Haematocrit Unit Converter
Haematocrit Unit Converter
Convert haematocrit between % and L/L — 45% is 0.45 L/L — and read it as what it is: a concentration that most analysers calculate rather than measure, and that fluid moves without a red cell being gained or lost.
Haematocrit converter
Fraction unitsHaematocrit 45% (the page opens in %, as most reports print it)
The fraction ladder
Haematocrit (L/L) = haematocrit (%) ÷ 100
and, on most analysers: haematocrit = MCV × RBC
- % and L/L
- the same dimensionless fraction on two scales. 45% is 0.45 L/L. A percentage is parts per hundred and L/L is litres of packed red cells per litre of whole blood, so the only difference is the factor of 100
- mL/dL
- identical to the percentage. A millilitre per decilitre is one part in a hundred, so a haematocrit of 45% is 45 mL/dL and no conversion is involved
- no molar unit
- a volume fraction has no molecular weight. There is no mmol/L haematocrit, and a report offering one has confused it with haemoglobin
- calculated, not measured
- modern analysers do not spin a sample down. They multiply the mean cell volume by the red cell count, so the haematocrit inherits both of those measurements' errors — and any cause of a spurious MCV, such as cold agglutinins or marked hyperglycaemia, moves the haematocrit with it
Worked example
Haematocrit 45% (the page opens in %, as most reports print it)
45 ÷ 100 = 0.45 L/L
45% = 45 mL/dL — a millilitre per decilitre is one part in a hundred, so those two never differ
0.45 L/L sits inside the 0.40–0.54 L/L interval quoted here for adult men
On most analysers that number was never measured: it is the MCV multiplied by the red cell count, so it is a derived value carrying two other measurements' errors
The same haematocrit in every unit
| Unit | Relationship to L/L | 0.45 L/L is |
|---|---|---|
| L/L | The SI convention — litres of red cells per litre of blood | 0.45 |
| proportion | Identical; the bare decimal some reports print with no unit | 0.45 |
| % | × 100 | 45.0 |
| mL/dL | × 100; a mL/dL is one part in a hundred, so identical to % | 45.0 |
| PCV | Not a unit — packed cell volume is the older name for the same measurement | 0.45 or 45% |
Things that move the haematocrit without changing red cell mass
| Situation | Direction | Why |
|---|---|---|
| Dehydration | Rises | Plasma volume falls, cells are concentrated into less fluid |
| Crystalloid or colloid resuscitation | Falls | Plasma volume expands, the same cells are diluted |
| Pregnancy | Falls | Physiological plasma expansion outpaces the rise in red cell mass |
| The first hours of acute haemorrhage | Unchanged | Whole blood is lost in the proportion it is stored in; the fall appears over hours as plasma volume is restored |
| Diuresis | Rises | Plasma volume contracts |
| Spurious MCV — cold agglutinins, marked hyperglycaemia | Either | Analysers calculate haematocrit as MCV × RBC, so an artefactual MCV carries straight through |
A calculated concentration, in two units that differ by a factor of 100
The arithmetic is the easy half. Haematocrit is a dimensionless fraction — the proportion of whole blood occupied by red cells — so the SI form in litres per litre and the conventional percentage differ only by a factor of 100. A haematocrit of 45% is 0.45 L/L, and mL/dL is the percentage under another name, since a millilitre per decilitre is one part in a hundred. Some reports print the bare decimal with no unit at all, which is the same proportion again. There is no molar unit: a volume fraction has no molecular weight, and a report offering haematocrit in mmol/L has confused it with haemoglobin.
The harder half is what the number is. On almost every modern analyser the haematocrit is not measured. The instrument counts the red cells and sizes them, and multiplies the mean cell volume by the count; the spun microhaematocrit that gave the measurement its name is now a reference method rather than a routine one. That matters because the calculated value inherits the errors of both inputs. Anything that makes the MCV spurious — cold agglutinins clumping cells so they are counted as one large one, marked hyperglycaemia swelling cells in the diluent — moves the haematocrit with it, and the plasma trapping that makes a spun value read slightly high is absent, so the two methods do not agree exactly even when both are working.
More importantly, a haematocrit is a concentration. It is red cells as a fraction of whatever plasma volume happens to be present, so the denominator moves it as readily as the numerator does. It falls with crystalloid resuscitation, with the plasma expansion of pregnancy and with fluid overload, in each case without a single red cell having been lost. It rises with dehydration and diuresis without a single one having been gained. A haematocrit taken during a resuscitation is partly a report on the fluid that has been given.
The most dangerous version of that is acute haemorrhage. Whole blood is lost in the proportion it is stored in, so in the first hours after a major bleed the haematocrit can be entirely normal — cells and plasma have gone in the same ratio, and nothing has yet diluted what remains. The fall appears over hours as plasma volume is restored, from the interstitium or from the giving set. A normal haematocrit in a shocked patient is therefore expected rather than reassuring, and the decision to transfuse is made on the patient, the losses and the trend, not on a single number.
Frequently asked questions
How do I convert haematocrit from % to L/L?
Divide by 100. A haematocrit of 45% is 0.45 L/L. Going the other way, multiply the L/L figure by 100. Millilitres per decilitre is the same number as the percentage, so 45% is also 45 mL/dL.
Is haematocrit measured or calculated?
Calculated, on almost every modern analyser: the mean cell volume multiplied by the red cell count. The spun microhaematocrit that named the test is now a reference method. The calculated value therefore carries the errors of both inputs, including any cause of a spurious MCV.
Is haematocrit the same as packed cell volume?
In current use, yes — haematocrit, Hct and PCV name the same quantity and reports use the terms interchangeably. Historically PCV meant the centrifuged value and haematocrit the analyser’s calculated one, and the two differ slightly because centrifugation traps a little plasma among the cells.
Why does the haematocrit fall after fluid resuscitation?
Because it is a concentration. Giving crystalloid expands the plasma volume, so the same red cells occupy a smaller fraction of a larger total. Nothing has been lost. The same mechanism lowers it in pregnancy, and dehydration raises it for the mirror-image reason.
Can the haematocrit be normal straight after a major bleed?
Yes, and it usually is. Whole blood is lost in the proportion it is stored in, so the ratio of cells to plasma is unchanged at first. The haematocrit falls over the following hours as plasma volume is restored, which is why it lags the bleed and should not be used to decide whether one is happening.
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References
- Tefferi A, Hanson CA, Inwards DJ. How to interpret and pursue an abnormal complete blood cell count in adults. Mayo Clin Proc. 2005;80(7):923–936.
- Bain BJ, Bates I, Laffan MA, eds. Dacie and Lewis Practical Haematology. 12th ed. Elsevier; 2017 — packed cell volume and the calculated haematocrit.
- Carson JL, Stanworth SJ, Guyatt G, et al. Red blood cell transfusion: 2023 AABB international guidelines. JAMA. 2023;330(19):1892–1902.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.