Age/sex category
adult female
Sodium level
Free water deficit

Free Water Deficit Calculator

By Małgorzata Koperska, MD

The free water deficit calculator finds the amount of water replacement needed to correct dehydration - or to reach the desired level of sodium in the blood serum. Read on to learn more about the water deficit and to understand the free water deficit formula used by the calculator.

Please, keep in mind that Omni Calculator is not a doctor. If you're unwell and you're worried that you might be dehydrated, contact a physician.

If you're a medical professional, you can find our anion gap calculator useful.

What is water deficit?

It's an estimated value used in medicine and nutrition to assess the volume of water required to correct dehydration during the initial stages of fluid-replacement therapy. Acute symptomatic hypernatremia (under 24 hours), should be corrected rapidly, whereas chronic hypernatremia (>48 h) should be corrected slowly due to the risks of brain edema.

How is free water deficit calculated?

To calculate free water deficit (FWD), it's necessary to estimate total body water (TBW) first. You can use our TBW calculator to find an accurate result, or use a simpler equation:

TBW = body weight (kg) * sex/age coordinate

The sex/age coordinate equals:

  • 60% for adult males;
  • 50% for adult females;
  • 50% for elderly males;
  • 45% for elderly females;
  • 60% for children.

Then, you can use the calculated value to move on to the final equation:

FWD = TBW * (( patient's sodium / desired sodium ) - 1)

The calculator uses 140 mEq/L as the default desired level of sodium. You can change it in the advanced mode.

Małgorzata Koperska, MD