Sodium Deficit Calculator — Hyponatraemia Treatment Planning
In hyponatraemia, the sodium deficit formula quantifies how many milliequivalents of sodium are missing and how much intravenous fluid is needed to reach a safe target. Enter the current and target sodium, weight, patient type, and replacement fluid to get the total deficit and required volume.
mEq/L
mEq/L
kg
Patient type
Replacement fluid
Total sodium needed to raise serum Na to the target level (assuming no ongoing losses)
- 1
Estimated total body water (TBW)
70 × 0.6 = 42 L - 2
Sodium deficit
42 × (135 − 120) = 630TBW × ΔNa — total milliequivalents of sodium needed to reach the target.
How does this calculator work?
Sodium deficit (mEq) = TBW × (target Na − current Na), where TBW = weight × 0.6 (adult males), 0.5 (adult females). Divide the deficit by the replacement fluid's Na concentration (mEq/L) and multiply by 1000 for the volume in mL. Correction rate must not exceed 10–12 mEq/L per 24 h. Ongoing losses and renal response will affect actual requirements — monitor frequently.
Formula
How this is calculated
The sodium deficit formula estimates the total amount of sodium (in milliequivalents) needed to raise the serum sodium from its current value to a target. It is derived from the dilution model: because sodium is distributed across total body water (TBW), raising the concentration by ΔNa requires adding TBW × ΔNa milliequivalents of sodium. TBW is estimated as a fixed fraction of body weight: 60% for adult males and children, 50% for adult females and elderly males, and 45% for elderly females (Adrogue-Madias 2000).
Once the deficit is known, the volume of replacement fluid required can be calculated by dividing the deficit by the sodium concentration of the chosen fluid (in mEq/L) and multiplying by 1000 to get millilitres. For example, a deficit of 200 mEq corrected with normal saline (154 mEq/L) requires 200/154 × 1000 ≈ 1300 mL.
Important caveats: this formula assumes no ongoing sodium losses (urine, sweat, fistula) and a static TBW, neither of which holds in practice. The real-world kidney response to volume loading — particularly in SIADH, where ADH prevents free-water excretion — means that saline infusion can paradoxically worsen hyponatraemia, as the kidney excretes the sodium while retaining the water. In SIADH, fluid restriction or vasopressin antagonists are usually more appropriate than sodium replacement. Always interpret the deficit calculation alongside the cause of hyponatraemia and monitor sodium frequently during treatment.
Frequently asked questions
It follows directly from the concept that serum sodium = total body sodium / total body water. If you want to raise sodium by ΔNa in a patient with TBW litres of body water, you need to add TBW × ΔNa milliequivalents of sodium (while keeping water constant). The formula Na_deficit = TBW × (Na_target − Na_current) is a simplified, clinically useful expression of this relationship.
Several factors can cause a discrepancy: ongoing urinary sodium losses, water intake or infusions diluting the gain, renal regulation of osmolality (particularly in SIADH), redistribution of sodium across compartments, and imprecise estimation of TBW. The Adrogue-Madias approach (used in the sodium change calculator) is more accurate because it accounts for the fluid added to TBW. Always monitor serum sodium every 2–4 hours during active correction.
3% hypertonic saline (513 mEq/L) is preferred when: (1) the patient has severe symptomatic hyponatraemia (seizures, coma, respiratory failure); (2) the sodium is very low (<120 mEq/L) and a rapid initial correction is needed; or (3) you need to deliver a large sodium load in a small volume to avoid fluid overload (e.g., in patients with heart failure or renal failure). It is typically administered centrally with close monitoring.
Also known as
TG we-Calculate Editorial Team. (2026). Sodium Deficit Calculator — Hyponatraemia Treatment Planning [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/sodium-deficit-calculator
TG we-Calculate Editorial Team. "Sodium Deficit Calculator — Hyponatraemia Treatment Planning." TG we-Calculate. 2026. https://we-calculate.com/calculator/sodium-deficit-calculator.
TG we-Calculate Editorial Team, "Sodium Deficit Calculator — Hyponatraemia Treatment Planning," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/sodium-deficit-calculator
@misc{wecalculate_sodium_deficit_calculator, title = {Sodium Deficit Calculator — Hyponatraemia Treatment Planning}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/sodium-deficit-calculator}}, year = {2026}, note = {TG we-Calculate} }
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