What is free water deficit?
Free water deficit is the volume of electrolyte-free water needed to bring an elevated serum sodium down to a chosen target. It is the core estimate behind a hypernatremia correction calculator and a practical dehydration fluid deficit calculator when the problem is pure water loss rather than isotonic volume loss alone.
Free water deficit in hypernatremia
Hypernatremia means serum sodium is high relative to body water. In clinical practice that almost always reflects a free water deficit (or inadequate free-water intake) rather than a primary “sodium overdose.” The deficit tells you how much fluid to correct sodium toward normal — usually with oral water or hypotonic IV fluid such as D5W — after any shock or hypovolemia has been addressed with isotonic resuscitation.
This page follows the same bedside approach used by major clinical calculators: estimate total body water, apply the classic free water deficit formula, then pace replacement with a safe sodium correction rate per day.
What the deficit represents
Sodium stays relatively fixed in the “tank”; free water is what is missing.
TBW fractions used here
Standard age/sex coordinates for this free water deficit calculator.
That is why free water deficit adults vs elderly estimates differ even at the same weight.
Free water deficit formula
The standard free water deficit formula (Adrogué–Madias style bedside estimate) is:
- TBW
- weight (kg) × age/sex fraction (0.6 / 0.5 / 0.45)
- Nacurrent
- measured serum sodium (mEq/L or mmol/L)
- Nadesired
- target sodium, commonly 140 mEq/L
- FWD
- liters of electrolyte-free water to reach the target
How to calculate free water deficit
- Pick the age/sex category (child, adult, or elderly) to set the TBW fraction.
- Multiply body weight in kilograms by that fraction — this is your total body water calculator step.
- Divide current sodium by desired sodium and subtract 1.
- Multiply by TBW to get the free water deficit in liters.
- Plan replacement with a sodium correction rate calculator mindset: chronic usually ≤0.5 mEq/L/hr and about ≤10 mEq/L/day.
Free water deficit calculation example
Example: 70 kg adult man, serum Na 156 mEq/L, goal 140 mEq/L.
- TBW = 70 × 0.6 = 42 L
- FWD = 42 × ((156 ÷ 140) − 1) = 42 × 0.1143 ≈ 4.8 L
- Sodium gap = 16 mEq/L → at ≤10 mEq/L/day, expect roughly 1.6 days of paced correction (plus ongoing losses).
Tap Example in the calculator to load these inputs instantly.
How to correct free water deficit
Clinical sequence
- Treat shock / hypovolemia with isotonic fluid first
- Then replace free water (oral water or D5W / hypotonic IV)
- Add ongoing urinary and insensible losses
- Recheck Na frequently and adjust the rate
Safe sodium correction rate per day
- Chronic: often ≤0.5 mEq/L/hr and ~10 mEq/L in 24 h
- Acute (<48 h): may correct faster (about 1–2 mEq/L/hr) if symptomatic
- Overly rapid falls risk cerebral edema
- The deficit formula is a snapshot — not a full fluid order
Hypernatremia correction timeline (visual)
Educational pacing bands used by this hypernatremia correction calculator.
Adults, elderly, and free water deficit calculator pediatric use
Children typically use a 0.6 TBW fraction — the same arithmetic as adults, but fluid orders and monitoring follow pediatric protocols. Elderly patients use lower TBW fractions, so the calculated deficit is smaller for the same weight and sodium. In all groups, edema, obesity, amputation, and large fluid shifts can make weight-based TBW less accurate.
FAQ
What is free water deficit?
It is the estimated liters of electrolyte-free water needed to dilute high serum sodium down to a target value in hypernatremia or pure-water dehydration.
What is the free water deficit formula?
FWD (L) = TBW × ((current Na ÷ desired Na) − 1), with TBW = weight × age/sex fraction.
How to calculate free water deficit step by step?
Estimate TBW, divide current sodium by the goal, subtract 1, multiply by TBW. Use the Example button for a worked 70 kg / Na 156 case.
How much fluid to correct sodium?
Start with the calculated free water deficit, then add ongoing losses. Oral water or D5W are common free-water sources; 0.45% saline provides only partial free water per liter.
What is a safe sodium correction rate per day?
Chronic hypernatremia is often corrected by about ≤10 mEq/L in 24 hours (roughly ≤0.5 mEq/L/hr). Acute symptomatic cases may allow faster correction with intensive monitoring.
How do free water deficit adults vs elderly compare?
Elderly men use 0.5 and elderly women 0.45 TBW fractions, so estimated deficits are lower than for younger adults at the same weight.
Can I use this as a free water deficit calculator pediatric tool?
Yes for the arithmetic (usually 0.6 × weight). Therapy decisions still need pediatric dosing, monitoring, and institutional protocols.
Is this a total body water calculator too?
Yes. Results show estimated TBW before the deficit so you can see both steps of the equation.
How to correct free water deficit safely?
Resuscitate volume first if needed, replace free water gradually, account for ongoing losses, and recheck sodium so the fall stays within a safe correction band.
Is this the same as a creatinine clearance calculator?
No. This tool estimates water needed for sodium correction. For Cockcroft–Gault CrCl, use our creatinine clearance calculator; for BUN/Cr pattern clues, use the BUN creatinine ratio calculator.
