Joints, grams and potency

If you're smoking a joint to enjoy the psychoactive effects of THC, then you might want to know how much of the potent cannabinoid is in a typical smoke. Here you'll learn how to calculate the amount of THC in a joint and understand what percentage of the cannabinoid your body is actually processing. 
The weight of the joint (in milligrams) multiplied by potency/THC percentage equals total milligrams of THC. 
One gram = 1000 milligrams. 
Example: The difference between a half gram and full gram joint of the same 20% THC flower:
1 gram: 1000 mg x 0.20 = 200 mg
Half gram: 500 mg x 0.20 = 100 mg
If you'd rather skip the math equations, a modern joint may contain between 60 to 150 mg of THC or more, representing a sharp increase from the approximately 10 mg common in the 1970s. 
The above formulas tell us how much THC is in a joint, but they tell us nothing about how much your body is absorbing from the joint. According to Rae, this number will depend on several factors that differ among individuals. These factors include:

• Composition of the joint, i.e., ratio of THC to CBD
• How much THC is wasted in sidestream smoke which wafts off at the end of a joint without being consumed, unlike mainstream smoke which is what you directly inhale and exhale
• Smoking topography, aka your puff technique
• Bioavailability (the proportion of THC that is able to activate the brain, after being introduced into the body and entering the bloodstream)

Rae elaborated on the key components of mainstream and sidestream smoke, saying, “The ratio of mainstream to sidestream smoke produced by a joint is highly variable because of differences in puff techniques. There are several factors in an individual puff. The volume of smoke per puff is typically about 35 milliliters but can be as big as 55 milliliters (for context, the volume of a shot glass is about 40 milliliters). This volume is determined by the force and amount of time a person inhales.”
Mainstream smoke is also affected by the frequency of inhalations, as Rae said, “Puffing more frequently increases the amount of THC consumed in mainstream smoke, and thus diminishes the amount of THC wasted in sidestream smoke.”
Ultimately, bioavailability is calculated by measuring the THC in the blood, and comparing that to how much THC you started with in the joint. Referencing research published in 2007 in the journal Chemistry & Biodiversity, Rae added, “Bioavailability is highly variable for inhalation: experiments in humans show that it ranges from 2% to 56% depending on the person.”
To illustrate bioavailability, Rae provided the following examples using one gram joints and half gram joints.

• One gram joint = 200 mg THC. On the low end, 2%: 200 x 0.02 = 4 mg. On the high end, 56%: 200 x 0.56 = 112 mg.
  The bioavailable THC in a 1 gram joint with 20% THC is between 4  and 112 milligrams.
  Half gram joint = 100 mg THC. On the low end, 2%: 100 x 0.02 = 2 mg. On the high end, 56%: 100 x 0.56 = 56 mg.
  The bioavailable THC in a half-gram joint with 20% THC is between 2 and 56 milligrams