Our calculators have been validated by our own extensive testing. Internally our calculators use tables which are identical to the ones specified by the standards. We have launched our calculators in the year 2000 (see http://www.sqconline.com/about-sqc-online). In the past 23 years the calculators have been used and scrutinized by thousands of users (including government agencies and many medical and pharmaceutical companies) who often double-check our results against the tables.
Thus, while we cannot guarantee that our website is 100% free of errors (see the disclaimer here http://www.sqconline.com/disclaimer) we are absolutely confident that using our calculators is a much safer alternative to the error-prone usage of the printed standards.