Free Water Deficit Calculator

Created by Małgorzata Koperska, MD
Reviewed by Bogna Szyk and Steven Wooding
Last updated: Aug 08, 2022

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 water deficit and understand the calculator's free water deficit formula.

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

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

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?

Before we can calculate free water deficit (FWD), we must first estimate total body water (TBW). 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; and
  • 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.

💡 If you are interesting in how much you should be drinking each day, check out our water intake calculator 💦

Małgorzata Koperska, MD
Weight
lb
Age/sex category
Adult female
Sodium level
mEq/L
Free water deficit
liters
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