Katz correction: measured Na + 1.6 × (glucose − 100) ÷ 100, with glucose in mg/dL. Some units use the steeper Hillier factor of 2.4; this page shows the Katz arithmetic it is doing.
A correction, not a diagnosis. It does not tell you the volume state, the tonicity or the rate at which sodium may be moved — and over-rapid correction is the harm that matters here.
It is a calculator, not a clinician. Nothing here is medical advice, and the person prescribing owns the decision.