Haematocrit from Haemoglobin Calculator
Haematocrit from Haemoglobin Calculator
Estimate the haematocrit as three times the haemoglobin — the rule of three — and see the anaemias in which that approximation stops holding.
Estimated haematocrit
Hb × 3 → HctHaemoglobin 13.8 g/dL
The rule of three
and, in the full rule: haemoglobin (g/dL) ≈ 3 × red cell count (×10¹²/L)
- hb
- haemoglobin in g/dL. If your report is in g/L, divide by 10 before entering it
- × 3
- follows from haematocrit = haemoglobin ÷ MCHC, with a normal MCHC of roughly 33 g/dL. The factor is really 100 ÷ MCHC, and it is only 3 when the MCHC is normal
- MCHC
- mean corpuscular haemoglobin concentration — the haemoglobin per unit of packed red cell volume. Any departure from about 33 g/dL breaks the approximation
Worked example
Haemoglobin 13.8 g/dL
13.8 × 3 = 41.4%
Between 36 and 48 → within the usual adult range
This is an estimate: use the analyser's measured haematocrit when one is reported
The full rule of three
| Relationship | Example | Holds when |
|---|---|---|
| Haemoglobin ≈ 3 × red cell count | RBC 4.6 ×10¹²/L → Hb ≈ 13.8 g/dL | MCH is normal, around 30 pg |
| Haematocrit ≈ 3 × haemoglobin | Hb 13.8 g/dL → Hct ≈ 41.4% | MCHC is normal, around 33 g/dL |
| Haematocrit ≈ 9 × red cell count | RBC 4.6 ×10¹²/L → Hct ≈ 41% | Both of the above hold |
Where the factor of three stops being three
| Situation | MCHC | True haematocrit |
|---|---|---|
| Iron deficiency, thalassaemia trait | Low (hypochromic cells) | Higher than 3 × Hb — the rule underestimates it |
| Normal red cell indices | About 33 g/dL | Close to 3 × Hb |
| Hereditary spherocytosis | High | Lower than 3 × Hb — the rule overestimates it |
| Cold agglutinins, severe lipaemia | Spuriously high | Unreliable — the analyser result needs review, not an estimate |
What the rule of three assumes, and when it fails
This is an approximation, not a measurement. It works because the haematocrit is the haemoglobin divided by the mean corpuscular haemoglobin concentration, and a normal MCHC is about 33 g/dL — so the multiplier is really 100 divided by the MCHC, and it equals three only when the MCHC is normal. Stated that way, the limitation of the rule becomes obvious: it holds in exactly the patients whose red cell indices are unremarkable, and drifts in the patients whose indices are not.
The direction of the error is worth getting right. In iron deficiency and thalassaemia the cells are small and pale, the MCHC is reduced, and the haematocrit falls proportionally less than the haemoglobin — so the true haematocrit is higher than three times the haemoglobin and the rule underestimates it. A raised MCHC pushes it the other way: in hereditary spherocytosis, and spuriously with cold agglutinins or severe lipaemia, the true haematocrit is lower than three times the haemoglobin. Assuming the error runs in the opposite direction is a common and avoidable mistake.
The full rule of three has a second limb that is often forgotten: the haemoglobin in g/dL is roughly three times the red cell count in ×10¹²/L, which in turn assumes a normal mean corpuscular haemoglobin of about 30 pg. Chained together, the two give a haematocrit of roughly nine times the red cell count. The chain is most useful as an internal consistency check on a full blood count: when the three numbers do not line up, either the indices are genuinely abnormal or one of the results is spurious, and both are worth knowing.
In practice, modern analysers either measure the haematocrit directly by impedance summation of red cell volumes or compute it as the red cell count multiplied by the MCV, and that value should be used whenever it is available. The rule of three earns its place when a haematocrit has not been reported, when a point-of-care haemoglobin is all you have, or as a rapid check that a reported set of numbers is internally coherent. It is not the value on which to base a transfusion or venesection decision.
Frequently asked questions
How do you estimate haematocrit from haemoglobin?
Multiply the haemoglobin in g/dL by three. A haemoglobin of 13.8 g/dL gives an estimated haematocrit of about 41.4%. It is an approximation, and the measured haematocrit should be used when one is reported.
Why the factor of three?
Because haematocrit equals haemoglobin divided by the mean corpuscular haemoglobin concentration, and a normal MCHC is about 33 g/dL. The multiplier is really 100 divided by the MCHC, so it is three only when the MCHC is normal.
When does the rule of three break down?
Whenever the MCHC is abnormal. In iron deficiency and thalassaemia the MCHC is low and the true haematocrit is higher than three times the haemoglobin. With a high MCHC, as in hereditary spherocytosis, it is lower.
What is the full rule of three?
Haemoglobin in g/dL is roughly three times the red cell count in ×10¹²/L, and the haematocrit is roughly three times the haemoglobin. Together they give a haematocrit of about nine times the red cell count.
Should I use this instead of the analyser's haematocrit?
No. Analysers measure or compute the haematocrit directly and that value is the one to use. This estimate is for occasions when no haematocrit has been reported, or as a quick check that a full blood count is internally consistent.
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References
- Bain BJ, Bates I, Laffan MA, eds. Dacie and Lewis Practical Haematology. Elsevier — red cell indices, packed cell volume and automated blood counting.
- Clinical and Laboratory Standards Institute. Reference and Selected Procedure for the Quantitative Determination of Hemoglobin in Blood. CLSI standard H15.