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The "rule of three" is a widely-taught rough clinical approximation: Hematocrit (%) is roughly 3x Hemoglobin (g/dL) when red cells are of normal size and hemoglobin content. This is an approximation only, commonly used as a quick sanity check that a reported Hct and Hgb are internally consistent — it is not a substitute for the actual lab-measured value, and can be inaccurate with abnormal red cell size/content. For general reference and educational purposes only — not a medical diagnosis.
How it works
Choose which value you know (hemoglobin or hematocrit) and enter it. The tool multiplies or divides by 3 to estimate the other value.
- Enter which value do you know?.
- Enter known value.
- Click Calculate to see your results.
Examples
Hemoglobin 15 g/dL
Estimated Hematocrit ≈ 15 x 3 = 45%.
Who should use it
- Quickly sanity-checking a lab report.
- Hematology education.
Industry applications
- Hematology education
- Clinical teaching reference
Advantages
- Simple, widely-taught rule.
- Useful quick sanity check on lab report consistency.
Limitations
- Inaccurate in microcytic, macrocytic, or other abnormal red-cell conditions.
- Not a substitute for an actual lab measurement.
Common mistakes to avoid
- Treating the rule-of-three estimate as an exact, precise conversion rather than a rough approximation.
Best practices
- Use this only as a quick sanity check — rely on the actual lab-measured value for real decisions.
Tips
- If your actual Hct and Hgb noticeably deviate from the rule-of-three estimate, it may hint at an abnormal red cell size or hemoglobin content — but only a doctor can properly interpret this.