Case 1
- Body weight
- 70kg
- Patient group
- Adult male (60 % water)
- Current serum sodium
- 160mmol/l
- Target serum sodium
- 140mmol/l
6.00l
Open with these valuesFree Water Deficit Calculator
6.00L
Result: 6.00 LTotal body water is body weight times a group fraction, and the deficit is that water times (current sodium ÷ target sodium − 1). A 70 kg adult man at 160 aiming for 140 is short about 6 litres. It sizes the existing gap only — not ongoing losses, and not a treatment plan.
Worked examples
Case 2
- Body weight
- 80kg
- Patient group
- Adult female (50 % water)
- Current serum sodium
- 158mmol/l
- Target serum sodium
- 140mmol/l
5.14l
Open with these valuesCase 3
- Body weight
- 20kg
- Patient group
- Child (60 % water)
- Current serum sodium
- 150mmol/l
- Target serum sodium
- 140mmol/l
0.86l
Open with these valuesHow it's calculated
Deficit = weight × water fraction × (current Na ÷ target Na − 1)
- StepEnter body weight and pick the patient group — it sets the water fraction, 0.6 to 0.45.
- StepEnter the most recent measured serum sodium as the current value.
- StepEnter the sodium you are aiming for; correction is gradual, so this is often an interim goal.
- ResultRead the deficit in litres, then add maintenance and ongoing losses separately.
What this number means
The free water deficit is the volume of electrolyte-free water the body is missing when serum sodium sits above the goal; replacing that water dilutes the sodium back down. It is built in two steps. Total body water is body weight times a group fraction — 0.6 for adult men and children, 0.5 for adult women and older men, 0.45 for older women — and the deficit is that water times (current sodium ÷ target sodium − 1). Take the preset case, a 70 kg adult man at 160 mmol/L aiming for 140: 70 × 0.6 gives 42 litres of body water, 160 ÷ 140 − 1 gives 0.1429, and the product is 6.00 litres. Only the 0.6 and the 0.5 come from the cited reference; the age-adjusted fractions are common clinical approximations with real spread, and the result carries that spread with it. Read the figure as a volume and nothing more. It sizes the existing shortfall, not the day's fluids: maintenance, urine output, insensible losses through skin and breathing and any gastrointestinal losses are added separately. It says nothing about pace either, and pace is where the risk sits — lowering sodium too fast risks cerebral edema, particularly when the hypernatremia is chronic, and common guidance limits correction to roughly 8 to 10 mmol/L per 24 hours. That pace is a clinical decision this calculator does not make, and this number replaces neither repeated measurements nor the treating clinician.
This number is not a fluid plan
It sizes the existing shortfall only. A complete plan adds maintenance, urine output, insensible losses through skin and breathing and any gastrointestinal losses on top of it.
The deficit is a volume, not a pace
Lowering sodium too fast risks cerebral edema, especially when the hypernatremia is chronic, and common guidance limits correction to roughly 8 to 10 mmol/L per 24 hours. The pace is a clinical decision that this calculator does not make.
Where the water fractions come from
The 0.6 for adult men and the 0.5 for adult women come from the cited reference. The age-adjusted fractions, 0.5 for older men and 0.45 for older women, are common clinical approximations with real spread.
Sodium units need no conversion
For sodium, mmol/L and mEq/L are the same number. A report in either unit can be entered as it stands.
Commonly misread
The deficit is 6 litres, so 6 litres of water replaces it today.
The deficit is the existing shortfall only, and maintenance and ongoing losses come on top of it. How fast it is replaced is a clinical decision, not part of this number.
The target sodium is always 140.
Because correction is deliberately gradual, an interim 24-hour goal is often the value that belongs in the target field.
The water fraction is a measured property of the patient.
It is a group approximation applied to body weight, and only the 0.6 and 0.5 figures come from the cited reference. The age-adjusted ones carry real spread.
Reference table
| Group, weight, Na now → target | Body water | Deficit |
|---|---|---|
| Adult male 70 kg, 140 → 140 | 42 L | 0.00 |
| Child 20 kg, 150 → 140 | 12 L | 0.86 |
| Elderly male 70 kg, 150 → 140 | 35 L | 2.50 |
| Elderly female 60 kg, 165 → 140 | 27 L | 4.82 |
| Adult female 80 kg, 158 → 140 | 40 L | 5.14 |
| Adult male 70 kg, 160 → 140 | 42 L | 6.00 |
| Adult male 80 kg, 170 → 145 | 48 L | 8.28 |
Questions
What is a free water deficit?
It is the volume of electrolyte-free water the body is missing when serum sodium sits above normal. Replacing that water dilutes the sodium back toward the goal. It is estimated as total body water times (current sodium ÷ target sodium − 1).
Where does the water fraction come from?
Body water lives mostly in lean tissue, so its share of body weight differs by group: about 0.6 in adult men and children, 0.5 in adult women and older men, and 0.45 in older women. The 0.6 and 0.5 figures come from the cited reference; the age-adjusted ones are common clinical approximations with real spread. Treat them as estimates, not measurements.
Which sodium values do I enter?
The most recent measured serum sodium as the current value, and the level you are aiming for as the target. Because correction is deliberately slow, clinicians often set a 24-hour interim goal rather than jumping to 140. For sodium, mmol/L and mEq/L are the same number.
How quickly should hypernatremia be corrected?
Slowly — lowering sodium too fast risks cerebral edema, especially when the hypernatremia is chronic. Common guidance limits correction to roughly 8 to 10 mmol/L per 24 hours. The deficit tells you the total volume; the pace is a clinical decision, and this calculator does not make it.
Does the deficit include ongoing losses?
No. It is the existing shortfall only. A complete fluid plan adds urine output, insensible losses through skin and breathing, and any gastrointestinal losses on top of this number.
Can I treat a patient from this number?
No. This is an educational estimate from a population formula, not medical advice and not a treatment plan. Real management depends on the cause, whether the hypernatremia is acute or chronic, volume status, ongoing losses and repeated measurements.
Sources and last check
Information, not medical advice.