Electrolyte Replacement Calculator

What this sodium estimate represents

This page calculates an estimated sodium deficit from body weight, a current serum sodium concentration, a higher target concentration, and a selected adult total-body-water factor. It is a simplified reference calculation for serum sodium values. It does not estimate sweat losses, sports-drink needs, or free-water deficit, and it does not select a treatment.

Total body water (TBW) is the estimated amount of water in the body, expressed here in liters. The result is an arithmetic estimate in millimoles (mmol), not a volume of fluid or a dose to administer. A target should come from the treating clinician; the calculator does not suggest one.

Inputs and result fields

Choose sodium, select male or female, and enter body weight in kilograms. Enter the current and clinician-specified target serum sodium concentrations in mmol/L. The page accepts only a target greater than the current value. The result shows the estimated deficit in mmol and the estimated TBW in liters.

Use values from the same laboratory report and keep the displayed units. Do not enter a sodium difference in place of the target concentration: enter the current value and target value separately. The page recalculates as the entries change and can be reset to clear the numeric fields.

The arithmetic behind the estimate

First, the page estimates TBW from weight using a fixed factor: 0.6 for the male selection or 0.5 for the female selection. It then multiplies that estimate by the difference between target and current serum sodium:

The concentration difference in mmol/L multiplied by liters gives mmol. The formula produces a positive value only when the target exceeds the current result; invalid, zero, or negative inputs do not produce an estimate. These fixed factors are a simplified adult assumption, not a personalized measurement of body water.

A worked example, with the right interpretation

Suppose the entered weight is 70 kg, the male factor is selected, current sodium is 125 mmol/L, and a clinician has specified a target of 130 mmol/L. The estimated TBW is 70 × 0.6 = 42 L. The concentration difference is 130 − 125 = 5 mmol/L, so the displayed arithmetic is 5 × 42 = 210 mmol.

This example demonstrates the calculation only. The number does not mean that 210 mmol should be given, nor does it specify a fluid, route, timing, or rate. Actual treatment depends on the clinical situation and monitored laboratory results.

How this differs from other sodium calculations

A sodium-deficit estimate uses the difference between a chosen target and the measured concentration, multiplied by estimated TBW. A free-water-deficit equation addresses a different question, commonly involving elevated sodium and an estimate of water needed relative to a reference concentration. The formulas and meanings of their outputs are not interchangeable.

This page has no potassium calculation, exercise duration, sweat estimate, supplement selector, saline-volume conversion, or correction-rate feature. Its result should be read only as the output of the stated sodium arithmetic.

Use limits and clinical context

Body-water fractions are approximations and can vary with age and individual circumstances. The page uses only two fixed selections and does not account for the full clinical context, causes of an abnormal sodium result, concurrent conditions, medications, or changes during care. Its estimate cannot diagnose hyponatremia or determine the appropriate target.

Clinical guidance treats sodium correction as a monitored process. A calculator output cannot establish an infusion amount, product concentration, administration route, correction speed, or treatment plan. Do not use this estimate to self-treat or to alter prescribed care; discuss laboratory results and targets with a qualified clinician.

Enter the laboratory values carefully

1. Select sodium and the applicable adult factor option.

2. Enter body weight in kilograms, then enter the current serum sodium from the report in mmol/L.

3. Enter the higher target concentration specified by the treating clinician. Do not choose a target based on the calculator.

4. Read the estimated deficit in mmol and estimated TBW in liters as separate outputs. Reset clears the values for another calculation.

Questions about reading the result

Do not use this as a treatment instruction

This educational arithmetic is not a diagnosis, prescription, or patient-specific treatment recommendation. Use a clinician-specified target and rely on qualified clinical assessment and monitoring for care decisions. The calculation does not provide correction speed or a fluid or medication dose. For urgent symptoms or a concerning laboratory result, contact a healthcare professional promptly.