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FWD = TBW × [(Current Na⁺ / Desired Na⁺) − 1]
The free water deficit represents the amount of water (in mL) needed to correct hypernatremia to the desired sodium level.
The free water deficit calculator is used in the management of hypernatremia to provide information on the volume status of the patient, based on age, gender, weight, plasma sodium, and overall fluid balance, and this estimation may help in deciding correct water replenishment by oral intake or via IV, depending on the case and whether it is less or more severe; the estimation provided by the calculator is not always accurate in patients who suffered recent significant gain or loss from fluid sources, especially in cases where oral fluid alone is not enough, and this tool is often used when evaluating patients in clinical management, while clinicians should note that each patient status may vary by condition and response.
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, and in practical use, this concept helps guide hydration decisions because acute symptomatic hypernatremia under 24 h should be corrected rapidly, whereas chronic hypernatremia >48 hours should be managed slowly due to the risks of brain edema, a distinction I have often seen emphasized when balancing safety and effectiveness in treatment planning.
How Does Our Free Water Deficit Calculator Work?
How to calculate free water deficit? Our Free Water Deficit Calculator estimates how much free water the body needs to safely correct hypernatremia (high serum sodium). The tool uses well-established clinical formulas and applies them step by step based on the values you enter, as shown in the screenshot.
Below is a clear breakdown of how the calculation of free water deficit works, followed by the exact calculation using the values visible in your example.
Step 1: Understanding the Inputs
The free water deficit calculation first collects key patient-specific values that directly affect body water balance:
- Sex: Female
- Age range: Adult
- Weight: 100 lbs
- Current serum sodium: 150 mEq/L
- Desired serum sodium: 140 mEq/L
Each of these values matters because total body water varies by sex, age, and body weight, and the sodium difference determines how much water is missing.
Step 2: Calculate Total Body Water (TBW)
Calculating free water deficit estimate, Total Body Water (TBW) using standard clinical constants.
TBW Formula
- Adult female: TBW = 0.5 × body weight (kg)
Convert weight from pounds to kilograms
- 100 lbs ÷ 2.2046 = 45.36 kg
Apply the TBW formula
- TBW = 0.5 × 45.36 = 22.68 L
This matches the screenshot result: Total Body Water = 22.68 L
Step 3: Apply the Free Water Deficit Formula
Once TBW is known, the calculator applies the standard Free Water Deficit (FWD) formula.
Free Water Deficit Formula
- Free Water Deficit = TBW × { ( Current Na / Desired Na ) −1}
Step 4: Perform the Calculation
Plug in the values:
- TBW = 22.68 L
- Current sodium = 150 mEq/L
- Desired sodium = 140 mEq/L
- 150 / 140 = 1.0714
- 1.0714 − 1 = 0.0714
- 22.68 × 0.0714 = 1.62 L
Step 5: Final Result
- Free Water Deficit = 1.62 L
- Converted to millilitres:
- 1.62 L = 1620 mL
This exactly matches the calculator output shown in the image: Free Water Deficit: 1620 mL
Why Each Input Matters
- Sex & age range: Determine the TBW multiplier
- Weight: Sets the baseline body water volume
- Current sodium: Reflects severity of hypernatremia
- Desired sodium: Sets the correction goal (commonly 140 mEq/L)
The calculator combines these factors to give a precise, clinically relevant estimate without manual math.
If You Don’t Use the Calculator
To get an accurate result manually, always follow this order:
- Convert weight to kilograms
- Calculate TBW using the correct multiplier
- Apply the free water deficit formula
- Convert litres to millilitres if needed
This ensures safe and consistent estimation, especially when planning fluid correction.
When to Use This Calculator
The Free Water Deficit Calculator should be used when you need to determine the amount of free water needed to treat hypernatremia in a patient with a high level of serum sodium (usually higher than 140 mEq/L). It proves to be the most useful in the first-time examination of dehydration, where the fluid replacement is planned, and determining whether the correction can be handled by oral intake or intravenous (IV) fluids is necessary.
This is a type of calculator that is frequently utilized by adults in inpatient, emergency, or clinical units to help facilitate the making of decisions in both less acute and more acute hypernatremia, as long as there are no rapid ongoing, extensive fluid losses or significant weight changes. Medical education and exam preparation can also be effectively used. The instrument can not be taken as the only reference in patients with unstable conditions, with recent marked weight gain/loss, kidney or heart disease, and in pediatrics, when particularized clinical judgment is of key importance.
FAQ's
This calculator is oriented mostly for adults. The body water percentages of children also vary depending on age and development, and applying the adult formula will result in misleading outcomes. In the case of pediatric patients, age-related clinical formulas and expert advice must be utilized.
The normal range of serum sodium is 136 to 145 mEq/L. A measurement that is higher than that is a sign of hypernatremia, whereas a lower value is a sign of hyponatremia. This calculator is intended for use when the serum sodium level is high.
No. This calculator should be used as a help, not as a replacement for clinical judgment. It does not consider continual fluid loss, comorbidity, drugs, or individual patient response. The medical professional must always make final decisions regarding the type of fluids, rate, and route of correction used.
A free water deficit shows the volume of water needed to correct dehydration. It means there isn’t enough body water to dilute sodium to normal levels, helping indicate how severe the water shortage is.
The free water deficit for a 70 kg adult man is about 2 litres when the measured sodium level is 147 mEq/L, assuming the ideal value is 140 mEq/L.