How this instrument works
Plasma is the straw-colored liquid component of blood, carrying dissolved proteins, clotting factors, electrolytes, and anything circulating in the bloodstream that is not a cell. It makes up most of blood by volume but not all of it — red cells, white cells, and platelets occupy the rest, and the share they take up is exactly what hematocrit measures. This instrument starts from Nadler's 1962 equation for total blood volume — the same formula behind this site's separate Blood Volume calculator — then subtracts out the hematocrit fraction to isolate plasma alone.
Nadler, Hidalgo and Bloch published two regressions in Surgery, one fitted to men and one to women, each built from height cubed and weight rather than a simpler linear combination, because vascular capacity scales more closely with body volume than with mass alone. Total blood volume comes out of that equation first; multiplying it by one minus the hematocrit fraction — entered here as a percentage and divided by 100 inside the arithmetic — removes the portion occupied by cells and leaves the plasma figure.
Total blood volume and plasma volume answer different clinical questions from the same starting number. Total volume matters for transfusion planning, fluid resuscitation, or sizing an extracorporeal circuit, where whole blood, cells included, is what is being moved or replaced. Plasma volume matters specifically where only the liquid fraction is relevant — estimating the distribution volume of a water-soluble drug, or interpreting a lab value reported per litre of plasma rather than per litre of whole blood — contexts where counting the red cells would give the wrong denominator entirely.
- Set Sex — Nadler's total blood volume equation uses separate constants for men and women.
- Enter Height in metres.
- Enter Weight in kilograms.
- Enter Hematocrit as a percentage; read Total blood volume and Plasma volume in litres, the second always smaller than the first by exactly the hematocrit's share.
Worked example — 1.75 m, 75 kg man, hematocrit 45%
A 1.75 m, 75 kg man has a hematocrit of 45%. Height cubed: 1.75 × 1.75 × 1.75 = 5.359375, times 0.3669 gives about 1.9664. Weight contributes 0.03219 × 75 ≈ 2.4143, and the male constant adds 0.6041. Summed, total blood volume comes to about 4.98 litres. Multiplying by (1 − 45/100), or 0.55, gives a plasma volume of about 2.74 litres — a little over half the total, since red cells and other formed elements make up the other 45%.
A smaller 1.65 m, 60 kg woman with a lower 40% hematocrit works out differently on both counts: total blood volume of about 3.77 litres, and plasma volume of about 2.26 litres once the 40% cell fraction is removed. A larger 1.90 m, 95 kg man with a higher 50% hematocrit has the biggest total blood volume of the three, about 6.18 litres, but because half of that volume is cells rather than liquid, his plasma volume comes to about 3.09 litres — proportionally the smallest plasma share of the three examples, despite having the largest body.
Questions
How is this different from this site's Blood Volume calculator?
They share the same starting point — Nadler's 1962 equation for total blood volume from height, weight, and sex — but this instrument takes one further step, multiplying that total by one minus the hematocrit fraction to isolate the plasma alone. The Blood Volume calculator answers 'how much blood in total'; this one answers 'how much of that is liquid, once the cells are set aside.'
Why does a higher hematocrit produce a lower plasma volume, even in a larger person?
Because hematocrit is the fraction of blood occupied by cells rather than liquid, and plasma volume is calculated as total blood volume multiplied by everything left over after that fraction is removed. A large person with a high hematocrit can have a bigger total blood volume than a smaller person with a lower hematocrit, yet end up with a similar or smaller plasma volume, because a bigger share of their larger total is cells rather than liquid.
What is plasma volume actually used for clinically?
It matters wherever a calculation needs the liquid compartment specifically rather than whole blood — estimating the distribution volume of drugs that stay largely within plasma, interpreting certain lab values reported per litre of plasma, or working out dilutional effects when large volumes of fluid or plasma products are given. Whole blood volume, by contrast, is the more relevant figure for transfusion thresholds or maximum allowable blood loss during surgery.
How accurate is a Nadler-derived plasma volume for one specific patient?
It inherits the accuracy of the underlying Nadler equation, which tracked directly measured blood volume to within roughly 10 percent for adults of ordinary build in its original validation, and layers on whatever measurement error exists in the hematocrit entered. Anyone unusually muscular, obese, dehydrated, pregnant, or acutely ill can sit outside that band on either the blood-volume or the hematocrit side, so the result is a planning estimate rather than a lab-grade figure.
Does dehydration or recent blood loss change this result?
Not automatically — this calculator only sees height, weight, sex, and whatever hematocrit value is entered. If a current, freshly drawn hematocrit is entered, the plasma-volume figure will reflect that day's concentration reasonably well, since hematocrit itself shifts quickly with hydration status and blood loss. Entering an old or averaged hematocrit instead will understate or overstate how much has actually changed.
References
- Nadler SB, Hidalgo JH, Bloch T, 1962, Surgery — total blood volume regression equations (PubMed)
- Sandefur CC, Anjum F — Plasma Volume Study, StatPearls / NCBI Bookshelf
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.