A Free Water Excess Calculator is a clinical tool used by healthcare professionals to estimate the volume of surplus “free water” (water without electrolytes) in a patient’s body that is causing a low sodium level in the blood (hyponatremia). Hyponatremia is a common electrolyte disorder that occurs when the body’s water content is too high relative to its sodium content. This calculator uses a patient’s body weight, sex, and current serum sodium level to quantify the amount of excess water that needs to be removed or restricted to safely raise the sodium concentration back toward a normal range. Consequently, it is a crucial tool for guiding treatment strategies, such as fluid restriction or the use of diuretics, in patients with dilutional hyponatremia.
formula of Free Water Excess Calculator
The calculation of free water excess involves first estimating the patient’s Total Body Water (TBW) and then determining how much of that water is in excess.
Free Water Excess Formula
This is the primary formula used to estimate the total volume of excess free water in the body that is causing the dilution of serum sodium.
Formula:
Free Water Excess (in Liters) = Total Body Water (TBW) * (1 – (Current Serum Sodium / Desired Serum Sodium))
- Free Water Excess: The final calculated volume of excess water, expressed in liters.
- Total Body Water (TBW): An estimate of the total water content in the body. This requires a separate calculation (see below).
- Current Serum Sodium: The patient’s current, measured sodium level from a blood test (in mEq/L or mmol/L).
- Desired Serum Sodium: The target sodium level for correction (e.g., a safe level like 125 or 130 mEq/L).
Total Body Water (TBW) Calculation
Total Body Water is a required sub-calculation. It is estimated as a percentage of the patient’s body weight. The percentage used depends on the patient’s age and sex.
Formula:
Total Body Water (in Liters) = Body Weight (in kg) * Water Percentage
The standard coefficients for water percentage are:
- Adult Men: 60% (0.60)
- Adult Women: 50% (0.50)
- Elderly Men: 50% (0.50)
- Elderly Women: 45% (0.45)
- Children: 60% (0.60)
Total Body Water (TBW) Estimation Table
This table provides quick estimates of Total Body Water (TBW) in Liters for various body weights and patient categories. This is the first step in any free water excess calculation.
| Body Weight | Adult Man (60%) | Adult Woman (50%) | Elderly Man (50%) | Elderly Woman (45%) |
| 50 kg (110 lbs) | 30 L | 25 L | 25 L | 22.5 L |
| 60 kg (132 lbs) | 36 L | 30 L | 30 L | 27 L |
| 70 kg (154 lbs) | 42 L | 35 L | 35 L | 31.5 L |
| 80 kg (176 lbs) | 48 L | 40 L | 40 L | 36 L |
| 90 kg (198 lbs) | 54 L | 45 L | 45 L | 40.5 L |
| 100 kg (220 lbs) | 60 L | 50 L | 50 L | 45 L |
Example of Free Water Excess Calculator
A clinician is treating a 70-year-old male patient who weighs 80 kg and presents with a very low serum sodium level.
First, the clinician gathers the necessary patient data.
- Patient Category: Elderly Male
- Body Weight: 80 kg
- Current Serum Sodium: 115 mEq/L
- Desired Serum Sodium: The initial target is to raise the sodium to a safer level of 125 mEq/L.
Step 1: Calculate the Total Body Water (TBW).
For an elderly male, the water percentage is 50% (or 0.50).
TBW = Body Weight (kg) * Water Percentage
TBW = 80 * 0.50
TBW = 40 Liters
Step 2: Calculate the Free Water Excess.
Free Water Excess = 40 * (1 – (115 / 125))
Free Water Excess = 40 * (1 – 0.92) = 3.2 Liters
Therefore, the patient has an estimated free water excess of 3.2 Liters. The clinician would use this information to guide a treatment plan, such as restricting daily fluid intake, to help the body slowly eliminate this excess water.
Most Common FAQs
A free water excess occurs when the body takes in or retains more water than it excretes. This can be caused by a variety of conditions, including certain medications, heart failure, kidney problems, liver disease, or a hormonal disorder called the Syndrome of Inappropriate Antidiuretic Hormone (SIADH), which causes the body to hold onto water.
Similar to correcting high sodium, correcting low sodium too quickly can be dangerous. The brain adapts to a low-sodium environment. If the sodium level is raised too fast, it can cause water to be pulled out of brain cells too rapidly, leading to a severe neurological condition called osmotic demyelination syndrome (ODS). For this reason, treatment is always done gradually and with careful monitoring.
No, this calculator is an educational tool for informational purposes only. The diagnosis and treatment of hyponatremia and other electrolyte imbalances are complex medical issues that must be managed by a qualified healthcare professional. Do not use this tool to self-diagnose or make any decisions about your health or treatment.