IUPAC defines the temperature programmed chromatography Kovats index equation:
* &  retention times of trailing and heading n-alkanes, respectively.
NOTE: TPGC index does depend on temperature program, gas velocity and the column used !
ASTM method D6730 defines the temperature programmed chromatography Kovats index equation:
Measured Kovats retention index values can be found in ASTM method D 6730 databases.
An extensive Kovats index database is compiled by NIST [https://webbook.nist.gov/chemistry/].
The equations produce significant different Kovats indices.