Sodium Deficit Calculator

About the Sodium Deficit Calculator

Hyponatremia — serum sodium below 135 mEq/L — is a common inpatient electrolyte problem. Calculating the sodium deficit is one way to organize the replacement discussion. The deficit formula (TBW × desired change in sodium) tells you how many milliequivalents of sodium correspond to the chosen change in serum sodium, which can then be translated into reference volumes for 3% saline, normal saline, or oral NaCl tablets.

This calculator estimates total body water using simple age- and sex-adjusted TBW factors, computes the sodium deficit, and converts it to common solution equivalents. It also generates a stepwise day-by-day worksheet that respects usual 24-hour correction limits (8–10 mEq/L per day), because the main risk in hyponatremia correction is overcorrection rather than undercalculation.

The page is best read as a reference worksheet, not a stand-alone order set. Mild chronic hyponatremia, hypovolemic hyponatremia, SIADH, and acute symptomatic hyponatremia are managed differently, so the numbers here need to be paired with the actual bedside cause and serial lab response.

When This Page Helps

Hyponatremia review depends on turning a lab value into a controlled correction worksheet. This calculator keeps the serum sodium, target sodium, TBW estimate, and solution equivalents together so the team can discuss replacement without losing track of the daily safety limit.

How to Use the Inputs

1. Enter the current serum sodium from the most recent lab draw.
2. Enter your target sodium — many teams choose a conservative first-day target rather than full normalization.
3. Enter the patient's weight (kg or lbs) and select sex and age for TBW estimation.
4. Review the total sodium deficit in mEq and the volume of each replacement solution.
5. Check the stepwise correction worksheet and the continuous-rate reference line if relevant.
6. Monitor serial sodium levels and recalculate after every new lab draw.

Example Calculation

Tips & Best Practices

• Start with a conservative target — a partial first-day correction is often enough to improve symptoms without overshooting.
• Recheck sodium frequently during active correction. Do not rely on the initial calculation alone.
• The same sodium value can reflect very different problems in SIADH, hypovolemia, hypervolemia, or acute symptomatic hyponatremia.
• The page shows continuous-rate equivalents, but acute bolus pathways are usually handled separately from slow correction plans.
• In edematous patients (CHF, cirrhosis), the simple TBW estimate may overstate the practical sodium need.
• Account for planned potassium replacement because it also changes effective tonicity.

Choosing Between 3% NaCl and Normal Saline

Hypertonic saline and normal saline are not interchangeable, even though this page converts the sodium deficit into both. Hypertonic saline gives more sodium per mL and is the common reference fluid when the team is reviewing severe or symptomatic hyponatremia. Normal saline is more often discussed in hypovolemic hyponatremia, where restoring volume can help suppress ADH and improve sodium indirectly.

Fluid Restriction: The Forgotten Pillar

In euvolemic hyponatremia (especially SIADH), fluid restriction often matters as much as sodium replacement. That is one reason a sodium-deficit number by itself is never the whole plan. Some patients improve mainly by reducing free-water intake, while others need a combination of oral sodium, loop diuretics, or a more specialized pathway.

When the Calculation Doesn't Match Reality

If sodium does not rise as predicted, think about ongoing water retention, unmeasured sodium losses, medication effects, or endocrine causes such as adrenal insufficiency. The deficit formula is only a guide; the serial sodium trend is the real feedback loop.

Frequently Asked Questions

• What is the sodium deficit formula?

  Sodium deficit = TBW × (target Na − measured Na). TBW is estimated as body weight × a sex/age factor (0.45–0.6). The result in mEq tells you how much sodium to replace. This is an estimate — real-world sodium is affected by ongoing losses, oral intake, and kidney handling.

• Can I give the entire deficit at once?

  For chronic hyponatremia, no. The whole point of the worksheet is to show the total sodium equivalent while keeping the daily change within the usual 8–10 mEq/L safety ceiling. The deficit often needs to be spread across more than one day.

• When should I use salt tablets vs IV?

  This page shows the sodium equivalents for both oral and IV options, but the right route depends on the cause of hyponatremia, symptom burden, oral tolerance, and the local treatment pathway. The calculator is better at showing equivalence than choosing the route for you.

• Why does the formula underestimate true needs?

  The formula assumes a closed system — no ongoing sodium losses. In reality, patients continue to lose sodium through renal excretion, GI losses (vomiting, diarrhea), and insensible losses. The calculated deficit is a starting point; serial monitoring allows real-time adjustment.

• How does TBW change with edema or dehydration?

  In edematous patients (heart failure, cirrhosis), total body water is increased but effective circulating volume is low — the standard formula overestimates replacement needs. In dehydrated patients, TBW is decreased, and the formula underestimates. Clinical judgment must adjust the calculation.

• What if the patient also needs potassium?

  Potassium is an effective intracellular osmole. Giving KCl raises serum sodium by approximately the same amount as an equivalent mEq of NaCl. Account for planned K replacement when calculating sodium correction rate to avoid exceeding 24h limits.