Estimate plasma volume from weight and hematocrit using male- and female-coefficient models.
Model inputs
Enter weight and hematocrit
Plasma
Model summary
Estimated plasma volumes
The estimates will appear here
Enter hematocrit as a percentage from 0 to 100.
Male-coefficient estimateweight kg × blood-volume coefficient × (1 − hematocrit)
Male coefficient
0 mL
Female coefficient
0 mL
Educational estimate only. It is not medical advice, a diagnosis, or a substitute for assessment by a qualified healthcare professional. These are model estimates, not measured plasma volume or guidance for fluids or transfusion.
This page gives a rough plasma-volume estimate from body weight and hematocrit. It first estimates total blood volume with simple weight factors—70 millilitres per kilogram for males and 65 millilitres per kilogram for females—then removes the fraction attributed to red blood cells. Both estimates are displayed because the form does not ask for sex.
This is educational arithmetic, not a measured plasma volume. Body size, pregnancy, illness, fluid balance and other clinical factors can make a simple millilitres-per-kilogram estimate unsuitable. A clinician should decide whether any estimate is relevant to an individual case.
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How to Use This Calculator
Enter body weight and choose kilograms, pounds or stone. The page converts pounds and stone to kilograms before applying its blood-volume assumptions.
Enter hematocrit as a percentage. For example, enter 42 for 42%, not 0.42. The form accepts values from 0 through 100.
Select Calculate. The result shows separate male and female estimates in millilitres.
The two numbers come from different fixed blood-volume factors. They are broad population rules and do not measure the person’s actual circulating volume. Use consistent, current measurements if you are checking the arithmetic with a health professional.
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Formula Used
Male estimate = weight in kg × 70 mL/kg × (1 − hematocrit ÷ 100).
Female estimate = weight in kg × 65 mL/kg × (1 − hematocrit ÷ 100).
The final factor represents the non-red-cell share of estimated blood volume. The calculation assumes hematocrit is a percentage and does not apply any correction for trapped plasma, body composition or changes in red-cell volume.
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Variables Explained
Input or value
Meaning
Weight
Body mass in the selected unit. Pounds are converted with 0.45359237 kg per pound; the Stone option is converted with 6.35029318 kg per stone.
Hematocrit (%)
The percentage of blood volume occupied by red blood cells in the sample. It must be entered on the 0–100 percentage scale.
70 or 65 mL/kg
The page’s fixed total-blood-volume assumptions for the male and female estimates.
Plasma volume
The calculated non-red-cell portion of estimated blood volume, displayed in millilitres.
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Worked Example
For a weight of 70 kg and hematocrit of 40%, the non-red-cell fraction is 1 − 0.40 = 0.60.
Male estimate: 70 × 70 × 0.60 = 2,940 mL.
Female estimate: 70 × 65 × 0.60 = 2,730 mL.
The page returns “2940 mL” and “2730 mL.” Those values illustrate the selected assumptions. They should not be read as a laboratory measurement or as evidence that one estimate applies to a particular patient.
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Why Use This Calculator?
The result is best understood as a transparent estimate built from two inputs and a fixed factor. Clinical blood-volume methods can account for height, body composition or other characteristics, and direct measurement can differ from any formula. Hematocrit itself can change with hydration and sampling conditions.
Do not use this number to decide on transfusion, fluid treatment, medicine dosing or whether to seek care. Those decisions need clinical assessment, measurements and context. If a result is being used in study or research, record the exact formula rather than calling it a universal plasma volume.
Because the calculation returns millilitres, a value such as 2,940 mL can also be written as 2.94 L. Changing the display unit does not improve the estimate. Keep units visible whenever discussing the figure so it is not mistaken for total blood volume.
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Common Mistakes
Enter hematocrit as 40 for 40%; entering 0.40 would mean 0.40% on this form.
Choose the correct weight unit. The option labelled Stone is a unit of 14 pounds, not a US ton.
Do not treat the male and female figures as a range around a measured value. They are separate rule-based estimates.
A current scale weight and a hematocrit taken at another time may reflect different fluid states.
Do not infer a diagnosis from a high or low estimate. The page does not compare the result with a clinical reference interval.
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Frequently Asked Questions
Is this a direct measurement of plasma volume?
No. It is an estimate made from weight, hematocrit and fixed blood-volume factors. Direct measurements and more detailed equations may produce different values.
Why are two results shown?
The page applies 70 mL/kg for the male estimate and 65 mL/kg for the female estimate. The form does not choose which assumption fits the person.
How should I enter a hematocrit of 42 percent?
Enter 42. The form is explicitly a percentage field and divides the number by 100 during the calculation.
Can this estimate guide a transfusion?
No. Transfusion and fluid decisions require a qualified clinician, appropriate testing and the full clinical situation. This simple estimate cannot supply that judgment.
Why might dehydration affect interpretation?
Fluid balance can change plasma volume and hematocrit. A value measured during one fluid state may not represent another time, even when body weight looks similar.
Does the Stone choice mean a US ton?
No. Despite the stored legacy option name, the visible choice is Stone. One stone is converted as 6.35029318 kilograms.
Can children use the same assumptions?
The displayed factors are simple adult-style rules and may be inappropriate for children or infants. A pediatric clinician should select an age-appropriate method.