Decarboxylation is the process whereby the acid cannabinoids (such as THCA) produced by the plant are heated to "activate" them by converting to the neutrals (such as THC). In the process, each acid molecule loses CO2 (carbon dioxide) which leaves as a gas. In concentrates, one can often see the bubbles during this process. Since the acid molecules lose mass (CO2 has a molar mass of 44 D), the percent by weight of THC will always be lower than the percent by weight of THC. For THCA--->THC, the conversion results in THC (MW = 314.4 g/mol) from THCA (MW = 358.4 g/mol) so the ratio is 0.88. But that's not all that happens. In addition to losing CO2 and forming THC, THCA also forms other byproducts. In most instances, a good estimate of the total conversion efficiency is about 75%, but this will depend somewhat on temperatures and times used to decarboxylate.
What is "Total THC"?
The term "Total THC" does not have a well-defined meaning. It is calculated in different ways, producing very different results.
Under some circumstances, THCA and THC are simply added together without modification to get a "Total THC" value.
Total THC = THCA + THC
If THCA = 15% and THC = 3%, then "Total THC" = 15 + 3 = 18% by wt
This overestimates the potential amount of THC obtainable by heating because it does not: 1) take into account the decarboxylation and concomitant loss of mass (358.4 --> 314.4, ratio = 0.88 applied to the THCA value) and also the inefficiency of the reaction. The rationale here is that only measured values are used (no other calculations) and, basically, that this gives the highest value for "Total THC."
In other circumstances, "Total THC" is calculated using
Total THC = (0.88 x THCA) + THC
If THCA = 15% and THC = 3%, then "Total THC" = (0.88 x 15) + 3 = 16.2% by wt
This also overestimates the potential amount of THC obtainable by heating because, while it does take into account the decarboxylation and concomitant loss of mass (358.4 --> 314.4, ratio = 0.88 applied to the THCA value), it does not take into account the inefficiency of the reaction. The rationale for using this method is that this method uses only measured values plus an exact theoretical calculation that doesn't change.
Under other circumstances, one wants to have a more accurate estimate of the actual potential THC produced after decarboxylation (i.e., to help gauge process efficiency, cost per mg of THC, etc.). In that case, experimental evidence (some cited in peer-reviewed literature) shows that, at most, about 75% of the THCA molecules end up as THC molecules. Thus, the following equation can be used to obtain a realistic estimate of the amount of THC potentially obtained.
Total THC = (0.75 x 0.88 x THCA) + THC
If THCA = 15% and THC = 3%, then "Total THC" = (0.75 x 0.88 x 15) + 3 = 12.2% by wt
This last method, however, uses an estimated conversion efficiency that can vary due to temperatures, timing, and method of heating. Thus, it could be in error if the methodology used is significantly different than that used to estimate the efficiency in the first place. Most times when we calculate this efficiency experimentally at Steep Hill, it comes out at about 75%, which is similar to an estimate form the literature at 72%.