Sodium Deficit Calculator

Enter Calculation

Patient Type (TBW factor) Adult Male / Child (0.6) Adult Female / Elderly Male (0.5) Elderly Female (0.45)

Body Weight (kg)

Actual Serum Sodium (mEq/L)

Target Serum Sodium (mEq/L)

Results

Sodium Deficit

840

mEq of sodium

Total Body Water	42 L
Sodium Gap (target − actual)	20 mEq/L

What Is the Sodium Deficit Calculator?

This tool estimates the total sodium deficit (in milliequivalents, mEq) in a patient with hyponatremia. It multiplies an estimate of total body water (TBW) by the difference between the desired (target) and measured (actual) serum sodium concentration. It is intended as an educational and clinical estimation aid only and does not replace bedside judgement, repeat lab measurements, or correction-rate safety limits.

How to Use It

Select the total body water factor that matches your patient: roughly 0.6 for adult males and children, 0.5 for adult females and elderly males, and 0.45 for elderly females. Enter the patient's body weight in kilograms, the actual (measured) serum sodium, and the target serum sodium you wish to reach. The calculator returns the sodium deficit in mEq plus the intermediate total body water and sodium gap.

The Formula Explained

The relationship used is $$\text{Na deficit} = (\text{TBW factor} \times \text{weight}) \times (\text{target Na} - \text{actual Na})$$. The first part, \(\text{TBW factor} \times \text{weight}\), estimates total body water in litres. Multiplying litres by a concentration difference in mEq/L gives the total milliequivalents of sodium required to shift the serum concentration to the target.

The sodium deficit combines total body water with the gap between target and actual serum sodium.

Worked Example

A 70 kg adult male (TBW factor 0.6) has an actual serum sodium of 120 mEq/L and a target of 140 mEq/L. Total body water = \(0.6 \times 70 = 42\) L. Sodium gap = \(140 - 120 = 20\) mEq/L. Deficit = \(42 \times 20 =\) 840 mEq of sodium.

The deficit depends on the difference between target and actual serum sodium.

FAQ

Does this tell me the infusion rate? No. It estimates total deficit only. Correction must respect safe rates (commonly \(\leq\) 8–10 mEq/L in 24 hours) to avoid osmotic demyelination.

Which TBW factor should I choose? Use 0.6 for most males/children, 0.5 for females and elderly males, and 0.45 for elderly females, reflecting lower body-water fraction with age and sex.

Can the result be negative? Yes — if the target is below the actual sodium, the deficit becomes negative, indicating a surplus relative to your target.