV the reverse (B12/folate deficiency, alcohol, liver disease). Spurious gaps also come from cold agglutinins, lipemia, and very high WBC counts interfering with counters. The converter gives the expected value; the lab gives the measured one; the gap between them is where the diagnosis sometimes hides.

Beyond the Conversion

Neither number diagnoses a cause: Hgb 9 needs iron studies, B12/folate, reticulocyte count, and bleeding history, beyond a transfusion threshold. Transfusion decisions use symptoms and context more than the number (most stable patients tolerate 7-8 g/dL; cardiac disease raises the threshold). Athletes and altitude residents run higher baselines; dehydration falsely raises both (hemoconcentration). Treat the conversion as vocabulary, and the clinical picture as the sentence.

Anemia Patterns the Rule of Three Reveals

The rule of three is most useful as a pattern check on the full CBC. Microcytic pattern: Hgb low, MCV under 80, Hgb running below Hct/3: think iron deficiency (low ferritin), thalassemia trait (normal/high RBC count despite low MCV), or anemia of chronic disease. Macrocytic: MCV over 100, Hgb above Hct/3 sometimes: B12 or folate deficiency, alcohol, liver disease, hypothyroidism, certain medications. Normocytic with both falling together: acute blood loss, CKD (low erythropoietin), or marrow problems. The reticulocyte count then separates production failure (low retics) from loss or destruction (high retics). This is the standard diagnostic cascade every low Hgb triggers, and the rule of three is its first step: it tells you whether the red cells are the expected size before you order a single additional test.

When in doubt, repeat the CBC: a single abnormal value with no symptoms often normalizes, while a persistent abnormality across two draws earns the workup. Treat the patient, confirm the lab, and let persistent numbers earn their workup.

Special Populations and Pitfalls

Newborns run high (Hgb 14-24, Hct 45-65) and fall to a physiologic nadir at 2-3 months; children's ranges climb with age, so always use age-specific references. Athletes using altitude training or (illicitly) erythropoietin push Hct toward 50+; endurance athletes can also dilute it (sports anemia, actually plasma expansion). Smokers run 1-2 points higher from chronic carbon monoxide. Spurious results come from cold agglutinins (falsely low RBC, high MCV), lipemia, and WBC counts over 50,000 interfering with counters. And remember hemodilution vs hemorrhage in acute bleeding: immediately after blood loss, Hct and Hgb look normal because whole blood was lost; they fall only after fluids shift or resuscitation dilutes, which is why vital signs and clinical picture lead early resuscitation, not the CBC.

For informational purposes only, not medical advice. Consult a healthcare professional.