Plasma volume is the liquid, cell-free portion of total blood volume — the complement to red cell volume. Clinicians and apheresis teams use estimated plasma volume to plan fluid-management decisions and to size plasma removal or exchange volumes. This calculator computes plasma volume from a blood volume and hematocrit you supply directly, or derives blood volume first via Nadler's formula from height, weight, and sex.

How the plasma volume formula works

Hematocrit is defined as the fraction of total blood volume occupied by red blood cells. Because blood volume has only two components — plasma and red cells — the plasma fraction is simply whatever's left: 1 minus the hematocrit fraction. Multiplying that fraction by total blood volume gives plasma volume: Plasma volume = Blood volume × (1 − Hct/100). Red cell volume follows directly as the remainder: Blood volume − Plasma volume, or equivalently Blood volume × (Hct/100).

Inputs and what they mean

Blood volume is entered in liters, either as a known or previously measured value, or estimated via the From Height/Weight tab using Nadler's formula — the same sex-specific equation used in this site's Blood Volume Calculator. Hematocrit is entered as a percentage (e.g. 45 for 45%, not 0.45). A typical adult hematocrit runs roughly 40-50% for men and 36-44% for women; values well outside that range (severe anemia, polycythemia, or a data-entry error) are flagged with a warning.

Limits and edge cases

This calculator uses the venous hematocrit exactly as entered. The true whole-body hematocrit is typically slightly lower than venous hematocrit — red cells aren't distributed perfectly evenly between large vessels and the microcirculation — and precise research applications sometimes apply a correction factor (the F-cell ratio, roughly 0.91) to convert venous to whole-body hematocrit. This calculator does not apply that correction, which is a reasonable simplification for routine fluid-management and apheresis planning but a limitation for research-grade precision. As with any formula-based estimate, the result is a planning figure, not a substitute for direct measurement or clinical judgment.