Dear Herb: How do I calculate THC from THCA?

Hey there, time traveller!
This article was published 31/07/2019 (1338 days ago), so information in it may no longer be current.

Dear Herb: How do people calculate the proportion of THC in cannabis from the THCA? Is there an equation for this?

I just read your article on the two different numbers for THC on cannabis packaging — brilliant, by the way — and I feel like the THCA content is almost more important since it contains the real information in its raw form instead of a “potential” THC content. Especially if a big amount of it is destroyed during combustion/decarboxylation. What are your thoughts on that? — Cannabis Calculations

Dear Calculations: There is indeed an equation to determine how much of the THCA in dried cannabis converts to THC after decarboxylation. First, let’s quickly review what that jargon means, for readers who don’t know much about the relationship between THCA and THC.

THCA, short for tetrahydrocannabinolic acid, is the chemical precursor to THC (delta-9-tetrahydrocannabinol), a key cannabinoid produced by the cannabis plant. Cannabis doesn’t actually produce THC directly — instead, it produces THCA, which is found in fresh cannabis bud. THCA is converted to THC via a chemical reaction called decarboxylation.

In the case of cannabis, the vast majority of decarboxylation is accomplished with the application of heat (i.e. smoking, vaporizing, or cooking). But some decarboxylation also occurs, slowly, as the cannabis bud is dried and cured. That’s why legal cannabis packaging in Canada lists two separate numbers for THC: the smaller number is the lesser amount of activated THC already contained in the dried cannabis bud before decarboxylation, and the larger number, sometimes called “total THC,” is the total proportion of THC that cannabis ought to yield after decarboxylation. In short, it’s the larger number that matters to consumers.

Back to the question. The formula used to calculate the conversion rate of THCA into total THC is:

Total THC = THC + (THCA × 0.877)

This webpage offers a very good explanation of the basic chemistry behind that equation, and it comes down to this: By molecular weight, THCA is roughly 87.7 per cent THC. The remainder of the weight of THCA is called the carboxyl group, which is released in the form of carbon dioxide gas during the decarboxylation process. In other words, about 87.7 per cent of the THCA in dried cannabis becomes THC when it’s burnt or otherwise decarboxylated. Add that to the small amount of already-decarboxylated THC in your dried cannabis bud, and you’ve got your total potential THC yield. (This assumes that 100 per cent of the THCA is decarboxylated into THC.)

In response to the second part of your question: In my opinion, many cannabis consumers are likely already confused by the inclusion of two different THC numbers on cannabis packaging. Listing a third number for THCA might make things even more confusing, especially since most consumers aren’t particularly interested in THCA beyond the amount of THC it yields when decarboxylated.

But if you really want to know how much THCA is contained in your dried cannabis, you can calculate it using the two THC numbers listed on legal cannabis packaging. The equation would be:

THCA = (Total THC − THC) ÷ 0.877

The same equations can be applied to calculate the yield of total CBD from CBDA, and vice versa.

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