This free water deficit calculator estimates the FWD value based on measured plasma Na+ and the gender, weight and age category of the patient. Discover more about the body water Na+ balance in the form below.

Patient Gender:*
Age Category:*
Patient weight:*
Measured Na+ (mEq/L):*
Ideal Na+ (mEq/L):*

How does this free water deficit calculator work?

This health tool estimates the free water deficit according to the plasma Na+, the chosen ideal Na+ value and suggests how much of this deficit should be corrected.

The form is simple and comprises of five fields, 2 where you select the gender and age category (young, adult and elderly) and three more to input weight in either lbs or kg and the measured Na in mEq/L.

For the ideal Na, the field is by default completed with the standard Na of 140mEq/L but even this area can be customized according to needs with any Na value.

This free water deficit calculator addresses the water Na balance and is often used in the management of hypernatremia. The formulas used to calculate the amount of free water required to balance the deficit are:

Free water deficit = TBW x (measured Na+/ideal Na+ – 1)


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

Correction factors:

Case Factor
Young Female 0.5
Adult Female 0.5
Elderly Female 0.45
Young Male 0.6
Adult Male 0.6
Elderly Male 0.5

Taking as example the case of an adult male, weighing 97kg and a measured Na+ of 117mEq/L. The water deficit formula is:

FWD = 0.6 x 97 x (117/140 -1)

FWD = -9.56 L

Hypernatremia medical implications

■ One of the most common causes of this condition is due to dehydration through loss from the gastrointenstinal tract, skin or urine. Hypovolemia on the other side, comprises of loss of both water and Na+.

■ The four steps in the management of hypernatremia are the recognition of symptoms, diagnosis and identification of cause(s), volume disturbance correction and hypertonicity correction.

■ The treatment comprises of a sustained decrease in serum sodium and intake of free water, either orally or parenterally. Acute hypernatremia needs rapid correction while hypernatremia needs a more slow rate of correction due to the brain edema risk.

■ Hypernatremia accompanied by diabetes, hyperglycemia needs to be monitored and if the correction fluid contains glucose there should be insulin dosage as well.

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

■ The correction of hypokalemia and hypercalcemia in diabetes insipidus are essential to the treatment with vasopressin.


1) Adroque HJ, Madias NE. (2000) Hypernatremia. N Enlg J Med; 342(20): 1493-9.

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

18 Jul, 2015