Free Water Deficit Correction Rate Calculator

Estimate the free water deficit and required infusion rate to correct hypernatremia.

Current Serum Sodium (mmol/L or mEq/L):

Target Serum Sodium (mmol/L or mEq/L): Target should generally not be >10-12 mmol/L lower than current per 24h.

Patient Weight (kg):

Patient Constitution (estimates TBW): Adult Male (0.6) Adult Female (0.5) Elderly Male (0.5) Elderly Female (0.45) Child (0.6)

Planned Correction Timeframe (Hours):

function calculateFWD() { var currentNa = parseFloat(document.getElementById('currentNa').value); var targetNa = parseFloat(document.getElementById('targetNa').value); var weight = parseFloat(document.getElementById('patientWeight').value); var tbwFactor = parseFloat(document.getElementById('patientType').value); var hours = parseFloat(document.getElementById('correctionHours').value); var resultDiv = document.getElementById('fwdResult'); resultDiv.style.display = 'block'; if (isNaN(currentNa) || isNaN(targetNa) || isNaN(weight) || isNaN(hours) || weight <= 0 || hours <= 0) { resultDiv.innerHTML = 'Please provide valid positive numerical inputs for all fields.'; return; } if (currentNa <= targetNa) { resultDiv.innerHTML = 'Current Sodium must be higher than Target Sodium to calculate a deficit (Hypernatremia).'; return; } // Step 1: Estimate Total Body Water (TBW) var tbw = weight * tbwFactor; // Step 2: Calculate Free Water Deficit (FWD) in Liters // Formula: FWD = TBW * ((Current Na / Target Na) – 1) var fwdLiters = tbw * ((currentNa / targetNa) – 1); var fwdML = fwdLiters * 1000; // Step 3: Calculate Hourly Rate var hourlyRateMl = fwdML / hours; resultDiv.innerHTML = '

Calculation Results

' + 'Estimated Total Body Water: ' + tbw.toFixed(1) + ' L' + 'Total Free Water Deficit: ' + fwdLiters.toFixed(2) + ' Liters (' + Math.round(fwdML) + ' mL)' + 'Required Correction Rate: ' + Math.round(hourlyRateMl) + ' mL/hr' + 'Note: This assumes administration of free water (e.g., D5W) and accounts only for existing deficit, not ongoing losses or maintenance needs. Clinical monitoring is essential to ensure safe correction rates.'; }

Understanding Free Water Deficit and Safe Correction in Hypernatremia

Hypernatremia is a medical condition characterized by a high concentration of sodium in the blood (serum sodium typically >145 mmol/L). It generally occurs because the body has lost more water than sodium, resulting in a "free water deficit." Correcting this deficit involves administering hypotonic fluids (fluids with a lower solute concentration than normal body fluids), essentially replacing the missing free water.

This calculator uses standard medical formulas to estimate the total amount of free water required to lower serum sodium to a desired target level and calculates the necessary hourly infusion rate to achieve that goal over a specific timeframe.

The Formulas Behind the Calculation

The estimation of free water deficit relies on understanding the patient's Total Body Water (TBW) relative to their sodium concentration. The process involves three main steps:

Estimating Total Body Water (TBW): TBW is estimated as a fraction of total body weight, which varies by age and biological sex due to differences in muscle mass and body fat.
- Adult Men and Children: ~60% of weight (Weight kg x 0.6)
- Adult Women and Elderly Men: ~50% of weight (Weight kg x 0.5)
- Elderly Women: ~45% of weight (Weight kg x 0.45)
Calculating Free Water Deficit (FWD): This formula determines how much water is missing relative to the current sodium level.
FWD (Liters) = TBW × [(Current Na+ / Target Na+) – 1]
Determining Correction Rate: The total deficit is converted to milliliters and divided by the desired correction hours to find the infusion rate.
Rate (mL/hr) = (FWD in Liters × 1000) / Correction Hours

Clinical Importance: The Danger of Rapid Correction

While calculating the deficit is essential, the rate of correction is critical for patient safety. Correcting chronic hypernatremia too rapidly can lead to severe neurological complications, most notably cerebral edema (swelling of the brain), which can be fatal.

Current medical guidelines generally recommend that serum sodium should not be lowered by more than 10 to 12 mmol/L in a 24-hour period (roughly 0.5 mmol/L per hour). This calculator allows you to input a timeframe to help plan a safe correction schedule. The target sodium input should reflect a safe goal for the chosen duration.

Example Scenario

Consider a 70 kg adult male admitted with a serum sodium of 160 mmol/L. The clinical team decides to lower his sodium to 150 mmol/L over the next 24 hours to ensure safety.

Current Na+: 160 mmol/L
Target Na+: 150 mmol/L
Weight: 70 kg
Constitution: Adult Male (Factor 0.6)
Timeframe: 24 Hours

Using the calculator:
TBW = 70 * 0.6 = 42 Liters.
FWD = 42 * [(160 / 150) – 1] = 42 * [1.066 – 1] = 42 * 0.066 = 2.8 Liters (2800 mL) total deficit.
Correction Rate = 2800 mL / 24 hours = 117 mL/hr.

Disclaimer: This tool is meant for educational and estimation purposes only. It does not account for ongoing free water losses (e.g., through sweat, respiration, or dilute urine) or maintenance fluid requirements. Actual treatment requires frequent monitoring of serum electrolytes and clinical judgment by qualified medical professionals.