Strong Ion Difference Calculator

Calculate the apparent strong ion difference (SIDa) using the Stewart approach.

Aug 31, 2026 Fluids, Electrolytes & Acid Base #acid-base #strong-ion-difference #sid #stewart

Calculate

All fields required unless noted

Serum sodium (mEq/L)

Serum potassium (mEq/L)

Serum chloride (mEq/L)

Ionised calcium (optional) (mEq/L)

Magnesium (optional) (mEq/L)

Lactate (optional) (mEq/L)

Calculating...

Enter your values and press Calculate to see the result.

Formula & Clinical Notes

Formula

SIDa = (Na + K + Ca + Mg) − (Cl + Lactate)

Clinical Notes

The apparent strong ion difference is SIDa = (Na + K + Ca + Mg) − (Cl + Lactate), all in mEq/L. The normal SIDa is approximately 38–42 mEq/L. A reduced SIDa (acidifying) favours metabolic acidosis (e.g., lactic acidosis, hyperchloremia), while an increased SIDa (alkalinising) favours metabolic alkalosis. Note: ionised calcium in mmol/L must be multiplied by 2 to convert to mEq/L.

Sources

Stewart (1983) — Modern Quantitative Acid-Base Chemistry