Free Water Deficit Calculator

Calculator Form

Patient Profile

Custom TBW Coefficient

Used only when Custom Coefficient is selected.

Body Weight

Weight Unit

Current Serum Sodium (mEq/L)

Target Sodium (135 to 145)

Ongoing Free Water Losses per 24h (mL)

Hypernatremia Duration

Optional Notes

Reset

Example Data Table

Profile	Weight	Current Sodium	Target Sodium	Coefficient	Estimated TBW	Estimated Deficit
Adult Male	70 kg	154 mEq/L	140 mEq/L	0.60	42.00 L	4.20 L
Adult Female	60 kg	150 mEq/L	140 mEq/L	0.50	30.00 L	2.14 L
Elderly Female	55 kg	148 mEq/L	140 mEq/L	0.45	24.75 L	1.41 L
Child	25 kg	149 mEq/L	140 mEq/L	0.60	15.00 L	0.96 L

Formula Used

Free Water Deficit = Total Body Water × ((Current Sodium ÷ Target Sodium) − 1)

Total Body Water = Weight in kg × coefficient

Common coefficients in this tool: adult male 0.60, adult female 0.50, elderly male 0.50, elderly female 0.45, child 0.60.

The result estimates pure free water deficit.
Combined sodium and water loss can change real needs.
Ongoing losses, glucose, urine output, and volume status matter.
This tool also shows simple 24h, 48h, and 72h math plans.

How to Use This Calculator

Select the patient profile that best matches the case.
Enter body weight and choose kilograms or pounds.
Enter the current serum sodium concentration.
Set a target sodium between 135 and 145 mEq/L.
Add optional ongoing free water losses for a fuller estimate.
Choose the timing category for reference notes.
Press the calculate button to show results above the form.
Use the chart, CSV export, PDF export, and notes for review.

Frequently Asked Questions

1) What does free water deficit mean?

It estimates how much electrolyte-free water is missing relative to the sodium concentration entered. It is a math estimate, not a full treatment prescription.

2) When is this calculator useful?

It is useful when reviewing hypernatremia, dehydration patterns, or educational fluid-balance exercises. Real care decisions still require clinical assessment and repeat laboratory monitoring.

3) Why does the tool ask for sex or age profile?

The equation estimates total body water from body weight, and common coefficients vary by sex and older age group. That changes the final deficit value.

4) Why might the estimate be zero?

If the current sodium is not above the chosen target, the formula may produce a zero or negative value. The calculator displays zero instead of a negative deficit.

5) Does this replace clinician judgment?

No. Volume status, ongoing losses, kidney function, glucose, cause of hypernatremia, and treatment setting can all change what is actually appropriate.

6) Why are there 24h, 48h, and 72h volumes?

They show simple even-distribution math for the estimated deficit and optional ongoing losses. They are planning views, not automatic orders.

7) Can I use a custom coefficient?

Yes. Select the custom option if you want to apply an alternative total body water factor for educational comparisons or local protocol review.

8) What should I monitor besides the estimate?

Trend sodium values, intake, urine output, blood glucose, blood pressure, symptoms, and ongoing gastrointestinal, renal, or insensible losses.