How this calculator works
Use spot urine electrolyte concentrations from the same sample. The gap has historically been used as an indirect clue to urinary ammonium excretion during normal-anion-gap metabolic acidosis. It is unreliable with substantial unmeasured urinary ions or altered dietary electrolyte balance. This page reports the arithmetic without assigning a diagnosis. The result is an estimate for information only and is not medical advice.
Formula
UAG = urine Na + urine K - urine Cl, all in mmol/L (numerically equal to mEq/L for these ions).
Worked example
Urine Na 40, K 20 and Cl 80 give 40 + 20 - 80 = -20 mmol/L. Values 50, 30 and 60 give +20 mmol/L.
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Frequently asked questions
Can the gap be negative?
Yes. Chloride can exceed the sum of sodium and potassium, producing a negative gap.
Is ammonium measured here?
No. The gap is not a direct ammonium measurement and cannot quantify ammonium reliably in every setting.
Is this the same as the serum anion gap?
No. It uses urine potassium and chloride, while the usual serum equation uses bicarbonate and often excludes potassium.