The Body Surface Area Method

Estimating Body Surface Area

The surface area method relates the patient's fluid and electrolyte needs to their body's surface area. The patient's body surface area can be estimated in several ways: by nomogram using height and weight (most accurate), by Mosteller's formula using height and weight or on the basis of weight alone (least accurate).

Estimation based on weight alone (m²)
0-5 kg	0.05 * body weight (kg) + 0.05
6-10	0.04 * body weight (kg) + 0.10
11-20	0.03 * body weight (kg) + 0.20
21-30	0.02 * body weight (kg) + 0.40

Example 1.

A 25 kg child would have an estimated body surface area of:

0.02*25+0.40 = 0.9 m²

Mosteller's Formula
Surface Area (m²)= square root (kg*cm/3600)

Example 2.

A 47 kg girl with a height of 157 cm would have an estimated body surface area of:

sqrt(157*47/3600) = 1.43 m²

Example 3.

A 43 kg male with a height of 50 cm:

BSA of 0.9 m² according to the nomogram

Maintenance Fluid and Electrolyte Requirements

Accounting for Maintenance Fluid Requirements
Insensible Water Loss	600-900 mL/m²/day
Renal Function	600-900 mL/m²/day
Average	1500 mL/m²/day

Electrolyte Requirements
Sodium	52 mEq per m²
Postassium	32 mEq per m²
Chloride	32 mEq per m²

Example 4.

Using the surface area method, calculate the maintenance fluids and electrolytes for a 185 cm tall patient who weighs 55 kilograms:

The surface area is calculated as sqrt(55*185/3600) = 1.68 m²

Thus, this patient would require 2.5 liters of fluid per day, with electrolyte requirements of 87.4 mEq of Na, 53.8 mEq of K, and 53.8 of Cl. Each liter of maintenance fluids should contain 34.7 mEq of Na and 21.3 mEq of K. A reasonable approximation would be ¹/₄NS (38.5 mEq/L of Na) or even ¹/₃NS (51.3 mEq/L of Na). After the patient voids, 20 mEq/L of K can be added to the solution.

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Last modification: April 30, 1998