Calculate strong ion difference (SID) from Na, K and Cl, with optional lactate, calcium and magnesium, plus effective SID and the strong ion gap (SIG).

Blank optional lactate, calcium or magnesium is omitted, not assumed to be zero, and the SIDa is marked partial.

Optional measured ions — blank means omitted

Use ionized calcium, not total or albumin-adjusted calcium.

Use ionized magnesium. This tool does not estimate it from total magnesium.

SID (Strong Ion Difference) Formula

The following formula gives apparent SID (SIDa) in mEq/L when all ion concentrations are in mmol/L. Use ionized calcium and magnesium. The factors of 2 convert their divalent molar concentrations to charge equivalents; do not double values already expressed in mEq/L.

SIDa = (Na⁺ + K⁺ + 2 × Ca²⁺ + 2 × Mg²⁺) − (Cl⁻ + Lactate⁻)

Variables:

  • SIDa is the apparent Strong Ion Difference
  • Na⁺ is the concentration of sodium ions
  • K⁺ is the concentration of potassium ions
  • Ca²⁺ is the concentration of calcium ions
  • Mg²⁺ is the concentration of magnesium ions
  • Cl⁻ is the concentration of chloride ions
  • Lactate⁻ is the concentration of lactate ions

To calculate SIDa, add sodium and potassium, then twice ionized calcium and magnesium in mmol/L, and subtract chloride and lactate. A blank optional ion is omitted, not measured as zero. Full mode also estimates SIDe = HCO₃⁻ + albumin (g/L) × (0.123 × pH − 0.631) + phosphate (mmol/L) × (0.309 × pH − 0.469), then SIG = SIDa − SIDe. Albumin in g/dL is multiplied by 10 before this formula is used. The linear weak-acid approximation is limited here to pH 6.9–7.9.

What is a SID (Strong Ion Difference)?

Strong ion difference is the difference between strong-cation and strong-anion charge concentrations in Stewart’s acid–base framework. SIDa uses the measured ions included in the calculation; SIDe estimates the balancing charge from bicarbonate, albumin and phosphate. Their difference is the strong ion gap. With carbon dioxide and weak acids held constant, a higher SID tends to raise pH and a lower SID tends to lower pH. Actual clinical interpretation also requires these other variables, measurement quality and the full assessment; this calculator does not apply a diagnostic cutoff.

How to Calculate SID (Strong Ion Difference)?

The following steps outline how to calculate the SID (Strong Ion Difference).

  1. First, determine the concentration of strong cations in charge equivalents (mEq/L). If your calcium and magnesium are in mmol/L, multiply each by 2 to convert to mEq/L.
  2. Next, determine the concentration of strong anions in mEq/L.
  3. Next, subtract the concentration of strong anions from the concentration of strong cations.
  4. Finally, calculate the SID (Strong Ion Difference).
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

concentration of strong cations (mEq/L) = 10

concentration of strong anions (mEq/L) = 5. The illustrative charge difference is 10 − 5 = 5 mEq/L; these are arithmetic example values, not patient reference ranges.