What is the rule of three?

It is a bedside shortcut stating that the haematocrit in percent is roughly three times the haemoglobin in g/dL. So a haemoglobin of 10 g/dL corresponds to a haematocrit of about 30 percent, and a haematocrit of 45 percent suggests a haemoglobin near 15 g/dL.

Why offer a 2.94 factor as well?

Large datasets show the true haematocrit-to-haemoglobin ratio sits a little below three, around 2.9 to 3.0. Dividing or multiplying by 2.941 gives a slightly more accurate average estimate than the round number, though the difference is small.

Can I enter the haematocrit as a decimal?

Yes. The tool auto-detects a value below one as a fraction and multiplies by 100, so both 0.45 and 45 are read as a 45 percent haematocrit.

When does the conversion fail?

The ratio assumes normally sized red cells. In iron deficiency and thalassaemia the cells are small, and in B12 or folate deficiency they are large, so the haematocrit-to-haemoglobin relationship drifts away from three. Treat the estimate as approximate in any abnormal red cell morphology.

Should I make transfusion decisions from this?

No. This is an educational estimator for when only one of the two values is to hand. Transfusion and anaemia management should always use the laboratory-measured haemoglobin and the full clinical picture.

Haematocrit ↔ Haemoglobin Converter

Haemoglobin and haematocrit both describe how much red cell mass is in the blood, and clinicians often need to move quickly between them when only one is reported. This converter gives the familiar bedside estimate alongside a slightly more accurate regression, and it accepts haematocrit as either a percent or a fraction.

How it works

The two values are tightly linked because haematocrit is the volume fraction occupied by red cells, which carry the haemoglobin. The classic rule of three is the simplest expression:

HCT(%) ~= 3 x Hb(g/dL)      ->  Hb ~= HCT / 3

A more precise factor, drawn from large clinical datasets, sits just below three:

HCT(%) ~= 2.941 x Hb(g/dL)  ->  Hb ~= HCT / 2.941

The tool applies whichever direction you choose and reports both estimates so you can see the spread.

Example and notes

A haematocrit of 45 percent gives a haemoglobin of 15.0 by the rule of three and 15.3 by the regression, a clinically trivial difference. The relationship holds well in normal blood but shifts when red cell size is abnormal: small cells in iron deficiency and thalassaemia lower the ratio, large cells in B12 or folate deficiency raise it. For any real decision, especially transfusion, rely on the laboratory-measured value rather than a conversion.