Potassium Deficit Calculator

How it is calculated

The tool takes the gap between the target and the measured serum potassium and scales it by body weight: deficit (mmol) = max(0, target − measured) × weight × 0.4. Potassium levels are in mmol/L, which is numerically the same as mEq/L for potassium, and weight is in kilograms. The factor 0.4 is fixed in the tool, so it is a single linear rule of thumb: each 1 mmol/L of shortfall counts as 0.4 mmol per kg. If the measured value is at or above the target, the deficit is 0. Both potassium values must be above 0 and at most 15 mmol/L, and weight must be between 2 and 650 kg; anything outside these ranges shows no result. The tool also shows the drop (target minus measured, never below zero) and your weight rounded to whole kilograms; the calculation itself uses the weight as typed. It is an estimate for study and discussion, not a dosing instruction, and treatment decisions belong to a clinician.

Common mistakes

• Entering weight in pounds. The field is in kilograms, so a 154 lb person typed as 154 gives 61.6 mmol instead of 28.0, more than double the right answer for the same drop.
• Swapping measured and target values. If the measured value is higher than the target, the tool correctly shows 0.0 mmol, which can look like the entries were ignored.
• Typing a potassium level in the wrong unit or with a slipped decimal, such as 30 instead of 3.0. Levels above 15 show no result.
• Treating the figure as the dose to give at once. It is an estimated total shortfall, and replacement is normally given in steps with re-measurement, as decided by a clinician.
• Ignoring that the formula uses one fixed factor. Kidney function, acid-base status and ongoing losses change real needs and are not modelled.