Free Water Deficit Calculator

Created by Małgorzata Koperska, MD
Reviewed by Bogna Szyk and Steven Wooding
Last updated: Dec 11, 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 interested in how much you should be drinking each day, check out our water intake calculator 💦


How do I calculate free water deficit?

To calculate the free water deficit:

  1. Determine your patient's sex/age coordinate.
  2. Multiply it by their body weight to estimate the total body water.
  3. Divide the patient's blood sodium level by its desired value and subtract 1.
  4. Multiply the above result by the total body water - this is the free water deficit.

How do I interpret free water deficit?

The free water deficit tells you the volume of water needed to correct dehydration. In other words, it's an occurrence in which there isn't enough body water to dilute sodium to its normal levels.

What is the water deficit for a 70 kg adult man?

The free water deficit for a 70 kg adult man is 2 liters if we assume the measured sodium level to be 147 mEq/L, with the ideal value being 140 mEq/L.

How do I correct free water deficit?

Generally, the free water deficit is corrected by gradually decreasing the blood sodium concentration by:

  • 0.5 mEq/L per hour for chronic hypernatremia;
  • 1-2 mEq/L per hour in case of acute hypernatremia (onset sooner than 48 hours).
Małgorzata Koperska, MD
Age/sex category
Adult female
Sodium level
Free water deficit
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