The following code first appeared on my blog post Stand-alone error function erf. See that post for documentation. See also Relating erf and Φ.

import math def erf(x): # constants a1 = 0.254829592 a2 = -0.284496736 a3 = 1.421413741 a4 = -1.453152027 a5 = 1.061405429 p = 0.3275911 # Save the sign of x sign = 1 if x < 0: sign = -1 x = abs(x) # A&S formula 7.1.26 t = 1.0/(1.0 + p*x) y = 1.0 - (((((a5*t + a4)*t) + a3)*t + a2)*t + a1)*t*math.exp(-x*x) return sign*y

A&S refers to Handbook of Mathematical Functions by Abramowitz and Stegun.

This code is in the public domain. Do whatever you want to with it, no strings attached.