Calculate total sodium deficit in hyponatremia. Provides replacement volumes for 3% NaCl, normal saline, and oral NaCl tablets with a stepwise correction plan.
Hyponatremia — serum sodium below 135 mEq/L — is the most common electrolyte disorder in hospitalized patients, affecting up to 30% of ICU admissions. Calculating the sodium deficit is the first step in planning replacement therapy. The deficit formula (TBW × desired change in sodium) tells you how many milliequivalents of sodium the patient needs, which then converts to a specific volume of 3% hypertonic saline, normal saline, or oral NaCl tablets.
This calculator estimates total body water using age- and sex-adjusted Watson factors, computes the total sodium deficit, and converts it to practical volumes for each solution. Crucially, it also generates a stepwise day-by-day correction plan that respects safe 24-hour limits (8–10 mEq/L per day) to prevent osmotic demyelination syndrome. An infusion rate for 3% NaCl is provided so the order can be written immediately.
For mild, chronic hyponatremia (Na 125–134), oral NaCl tablets with fluid restriction may suffice. For moderate hyponatremia (Na 120–124), admission with intravenous correction is standard. For severe or symptomatic hyponatremia (Na < 120 or seizures), 3% NaCl with ICU monitoring is the treatment of choice.
Converting a serum sodium value into a concrete volume order (mL of 3% NaCl, rate per hour) eliminates guesswork at the bedside. The stepwise plan ensures the correction stays within safe limits across multiple days. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain. Use this clarification to avoid ambiguous interpretation.
Sodium deficit (mEq) = TBW × (Target Na⁺ − Current Na⁺). TBW = body weight(kg) × factor (Male <65y: 0.6; Male ≥65y: 0.5; Female <65y: 0.5; Female ≥65y: 0.45). Volume of 3% NaCl = deficit(mEq) / 513(mEq/L) × 1000(mL).
Result: Deficit = 420 mEq; 3% NaCl = 819 mL; NS = 2,727 mL
TBW = 70 × 0.6 = 42 L. Deficit = 42 × (130 − 120) = 420 mEq. Volume of 3% NaCl (513 mEq/L) = 420/513 × 1000 = 819 mL. This 10 mEq/L correction exceeds the safe 24h limit, so it should be split over 2 days: 8 mEq/L on day 1, then 2 mEq/L on day 2.
For symptomatic hyponatremia (seizures, severe confusion), 3% NaCl is the first choice because it provides 3.3× more sodium per mL than normal saline. This means smaller volumes, faster correction, and less fluid overload risk. Normal saline (0.9% NaCl, 154 mEq/L) is used for hypovolemic hyponatremia — patients who are sodium-depleted from vomiting, diuretics, or third-spacing. In these patients, volume resuscitation itself improves sodium by suppressing ADH and allowing renal water excretion.
In euvolemic hyponatremia (SIADH), fluid restriction is as important as sodium replacement. The typical order is 1.0–1.5 L/day total fluid intake. However, compliance is difficult because patients are thirsty. A practical approach: restrict free water but allow unrestricted solid food, combine with oral NaCl tablets, and add a loop diuretic (furosemide 20–40 mg) to promote electrolyte-free water excretion if necessary.
If sodium doesn't rise as predicted, consider: (1) ongoing renal-free-water retention (check urine osmolality — if >300 mOsm/kg, the kidneys are still concentrating urine and retaining water), (2) unmeasured sodium losses (GI, burns, drains), (3) medication effects (thiazides continue to cause losses until discontinued), or (4) adrenal insufficiency (cortisol is required for normal water excretion). The deficit formula is a guide; the serial sodium is the truth.
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.
Absolutely not for chronic hyponatremia. The correction must be limited to 8–10 mEq/L per 24h to prevent ODS. The deficit calculation tells you the total amount needed, which may need to be spread over 2–3 days. Only in acute, symptomatic hyponatremia (<48h onset) can faster correction be considered.
Oral NaCl tablets (1g = 17.1 mEq) are appropriate for chronic, mild hyponatremia (Na 125–134) due to SIADH. Typically 3–9g/day with fluid restriction. IV replacement (NS or 3% NaCl) is needed for moderate-to-severe hyponatremia or when the patient cannot take oral medications.
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.
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.
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.