Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
suppŭtātĭo (subp-), ōnis, f. [supputo], a reckoning up, computation, Vitr. 3, 1 med.; Arn. 2, 93: temporum, Aug. Civ. Dei, 18, 36: stadiorum, Mart. Cap. 6, § 609.