z = (x − μ) / σ. σ must be non-zero.
Standard score z = (x − μ) / σ with finite x, mean μ, and standard deviation σ ≠ 0. This is the textbook definition for a single observation relative to a stated distribution center and spread.
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.