Adult Dehydration - Fluid Calculation

Fluid replacement in a dehydrated adult has three components that must be calculated separately and then added together:

Total Fluid Required = Deficit + Maintenance + Ongoing Losses

Step 1 - Assess Degree of Dehydration

Classify by clinical signs and estimated % body weight lost:

Severity	% Weight Loss	Clinical Features
Mild	~3-5%	Thirst, dry mouth, dark urine, slightly reduced skin turgor
Moderate	~6-8%	Tachycardia, orthostatic hypotension, sunken eyes, poor skin turgor, oliguria
Severe	~9%+	Hypotension, confusion, tachycardia, anuria, shock

Adults manifest symptoms at a lower percent weight loss than infants because of smaller total body water (TBW) relative to body weight. - Brenner and Rector's The Kidney, p. 1698

Step 2 - Calculate the Fluid Deficit

Isotonic Dehydration (most common: vomiting, diarrhea, sweating)

Deficit (mL) = % dehydration × body weight (kg) × 10

Example: 70 kg adult, estimated 6% dehydration
Deficit = 6 × 70 × 10 = 4,200 mL

Hypertonic / Hypernatremic Dehydration (water loss > solute loss)

Use the free water deficit formula: - National Kidney Foundation Primer on Kidney Diseases, 8e

Water deficit (L) = TBW × [(plasma Na+ / 140) - 1]

Where:

TBW = 0.6 × lean body weight (kg) in men
TBW = 0.5 × lean body weight (kg) in women

Example: 70 kg male, Na+ = 160 mEq/L

TBW = 0.6 × 70 = 42 L
Deficit = 42 × (160/140 - 1) = 42 × 0.143 = ~6 L

Important: Replace 140 with your target Na+ in the formula if you want partial correction.

Alternative (pediatric-origin formula used in emergency settings): - Tintinalli's Emergency Medicine

Free water deficit (mL) = 4 mL × weight (kg) × [desired change in Na+ (mEq/L)]

Step 3 - Calculate Daily Maintenance Fluids

Standard adult maintenance requirement: 25-30 mL/kg/day (approximately 1,500-2,500 mL/day for most adults).

A practical bedside formula:

Body weight ≤10 kg: 100 mL/kg/day
10-20 kg: 1,000 mL + 50 mL/kg for each kg above 10
>20 kg: 1,500 mL + 20 mL/kg for each kg above 20

For a typical 70 kg adult:

1,500 + (20 × 50) = 2,500 mL/day

Step 4 - Account for Ongoing Losses

Add estimated ongoing losses (vomiting, diarrhea, fever, drains, etc.):

Fever: ~150-200 mL/day per degree above 37°C
Vomiting/diarrhea: measure or estimate (~200-300 mL/episode)
Surgical drains: measure and replace volume for volume

Step 5 - Choose the Fluid Type

Situation	Preferred Fluid
Isotonic dehydration (most adults)	Normal saline (0.9% NaCl) or Lactated Ringer's
Hypernatremia (water deficit)	D5W or 0.45% NaCl (hypotonic)
Hyponatremia with volume depletion	0.9% NaCl (isotonic) - avoid hypotonic
Hemodynamic instability (shock)	Isotonic saline or Ringer's - large bolus first

Key rule: If hemodynamically unstable, always correct volume status with isotonic saline first, before addressing the specific electrolyte disturbance. - National Kidney Foundation Primer, p. 3723

Step 6 - Rate of Fluid Administration

Isotonic dehydration:

Moderate dehydration: Replace deficit over 24 hours
- Give 50% of deficit in the first 8 hours + 50% over the next 16 hours
- Add maintenance to each time block
Severe/shock: 500-1,000 mL bolus stat; reassess after each bolus

Hypernatremia:

Reduce sodium no faster than 1 mEq/L/hour and no more than 10-12 mEq/L in 24 hours to avoid cerebral edema - Tintinalli's Emergency Medicine
Monitor serum Na+ every 2-4 hours

Hyponatremia:

Correct no faster than 8-10 mEq/L in 24 hours to avoid osmotic demyelination syndrome

Quick Reference Example

Scenario: 70 kg male, moderate isotonic dehydration (~6%), vomiting, HR 110, BP 100/70

Component	Volume
Fluid deficit (6 × 70 × 10)	4,200 mL
Maintenance (2,500 mL/day)	2,500 mL
Ongoing losses (estimated)	500 mL
Total over 24 hours	~7,200 mL

Rate:

Hours 0-8: Give 50% deficit (2,100 mL) + 1/3 maintenance (833 mL) = ~2,933 mL (~366 mL/hr)
Hours 9-24: Give remaining 50% deficit (2,100 mL) + 2/3 maintenance (1,667 mL) + ongoing losses = ~4,267 mL (~267 mL/hr)

Key Clinical Points

Weight loss is the gold standard for estimating deficit - but is often unavailable; use clinical signs instead.
Urine output is your best real-time marker of adequate resuscitation (target ≥0.5 mL/kg/hr in adults).
Reassess frequently - fluid calculations are estimates, not fixed prescriptions.
Elderly patients are at higher risk; dehydration can present as delirium without the classic signs of thirst.
Oral rehydration is preferred for mild to moderate dehydration if the patient can tolerate it.

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