What is the Rule of 3 in transfusion medicine?

The Rule of 3 states that each unit of packed red blood cells (pRBC) raises hemoglobin by approximately 1 g/dL and hematocrit by approximately 3% in a 70 kg adult with stable bleeding. It is a quick bedside estimation tool, not a replacement for post-transfusion lab values.

Does this apply to pediatric patients?

This calculator applies a weight-adjustment for patients who differ from the standard 70 kg reference. However, pediatric transfusions are typically ordered by volume (mL/kg), not units. Always follow your institution's pediatric transfusion protocol.

When does the Rule of 3 not apply?

Active hemorrhage, hemolysis, hemodilution, splenomegaly, or a patient who is not at steady state will invalidate the prediction. Post-transfusion CBC should always be checked after 15-30 minutes.

What is the relationship between Hgb and Hct?

Clinically, Hct (%) is approximately 3 times Hgb (g/dL). This calculator uses that relationship when you enter one value and it derives the other, and to show expected changes in both units.

Can I enter hematocrit instead of hemoglobin?

Yes. Use the Hct input field. The calculator will convert it to Hgb internally (Hgb = Hct / 3) and display results in both units.

Rule of 3 Hemoglobin

Name: Rule of 3 Hemoglobin
Availability: InStock
Author: Nham Vu

Enter current hemoglobin (or hematocrit), units of pRBC to transfuse, and patient weight to predict post-transfusion Hgb and Hct using the Rule of 3.

Patient & Transfusion Data

Current Hemoglobin (g/dL)

Enter Hgb or Hct — the other auto-fills.

Current Hematocrit (%)

Units of pRBC to Transfuse

Patient Weight

Transfusion Target Hgb (g/dL) (optional)

Enter patient data to predict post-transfusion Hgb and Hct.

Expected Hgb After

g/dL

Expected Hct After

Per-Unit Rise (Weight-Adjusted)

g/dL per unit

% Hct per unit

Cumulative Hgb by Unit

Units	Hgb (g/dL)	Hct (%)

Clinical note: The Rule of 3 is an estimation only. Active hemorrhage, hemolysis, or hemodilution invalidate this prediction. Always obtain a post-transfusion CBC 15-30 minutes after completion.

Summary

Enter current hemoglobin (or hematocrit), units of pRBC to transfuse, and patient weight to predict post-transfusion Hgb and Hct using the Rule of 3.

How it works

The basic Rule of 3 assumes a standard 70 kg adult. For weight-adjusted calculations, the expected Hgb rise per unit is scaled by the ratio of a standard 70 kg blood volume to the patient's estimated blood volume. The tool also converts between Hgb and Hct using the standard relationship Hct (%) = Hgb (g/dL) x 3.

Use cases

Quickly estimate post-transfusion Hgb before ordering blood products.
Plan transfusion targets for anemic patients in the ICU or perioperative setting.
Verify expected response after transfusing a specific number of pRBC units.
Teach medical students and residents the Rule of 3 at the bedside.
Cross-check weight-adjusted transfusion calculations in pediatric patients.

Frequently Asked Questions

Last updated: 2026-06-09 · Reviewed by Nham Vu