BYTETOOLS

Blood Volume Calculator

Estimate total blood volume with Nadler's equations and Gilcher's rule of five, plus red cell mass, plasma volume and maximum allowable blood loss.

Patient

5.32 L
Blood volume (Nadler)
5,319
Blood volume (mL)
2,394 mL
Red cell mass
1,773 mL
Max allowable blood loss

Nadler total blood volume

5.32 L

0.3669 × h³ + 0.03219 × w + 0.60410.3669 × 1.80³ + 0.03219 × 80.0 + 0.6041 = 5.319 L
Per kilogram66.5 mL/kg
Plasma volume (1 − Hct)2,925 mL
Red cell mass (TBV × Hct)2,394 mL

Gilcher's rule of five

HabitusMale mL/kgFemale mL/kgVolume at 80.0 kg
Obese65605,200 mL
Thin70655,600 mL
Normal75706,000 mL
Muscular80756,400 mL

Gilcher's rule of five is the quick bedside alternative to Nadler: pick the habitus, multiply by weight. For this patient it gives 75 mL/kg = 6,000mL, against Nadler's 66.5 mL/kg = 5,319 mL.

Maximum allowable blood loss

Formula5,319 × (45.0 − 30.0) ÷ 45.0
Maximum allowable blood loss1,773 mL
As a share of blood volume33.3%
Blood volume remaining at target3,546 mL of red-cell-equivalent circulating volume

This is the classic EBV × (Hct start − Hct target) ÷ Hct start form. It assumes isovolaemic replacement with crystalloid or colloid — that circulating volume is kept constant while red cells are diluted out — which is a simplification of real theatre losses.

Nadler's equations were derived by Nadler, Hidalgo and Bloch in 1962 from radioisotope dilution measurements in 224 healthy adults, so they describe healthy adult physiology. They are not validated in children, in pregnancy, in morbid obesity or in patients with large third-space losses, and paediatric practice normally uses a straight mL/kg rule instead (roughly 80–90 mL/kg in a neonate falling to 70 mL/kg by adulthood).

Estimates only, not medical advice, and never a transfusion decision on its own. Haematocrit lags acute bleeding by hours, so a maximum allowable blood loss figure calculated from an admission haematocrit will overstate the reserve in someone who is actively bleeding. Real transfusion thresholds are set by physiology, symptoms and local protocol — most modern guidance uses a restrictive haemoglobin trigger rather than a haematocrit target. Check every number against your own institution's policy.

What is the Blood Volume Calculator?

Nadler's equations estimate total blood volume in litres from height and weight: 0.3669 × height in metres cubed + 0.03219 × weight in kg + 0.6041 for men, and 0.3561 × height cubed + 0.03308 × weight + 0.1833 for women.

  • Nadler's male and female equations with the substitution shown in full
  • Gilcher's rule of five as a four-row comparison table
  • Red cell mass and plasma volume split out from the haematocrit
  • Maximum allowable blood loss to any target haematocrit
  • Metric and imperial entry with automatic conversion
  • Runs offline in the browser — no patient data is transmitted

How to use the Blood Volume Calculator

  1. 1

    Choose metric or imperial units, then enter height, weight and sex.

  2. 2

    Pick a body habitus — obese, thin, normal or muscular — which drives the Gilcher rule-of-five comparison only.

  3. 3

    Enter the starting haematocrit to break the volume into red cell mass and plasma volume.

  4. 4

    Enter a lower target haematocrit to get the maximum allowable blood loss.

  5. 5

    Read the Nadler volume, the Gilcher table and the blood loss calculation, each with its working shown.

About the Blood Volume Calculator

The ByteTools Blood Volume Calculator estimates total blood volume from height, weight and sex using Nadler's equations, then breaks it down into red cell mass and plasma volume from a haematocrit and works out the maximum allowable blood loss to a target haematocrit. Gilcher's rule of five is shown alongside as the quick bedside cross-check.

It is built for anaesthetics and transfusion teaching, for perfusion and apheresis arithmetic, and for anyone who needs the working laid out rather than hidden. Every derived figure prints the substitution used, and any step that would need a divide-by-zero renders a dash with an explanation instead.

All calculations happen in your browser and nothing is uploaded. These are estimates only, not medical advice: Nadler's coefficients were derived in healthy adults and are not validated in children, pregnancy or morbid obesity, and no transfusion decision should ever rest on a calculated number.

Frequently asked questions

How much blood does the average adult have?

Roughly 70 mL per kilogram in an adult man and 65 mL per kilogram in an adult woman, which works out at about 5 litres for a 70 kg adult. Nadler's equations refine that by taking height into account as well, because tall and short people of the same weight do not have the same blood volume.

What is Nadler's formula for blood volume?

For men, total blood volume in litres is 0.3669 × height in metres cubed, plus 0.03219 × weight in kilograms, plus 0.6041. For women it is 0.3561 × height cubed, plus 0.03308 × weight, plus 0.1833. Both come from radioisotope dilution measurements published in 1962.

How do you calculate maximum allowable blood loss?

Multiply estimated blood volume by the starting haematocrit minus the target haematocrit, then divide by the starting haematocrit. It assumes circulating volume is kept constant with crystalloid or colloid while red cells are diluted out, which is a simplification of what happens in theatre.

What is Gilcher's rule of five?

A bedside shortcut that assigns millilitres per kilogram by body habitus: 60 or 65 for an obese patient, 65 or 70 for a thin one, 70 or 75 for a normal build and 75 or 80 for a muscular one, with the lower number in each pair for women. It is faster than Nadler and usually close enough for a quick estimate.

Does this work for children?

No. Nadler's equations were derived in adults, and paediatric practice uses a straight millilitres-per-kilogram rule instead, roughly 80–90 mL/kg in a neonate falling toward the adult 70 mL/kg. Use a paediatric reference rather than this tool for anyone still growing.

Why is haematocrit unreliable in acute bleeding?

Haematocrit is a concentration, not a quantity, and it takes hours to fall after acute haemorrhage as fluid shifts into the vascular space. A maximum allowable blood loss figure calculated from an admission haematocrit therefore overstates the reserve in someone who is actively bleeding.

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