How this calculator works
The familiar ratio near three uses hematocrit expressed as a percentage and hemoglobin in g/dL. It corresponds to an MCHC near 33.3 g/dL, rather than a universal biological constant. A measured ratio is useful for checking consistency. Estimating one measurement from the other assumes the entered ratio is appropriate; it can fail with abnormal red cells or analytical interference. The result is an estimate for information only and is not medical advice.
Formula
Ratio = hematocrit (%) / hemoglobin (g/dL). Hematocrit (%) = ratio x hemoglobin. Hemoglobin = hematocrit (%) / ratio.
Worked example
Hematocrit 45% and hemoglobin 15 g/dL give a ratio of 3. Hematocrit 36% and hemoglobin 120 g/L (12 g/dL) also give 3.
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Frequently asked questions
Is a ratio of three always correct?
No. It is a rough consistency rule for blood with a usual mean corpuscular hemoglobin concentration.
Why convert hemoglobin to g/dL?
The numerical rule of three is defined with g/dL. Using g/L directly would make the ratio ten times smaller.
Can I replace a laboratory measurement?
No. The inverse modes are estimates under a stated ratio, not a measured blood count.