In the dried plant, THC is mostly present as THCA, an acid form that is not intoxicating. Only when it is heated, for example when smoked or vaporised, does part of the molecule split off, and THCA turns into THC.
Labs therefore measure both and add them together:
Total THC = THC + 0.877 × THCA
An example: if a sample contains 20 per cent THCA and 0.5 per cent THC, that makes 0.5 + 17.5 = 18.0 per cent total THC.
The factor 0.877 comes from chemistry: THCA loses weight when it converts, and of 100 parts of THCA, around 87.7 parts of THC remain. Total THC therefore shows the maximum amount of THC that can be produced if all the THCA is converted. The same calculation applies to CBD and its acid form CBDA.
This total value is what counts for an assessment. A low THC value on its own says little if there is a lot of THCA.
Remember: On lab reports, the total THC value is what counts for the risk, not the THC value alone.
More on this: What cannabis does in the body · Lab resultsDE