What the free water deficit is
Hypernatremia, a serum sodium above 145 mmol/L, almost always means the body has lost more water than salt. The free water deficit estimates how many liters of water would bring sodium back to normal.
The formula
Free water deficit = total body water × (sodium ÷ target − 1)
The target is usually 140 mmol/L. Total body water is a share of body weight:
| Patient | Share of weight |
|---|---|
| Adult man | 60% |
| Adult woman | 50% |
| Man over 65 | 50% |
| Woman over 65 | 45% |
| Child | 60% |
Worked example
A 154 lb (70 kg) man with a sodium of 155 mmol/L:
- Total body water: 70 × 0.6 ≈ 42 L.
- Deficit: 42 × (155 ÷ 140 − 1) ≈ 4.5 L.
- To lower sodium by 15 mmol/L at no more than 10 mmol/L per day: 36 hours minimum.
- Average rate over that time: 4,500 mL ÷ 36 h ≈ 125 mL/h, before ongoing losses.
How fast to correct
In hypernatremia that developed over more than a day or two, brain cells adapt by holding on to solutes. Lowering sodium too quickly lets water rush into them and can cause brain swelling and seizures. Most references advise a fall of no more than 10 to 12 mmol/L per 24 hours. Hypernatremia that developed within hours can be corrected faster under close monitoring.
What the formula leaves out
- Ongoing losses: urine (especially in diabetes insipidus or with high blood sugar), fever, sweating, diarrhea and breathing all add to the need.
- Sodium in the fluids given: half-normal saline delivers only half of its volume as free water.
- Body composition: the water share is lower in people with obesity and higher in very lean people.
This is why sodium is rechecked every few hours during treatment and the plan is adjusted to the results.
Common causes
Older adults who cannot drink on their own, infants, people with fever or diarrhea, diabetes insipidus and very high blood sugar are the classic causes of hypernatremia. Thirst is the main defense, so the condition usually reflects limited access to water.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.