Figuring out how much THC is actually consumed in a joint can be a bit intense. (Dean Mouhtaropoulos, Getty Images)

THC test time: How to calculate the amount of THC you’re toking in a joint

Ask the Cannabist: Here's a math problem your teacher never assigned

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Hey, Cannabist!
If I smoke two puffs off a joint that contains a gram of Kush that is rated 15 percent THC, how much THC did I just ingest in my body, and what unit of measurement is used? If it was 4:20 p.m. and I decided to smoke half of it, how much THC then? If I decided to go “all in” and leave nothing but a small roach, how much THC then?
–Magic Smoke Mathematician

Hey, Magic!

This is a fun question. Let’s do some calculating!

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

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.

If all of this info is too geeky for you, the short answer to the question is you ingested at least enough THC to get high! XO