Free Water Deficit Calculator

Estimates FWD based on patient data, measured and ideal plasma sodium.

In the text below the form there is more information on the formula and the variables employed.

The free water deficit calculator estimates FWD based on a formula published by Adrogué and Madias in 2000. The model accounts for patient age, gender and weight and for two plasma sodium values (the measured and the ideal one).

This offers information about body fluid status and guides management and treatment of hypernatremia (oral intake or IV).

Free water deficit = TBW x (Measured Na / Ideal Na – 1)


■ Ideal Na+ can be considered 140 mEq/L;

■ TBW is total body water = correction factor x weight in kg;

The correction factors depending on the patient type can be found in the text below the calculator.

Patient Gender
Age Category
Patient weight
Measured Na+ (mEq/L)
Ideal Na+ (mEq/L)
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Steps on how to print your input & results:

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.


Variables and formula

The free water deficit estimation offers information on the volume status of the patient based on age, gender, weight and plasma sodium.

Please note that the estimation provided by the calculator may not be correct in the case of patients who suffered recent significant weight gain or loss (especially from fluid sources).

FWD is often used in the management of hypernatremia when deciding fluid replenishment, in less severe cases just by oral intake, in more severe cases via IV.

The amount of free water required to balance the deficit is calculated via:

Free water deficit = TBW x (Measured Na / Ideal Na – 1)


■ Ideal Na+ can be considered 140 mEq/L;

■ TBW is total body water = correction factor x weight in kg;

The TBW correction factors are:

Patient Factor Patient Factor
Young female 0.5 Young male 0.6
Adult female 0.5 Adult male 0.6
Elderly female 0.45 Elderly male 0.5

This means that the free water deficit formulas for adults are:

Female FWD = 0.5 x Weight in kg x (Measured Na / Ideal Na – 1)

Male FWD = 0.6 x Weight in kg x (Measured Na / Ideal Na – 1)


Hypernatremia implications

Hypernatremia is caused by water loss either because of insufficient water intake, a rise in serum osmolarity or loss from gastrointenstinal tract, skin or urine.

Each litre of free water lost increases sodium by 3-5 mEq/L.

The four steps in the management of hypernatremia are:

1. Recognition of symptoms;

2. Diagnosis & identification of cause(s);

3. Volume disturbance correction;

4. Hypertonicity correction.

Acute hypernatremia requires rapid correction while hypernatremia needs a slower rate of correction to avoid cerebral edema.

Some of the correction infusates include:

■ 5% dextrose in water (D 5 W);

■ 2% sodium chloride in 5% dextrose in water (D 5 2NS);

■ Ringer's lactate solution;

■ 45% sodium chloride in water (0.45NS);

■ 9% sodium chloride in water (0.9NS).

Hypernatremia in the setting of volume overload in heart failure or pulmonary edema, may require correction through dialysis.

When hypernatremia occurs in the presence of diabetes, accompanied hyperglycemia needs to be carefully monitored. If the correction fluid contains glucose, insulin dosage may be required.

In hypervolemic patients due to impaired renal function, a loop diuretic increases sodium excretion.


Original source

Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000; 342(20):1493-9.

Other references

1. Cheuvront SN, Kenefick RW, Sollanek KJ, Ely BR, Sawka MN. Water-deficit equation: systematic analysis and improvement. Am J Clin Nutr. 2013; 97(1):79-85.

2. Barsoum NR, Levine BS. Current prescriptions for the correction of hyponatraemia and hypernatraemia: are they too simple? Nephrol Dial Transplant. 2002; 17(7):1176-80.

Specialty: Deficiency

Objective: Determination

No. Of Variables: 5

Year Of Study: 2000

Abbreviation: FWD

Article By: Denise Nedea

Published On: June 4, 2017 · 08:42 AM

Last Checked: June 4, 2017

Next Review: June 4, 2023