What is free water deficit?

Free water deficit is the amount of water needed to restore normal serum sodium. It's calculated based on total body water (TBW) and the ratio of current to normal sodium. Used primarily for hypernatremia management.

How fast should I correct hypernatremia?

Correct at 10-12 mEq/L per 24 hours maximum for chronic hypernatremia. Faster correction (1-2 mEq/L/hr) may be safe for acute (<24h) hypernatremia. Too rapid correction risks cerebral edema.

Why does TBW factor differ by sex?

Adult males have ~60% body weight as water (TBW = 0.6 × weight), females ~50% (TBW = 0.5 × weight). Elderly patients may have even lower percentages (0.5 males, 0.45 females).

Does this account for ongoing losses?

No. The calculated deficit is a snapshot. Ongoing losses (insensible losses, urine output, GI losses) must be added to the replacement plan. Monitor sodium levels and adjust accordingly.

What fluid should I use?

For hypernatremia: D5W or hypotonic saline (0.45% NS). Choice depends on patient volume status. Hypovolemic patients may need initial NS resuscitation before free water replacement.

What about hyponatremia?

This calculator is for hypernatremia (sodium >145). For hyponatremia, different formulas and considerations apply, including risk of osmotic demyelination syndrome with rapid correction.

Free Water Deficit

Q: Why does TBW factor differ by sex?

Adult males have ~60% body weight as water (TBW = 0.6 × weight), females ~50% (TBW = 0.5 × weight). Elderly patients may have even lower percentages (0.5 males, 0.45 females).

Q: Does this account for ongoing losses?

No. The calculated deficit is a snapshot. Ongoing losses (insensible losses, urine output, GI losses) must be added to the replacement plan. Monitor sodium levels and adjust accordingly.

Q: What fluid should I use?

For hypernatremia: D5W or hypotonic saline (0.45% NS). Choice depends on patient volume status. Hypovolemic patients may need initial NS resuscitation before free water replacement.

Q: What about hyponatremia?

This calculator is for hypernatremia (sodium >145). For hyponatremia, different formulas and considerations apply, including risk of osmotic demyelination syndrome with rapid correction.

Calculate fluid replacement needed for hypernatremia correction.

Patient Parameters

Sex

TBW factor: 60% of body weight

Weight

Total body weight. Used to calculate Total Body Water (TBW).

Current Sodium

mEq/L

Current serum sodium. Normal: 135-145 mEq/L. This calculator is for hypernatremia (>145).

Free Water Deficit

0.0 L

Water needed to normalize sodium

Moderate Hypernatremia

Total Body Water

42.0 L

Sodium Excess

15 mEq/L

Target Na (48h)

140 mEq/L

Max Rate

10-12 mEq/24h

Formula & Calculation

Free Water Deficit = TBW × [(Current Na / 140) - 1]

= 42.0 × [(155 / 140) - 1] = 0.00 L

• TBW (male) = Weight × 0.6 = 70 × 0.6 = 42.0 L

• TBW (female) = Weight × 0.5 = 70 × 0.5 = 35.0 L

• Elderly: Use 0.5 (male) or 0.45 (female)

⚠️ Remember: This is the deficit only. Add ongoing losses (insensible ~500mL/day, urine output, GI losses). Correct slowly: 10-12 mEq/L/day for chronic hypernatremia.

Frequently Asked Questions

How to Use

About the Fluid Deficit Calculator

What it calculates

This fluid deficit calculator applies a validated clinical scoring method used in healthcare settings. Free water deficit calculation.

When to use it

Use as a clinical decision support aid alongside professional judgment — not as a standalone diagnosis.

Example

Adjust the inputs above to model your specific scenario with the Fluid Deficit Calculator.

Explore More

Free Water Deficit

Patient Parameters

0.0 L

Moderate Hypernatremia

Formula & Calculation

About the Fluid Deficit Calculator

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