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Maintenance IV Fluids (4-2-1 Rule) Calculator

Calculates hourly fluid requirement based on body weight.

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Hourly Fluid Rate

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Overview

The 4-2-1 rule is a physiological heuristic used to calculate the hourly rate of maintenance intravenous fluids for pediatric patients based on their body weight. It estimates the volume required to replace daily insensible water loss and urinary output, ensuring electrolyte stability and hydration in patients who cannot take fluids orally.

Symbols

Variables

Rate = Hourly Fluid Rate, W = Patient Weight

Hourly Fluid Rate
Patient Weight

Apply it well

When To Use

When to use: This rule is primarily used for clinically stable pediatric patients who are fasting, such as those awaiting surgery or unable to tolerate oral intake. It assumes normal renal and cardiac function and should not be used in cases of significant dehydration or fluid overload.

Why it matters: Proper fluid management prevents life-threatening complications like hyponatremia, dehydration, and metabolic imbalances. In pediatrics, where body surface area to weight ratios are high, precision in calculating maintenance rates is critical for patient safety and recovery.

Avoid these traps

Common Mistakes

  • Using it for fluid resuscitation or bolus instead of maintenance.
  • Applying to severe kidney or heart failure patients without modification.

One free problem

Practice Problem

Calculate the hourly maintenance fluid rate for an infant weighing 7 kg.

Patient Weight7 kg

Solve for:

Hint: For the first 10 kg of body weight, the rate is 4 mL per kg per hour.

The full worked solution stays in the interactive walkthrough.

References

Sources

  1. Holliday, M. A., & Segar, W. E. (1957). The maintenance need for water in parenteral fluid therapy. Pediatrics, 19(5), 823-832.
  2. Nelson Textbook of Pediatrics
  3. Wikipedia: Holliday-Segar formula
  4. Basic Clinical Anesthesia by Stoelting and Miller
  5. Wikipedia: Maintenance intravenous fluid
  6. Kliegman, R. M., St. Geme, J. W., Blum, N. J., Shah, S. S., Tasker, R. C., & Wilson, K. M. (2020). Nelson Textbook of Pediatrics (21st ed.).
  7. The Harriet Lane Handbook: A Manual for Pediatric House Officers (22nd ed.). (2021). Elsevier.
  8. Advanced Paediatric Life Support: The Practical Approach (APLS) (5th ed.). (2017). Wiley Blackwell.