Enter the current sodium level, desired sodium level, and weight into the calculator to determine the free water excess. This calculator helps in managing fluid therapy in patients with conditions like hyponatremia.

Free Water Excess Formula

The following formula is used to calculate the free water excess:

FWE = ((Na / Na<sub>desired</sub>) - 1) * W

Variables:

  • FWE is the free water excess (liters)
  • Na is the current sodium level (mmol/L)
  • Nadesired is the desired sodium level (mmol/L)
  • W is the weight of the patient (kg)

To calculate the free water excess, subtract the desired sodium level from the current sodium level, divide by the desired sodium level, and then multiply by the patient’s weight.

What is Free Water Excess?

Free water excess is a measure used in the medical field to determine the amount of water without solutes that needs to be removed or added to bring a patient’s sodium level to a desired target. It is particularly useful in the management of electrolyte imbalances such as hyponatremia, where the sodium concentration in the blood is abnormally low.

How to Calculate Free Water Excess?

The following steps outline how to calculate the Free Water Excess:

  1. First, determine the current sodium level (Na) in mmol/L.
  2. Next, determine the desired sodium level (Nadesired) in mmol/L.
  3. Next, determine the weight of the patient (W) in kilograms.
  4. Next, gather the formula from above = FWE = ((Na / Nadesired) – 1) * W.
  5. Finally, calculate the Free Water Excess (FWE) in liters.
  6. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Current sodium level (Na) = 125 mmol/L

Desired sodium level (Nadesired) = 135 mmol/L

Weight of the patient (W) = 70 kg