Marijuana Math

Updated: Nov 11

Ha! Who knew that when you walk into a dispensary to buy marijuana/cannabis/weed you’d have to manage metric system conversions, percentages, fractions, multiplication and division. Happily you only need your cell phone’s calculator to figure out what you are buying (or scroll to the bottom for a marijuana-THC calculator!).

Let’s get right to it. You are looking at a wall of pre-rolled joints with fun names and percentages of THC or other cannabinoids. You totally understand a gummy or a chocolate that is labeled 5 milligrams of THC but what does a pre-rolled joint with 19% THC really mean?? How can you figure out the equivalent amount? Out comes the calculator as you dust off the high school math you never thought you’d use. First consider the size of the joint you’re buying. Let’s agree that you chose a 1-gram pre-rolled joint labeled 19% THC.

  1. Convert 1 gram to milligrams and you have a joint that = 1000 milligrams of cannabis

  2. You have a joint with 19% THC. Converting percentages to fractions gives you 19/100 = 0.19

  3. Multiply 1000 by 0.19 to determine that you have 190 milligrams of THC in the entire joint

Wow, that’s a lot if you compare it with the 5-milligram gummy, mint or chocolate. But, we aren’t done yet! When you light the joint, some of the cannabis goes up in smoke. Researchers who test the bioavailability of THC in smoked cannabis show that anywhere from 23 to 30% of the cannabis is lost in the burning process. Additionally, about 40 to 50% of the original THC content is lost to side stream smoke, essentially the smoke that is lost when the joint is lit but idle.

At a 25% loss simply due to lighting your joint and having the dried material burn, the bioavailability of THC in your joint is now only 75% of the original amount or 142.50 milligrams.

And now the last question, how frequently do you take a drag of that joint? How many puffs are you likely to take? The same researchers showed that people tend to take 2.6 puffs per minute accounting for that 40-50% of side stream smoke. With those numbers, the joint you purchased (1 gram at 19% THC) will likely pack approximately 71 milligrams of THC if you smoked the whole thing. How many puffs are in a typical joint? Well, that’s impossible to know for sure and it’s very individual.

As with all cannabis, the adage is: start low and go slow.

Now for the math problem you wish you had in high school 😊.

How many milligrams of THC would you expect to be in a joint that weighs 0.75 grams with a THC content of 12%. (Hint: Use 25% cannabis lost in the burning and 50% lost to side stream smoke)

A THC Calculator:

Perez-Reyes, M. Factors Influencing the bioavailability of THC. Department of Psychiatry UNC Chapel Hill. Research supported by the National Institute of Drug Addiction. Accessed:

  • Answer: 33.75 milligrams


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