Calculate The Amount Of THC In A Joint

Cannabis products are required to be tested and labeled for the amount of THC and CBD they contain. Because these two cannabinoids are typically the most abundant in cannabis products, it’s critical for consumers to understand what they are and how they work. Although cannabis packaging contains labels that indicate the amount of THC and CBD in products, these numbers can be difficult to read and interpret, especially for people new to cannabis.

Fortunately, the biology of cannabinoid production is constrained in interesting ways. Strain genetics limit cannabinoid production in cannabis flower. Although there are several different major cannabinoids produced by cannabis, broadly speaking, there are three major “chemotypes” defined by THC:CBD ratios:
  • THC-dominant strains contain mostly THC and little CBD and will have strong psychoactive effects. Most popular strains, such as Blue Dream, OG Kush, and Granddaddy Purple, fall into this category.
  • CBD-dominant strains contain mostly CBD and little THC and will have little or no obvious psychoactive effects. Charlotte’s Web and Remedy are popular examples of CBD-dominant strains.
  • Balanced strains fall somewhere in the middle. They have THC and some CBD, but generally not as much as THC- or CBD-dominant strains. The strains will get you high, but the effects will be noticeably different from THC-dominant strains. Examples include Cannatonic and Harlequin.

There are numerous factors to consider in determining how much THC a person actually consumes when taking a toke.

This topic has been researched by the analytical testing lab The Werc Shop, and they break it down in a comprehensive 2015 report titled “The Conversion and Transfer of Cannabinoids from Cannabis to Smoke Stream in Cigarettes.”

First, I hate to break it to you, but your Kush joint is not actually 15 percent THC. The THC in flower or bud is mostly in the non-psychoactive acid form, THCA. After the joint is sparked, the heat from burning converts THCA to psychoactive THC, a process known as decarboxylation.

According to The Werc Shop’s report, a calculation for theoretical maximum THC is needed, and here’s the formula they employed:

THC max = THC + THCA / 358.48 * 314.47
For simplicity, I don’t include the 1 percent to 3 percent THC possibly present in your gram of flower as this is an unknown. The calculation for the Kush is thus:

0 + 15 / 358.48 * 314.47 = 13.16 percent maximum THC
If you have a thorough lab analysis, you could include the THC in the formula and argue there is indeed 15 percent THC in the bud. My point is there is a conversion to factor in.

Next, let’s calculate how much THC is in the whole joint. The unit of measurement is milligrams. Your one-gram joint of Kush weighs 1000 milligrams. Since the strain contains 13.16 percent maximum THC, let’s round down to 13 percent. Multiply 13 percent THC by 1000 milligrams to get the number of milligrams in the joint:

.13 x 1000 = 130 milligrams THC

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Up in smoke

Now, for the puffing and passing. This is where the math starts to get fuzzy.

For smoking metrics, I turned to medical professor Mario Perez-Reyes in the Department of Psychiatry at University of North Carolina at Chapel Hill. According to his informative 1990 report “Marijuana Smoking: Factors That Influence the Bioavailability of Tetrahydrocannabinol,” one reason it’s hard to know exactly how much THC is consumed in a joint is because a good deal of THC is lost as the joint burns, awaiting the next puff. According to Perez-Reyes, as much as 40 percent to 50 percent THC is lost in this sidestream smoke.

Another reason for the fuzzy math is physiological differences in people.

I spoke with Jeff Raber, CEO of The Werc Shop, by phone for additional insights. Raber says: “There is no standard puff measurement. Lung capacity, how much you inhale, how long you hold your breath, how much THC is left in the exhale, varies greatly among individuals.”

Given these variables, it’s difficult to say how much THC is in a single puff. We can look at the whole joint and make some estimates of THC ingestion, based on Perez-Reyes’ findings.

If you smoke half of the Kush joint, the most THC you can get is 33 milligrams to 39 milligrams with a sidestream loss between 26 milligrams and 33 milligrams THC. If it’s 4:20 and you want to smoke the whole joint, you’ll be consuming approximately 65 milligrams to 78 milligrams THC.

Size matters

The one-gram joint is easy for calculating; however, it’s a fatty for just one person. Let’s get a couple more Kush joints circulating in this math sesh for comparison.

In an article by The Washington Post’s Christopher Ingraham, the average weight of a joint is reported to be .32 grams, according to a 2016 analysis published in the journal Drug and Alcohol Dependence.

To calculate maximum THC content of this average-sized joint, first convert to milligrams:

.32 grams = 320 milligrams
13 percent maximum THC of 320 milligrams =
.13 * 320 = 42 milligrams THC
Puffing on half of an average joint will get you 11 milligrams to 13 milligrams THC, with 9 milligrams to 11 milligrams THC going up in sidestream smoke. If you are smoking the entire joint, let’s assume you are using either a crutch or a roach clip to maximize the herb consumption in the joint, so your roach is just paper. The THC consumed in the average joint then is between 17 milligrams to 21 milligrams.

Another popular joint size, also from Ingraham’s report, is .75 grams, based on a survey by High Times with about 3,000 responses. Here’s the amount of Kush THC in the pot enthusiast’s joint size:

.75 grams = 750 milligrams
13 percent of 750 =
.13 x 750 = 97.5 milligrams THC
If you smoke half the joint, you’ll consume at most 25 milligrams to 30 milligrams THC. After smoking the whole joint, you’ll have between 49 milligrams and 59 milligrams THC with the sidestream loss of 39-49 milligrams THC.

To double-check my math, I asked Benjamin Dyhr, Metropolitan State University of Denver Associate Professor of Mathematics, to look at my calculations. He reports “they seemed correct.” All right!

Additional factors

One more reason for the fuzzy math is the changes of THC concentration within the joint as it is being smoked. Raber points out, “As the joint is burning, THC condenses toward the end of the joint and the THC levels will be slightly higher in the second half of the joint. Each puff is not the same.”

Raber also mentions that shape differences in joints and how they burn may make accurate measurements difficult: “Are they smoking a cone or a hand rolled joint that might have a fatter end?”

If you are interested in determining your own THC consumption per puff, Raber has an experiment for you: Do your own puff test.

Smoke a joint with a known weight and potency. Keep a tally of each puff you take. Then calculate your average puff: Use the max THC calculation and metric conversion to determine the total amount of THC in milligrams; consider the percentages of sidestream loss; and divide by the number of puffs to figure out the amount of THC in each toke.

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