Introduction
The Free Water Deficit Calculator tells you how much water the body is missing when blood sodium is too high. High sodium is called hypernatremia. It usually means the body lost water, not that it gained salt. This tool shows the water gap in liters, milliliters, and other units you pick.
To use it, enter the patient's weight (or total body water), the current sodium level, and the target sodium level. Pick the patient type: adult male, adult female, elderly male, elderly female, or child. Each type has its own total body water fraction, since bodies hold different amounts of water. The math uses this simple formula:
Free Water Deficit = Total Body Water × (Current Sodium ÷ Target Sodium − 1)
You also enter how long you plan to replace the water. The calculator then gives an hourly rate in mL/h and checks your speed against the safe limit of 10–12 mEq/L per 24 hours.1 Fixing sodium too fast can hurt the brain, so this check matters. Most plans spread the water over 48 to 72 hours.
You get a full step-by-step solution, a chart of the projected sodium over time, and a chart comparing rates at 24, 48, and 72 hours. Remember: this deficit does not include ongoing losses from urine, sweat, or breathing. Those need extra fluid on top. This tool is for learning and planning support, not a substitute for a doctor's judgment.
How to use our Free Water Deficit Calculator
Enter the patient's body water details, current and target sodium, and how long you plan to replace fluid. The calculator shows the free water deficit in liters, milliliters, and one unit you pick, plus the hourly rate and a safety check on correction speed.
Calculation Method: Pick "Weight-Based" to work out total body water from weight, or "TBW-Based" to type in a total body water value you already have.
Patient Type / Demographic: Choose adult male, adult female, elderly male, elderly female, or child. This sets the body water fraction used with weight.
Patient Weight: Type the weight and choose kg or lb. The number changes on its own when you switch units. This box only shows in Weight-Based mode.
Total Body Water (TBW): Type the known total body water and pick L, mL, fl oz, pt, or gal. This box only shows in TBW-Based mode.
Current Serum Sodium: Enter the patient's latest sodium level in mEq/L or mmol/L. The two units are the same number for sodium. Sodium also drives the Serum Osmolality Calculator.
Target Serum Sodium: Enter the sodium level you want to reach. It starts at 140 mEq/L, and you can change it.
Planned Replacement Duration: Enter how long the fluid will be given and pick hours, days, or minutes. Most plans use 48 to 72 hours.
Additional Output Volume Unit: Choose one more unit for the answer, such as fluid ounces or cups. Liters and milliliters always show.
Calculate and Reset: Results update as you type, but you can press Calculate to refresh them. Press Reset to bring back the starting values.
What Is Free Water Deficit?
Free water deficit is the amount of plain water the body is missing. When the body loses more water than salt, the sodium in the blood goes up. This is called hypernatremia (blood sodium above 145 mEq/L).1 The free water deficit tells doctors how many liters of water are needed to bring sodium back down to a safe level.
The Free Water Deficit Formula
The standard formula is:
Free Water Deficit (L) = Total Body Water × [(Current Sodium ÷ Target Sodium) − 1]
Total body water (TBW) is a share of body weight. That share changes with age and sex, because muscle holds more water than fat. The standard fractions are 0.6 in men and 0.5 in women,1 and 0.5 and 0.45 in elderly men and women.2
- Adult male: 60% of body weight (0.60)
- Adult female: 50% of body weight (0.50)
- Elderly male: 50% of body weight (0.50)
- Elderly female: 45% of body weight (0.45)
- Child: 60% of body weight (0.60)
For a child the calculator uses 0.60. Published values put total body water higher in the young — about 65% of body weight in children and 70% to 80% in infants — so a small child's true deficit can be larger than the number here.3 Example: a 70 kg adult male has about 42 L of body water. If his sodium is 158 and the goal is 140, his free water deficit is about 5.4 L.
Why High Sodium Happens
Blood sodium rises when water leaves the body or when a person does not drink enough. Common causes include:
- Fever, sweating, or hot weather
- Vomiting and diarrhea
- Diabetes insipidus or high blood sugar
- Water pills (diuretics)
- Older adults or babies who cannot ask for water
- Tube feeding without enough free water
Signs can include thirst, dry mouth, weakness, confusion, and in bad cases, seizures. Kidney function shapes how the body handles water, so the GFR Calculator is often reviewed alongside sodium.
Correcting Sodium Safely
Fixing sodium too fast is dangerous. Fast changes can make brain cells swell and cause cerebral edema or osmotic demyelination syndrome. Because of this, sodium should drop no faster than 10 to 12 mEq/L in 24 hours; the usual goal is a fall of no more than 12 mEq/L in 24 hours, and this calculator draws its safety line at the stricter 10.1 Most plans spread the water over 48 to 72 hours, with blood tests every few hours to check progress.
Things the Number Does Not Include
The free water deficit only covers water that is already missing. It does not count water the body keeps losing. You must also replace:
- Urine losses that keep happening, which you can measure with the Urine Output Calculator
- Insensible losses from breathing and skin (about 500–1000 mL a day)
- Extra losses from fever, diarrhea, drains, or burns
Water can be given by mouth, through a feeding tube, or by IV using D5W or half-normal saline, depending on the patient.
Important Note
This tool gives an estimate, not a treatment plan. Real body water can differ from the formula, and each patient needs lab checks and a doctor's judgment. Always work with a health care provider before giving fluids.