Clinical reference calculator

Free Water Deficit Calculator

Estimate free-water deficit in hypernatremia using body weight, age group, sex category, and sodium values.

Calculator inputs

Enter the available information

Hypernatremia

Equation sex category

Age group

Body weight

Measured sodium (mEq/L)

Target sodium (mEq/L)

Calculated result

Result summary

Your result will appear here

Complete every required field, then select the calculate button.

What is Free Water Deficit Hypernatremia?

This calculator estimates a free-water deficit from body weight, an assumed total-body-water fraction, current serum sodium, and a chosen sodium target. The result is the theoretical amount of water, in liters, associated with lowering sodium to that target if the assumptions held and there were no ongoing gains or losses.

Hypernatremia can reflect serious illness and overly rapid correction can cause harm. This estimate is for education and professional cross-checking; it is not a diagnosis, drinking instruction, infusion order, or replacement schedule. Fluid type, correction rate, circulation, urine and gastrointestinal losses, kidney function, glucose, volume status, and repeat laboratory results require clinical assessment. Seek urgent medical care for confusion, seizures, marked weakness, loss of consciousness, or other severe symptoms.

How to Use This Calculator

Select the category used for the total-body-water estimate: child, adult, or elderly, together with male or female.
Enter measured weight and select kilograms or pounds. Pounds are converted to kilograms.
Enter current and desired sodium in mEq/L or mmol/L. For sodium, the numerical values are equivalent in these units.
The page estimates total body water as weight in kilograms multiplied by 0.60 for children, 0.60 for adult males, 0.50 for adult females, 0.50 for elderly males, or 0.45 for elderly females.
It applies the water-deficit equation and shows liters to two decimal places.

The broad categories are approximations. Body composition, obesity, dehydration, age, and illness can make actual total body water differ substantially.

Formula Used

Estimated total body water (L) = weight (kg) × selected TBW fraction

Free-water deficit (L) = estimated total body water × [(current sodium ÷ desired sodium) − 1]

Because one liter of water has a mass close to one kilogram, total body water in kilograms is commonly treated numerically as liters for this estimate. The equation models a static relationship. It does not include continuing renal or extrarenal water loss, sodium gain, administered fluids, or the time over which correction occurs.

Variables Explained

Free-water-deficit inputs and assumptions
Input	Use in the estimate	Important limitation
Gender category	Selects an adult or elderly TBW fraction.	It is a population approximation, not a measurement of body water.
Age range	Selects child, adult, or elderly assumptions.	Broad labels do not capture infant physiology or individual body composition.
Weight	Current body mass, converted to kilograms when needed.	Choice of weight in obesity or major fluid shifts needs clinical judgment.
Current sodium	Measured serum sodium concentration.	Use a current, verified laboratory result.
Desired sodium	Target used only for the theoretical calculation.	It does not set a safe correction rate or treatment plan.
Estimated deficit	Static difference expressed in liters.	Ongoing losses and clinical response are not included.

Worked Example

For a 70 kg adult male, the page uses a total-body-water fraction of 0.60. With current sodium of 154 mEq/L and a selected target of 140 mEq/L:

Estimated TBW = 70 × 0.60 = 42 L

Deficit = 42 × [(154 ÷ 140) − 1] = 42 × 0.10 = 4.20 L

The displayed estimate is 4.2 liters. It does not mean that 4.2 liters should be given immediately or as water alone. A clinician would consider whether hypernatremia is acute or chronic, choose a fluid and correction rate, account for ongoing losses, and monitor sodium and the person’s condition.

Why Use This Calculator?

A positive estimate describes the modeled free-water shortfall relative to the selected target. It is sensitive to both the target and the assumed total-body-water fraction. Choosing a lower target increases the calculated amount; choosing a smaller TBW fraction lowers it. Neither adjustment resolves uncertainty about the person’s true body water.

The result should be viewed as one component of a monitored clinical plan. Standard discussions of hypernatremia emphasize cause, duration, symptoms, extracellular volume, and repeated sodium measurement. The calculated deficit omits ongoing urine, stool, skin, and respiratory losses as well as fluid already being administered. In hyperglycemia, clinicians may also use a glucose-corrected sodium. Pediatric care, severe symptoms, pregnancy, kidney failure, heart failure, and major fluid shifts require specialized assessment.

Common Mistakes

Treating the calculated liters as a direct prescription or immediate replacement volume.
Ignoring whether hypernatremia developed acutely or over a longer period.
Forgetting ongoing water losses, sodium gains, and administered fluid.
Using pounds as though they were kilograms or selecting the wrong unit.
Assuming the listed TBW fraction precisely represents an individual with obesity, low muscle mass, edema, or severe dehydration.
Choosing a target without considering safe correction and repeat monitoring.
Using an old or unverified sodium result when the clinical situation is changing.

Frequently Asked Questions

Is the result a fluid prescription?

No. It is a static mathematical estimate. Fluid type, amount, route, timing, ongoing losses, and monitoring must be decided by a qualified clinician.

Are mmol/L and mEq/L the same for sodium?

For sodium, which has a valence of one, the numerical concentration is the same in mmol/L and mEq/L. Keep the selected label consistent with the laboratory report.

Why does age and gender category change the answer?

Those choices select approximate total-body-water fractions. They reflect average body-composition differences but cannot measure an individual’s actual body water.

Does the equation include ongoing water loss?

No. Urine, diarrhea, sweating, fever-related loss, and respiratory loss after the measurement are outside this static estimate.

Can I use this calculator for an infant?

The broad child category does not capture age-specific infant physiology. Pediatric hypernatremia needs prompt assessment and age-appropriate specialist guidance.

Why can the true need differ from the estimate?

The TBW fraction may be inaccurate for the individual, and sodium changes with continuing losses, intake, solute balance, glucose, kidney handling, and treatment.

When is urgent medical care needed?

Hypernatremia with confusion, seizures, severe weakness, reduced consciousness, or significant illness needs urgent evaluation. Do not delay care to perform an online calculation.