Z-Score to percentile

The normal curve is computed in this tab: erf by its power series, the far tail by a continued fraction, and the inverse by the Acklam approximation with one refinement pass, so percentile-to-z round-trips close to machine precision. One honest limit: beyond about ±8.3 standard deviations the tail probability is smaller than a double can express next to 1, and it reads as zero even though the true tail is not. p values here are raw normal probabilities, not corrected for multiple comparisons, and this tool does not decide significance for you.