Recently I’ve been asked by several patients and also by an NGO for help with cannabis THC calculations. How do you calculate your dose? It occurred to me that this might be a topic that others would like to hear about. 

Say No To Smoking

Let’s start by being clear.  If you’re using cannabis for medical benefit, particularly on a regular or daily basis, smoking isn’t the best choice.  Cannabis smoke contains many of the same carcinogens, toxins, and small irritants that are found in tobacco smoke.

It’s true that long-term follow-up on cannabis smokers has shown little to no development of cancers or emphysema like we see in tobacco smokers.  Yet, given the similarities of toxins and the availability of cheap and safe flower vaporizers, I see no good reason anyone should smoke marijuana.  

If you are my patient and are currently smoking, please consider my recommendation to switch to a flower vaporizer.  We have a highly regarded machine available.  If you are outside of Massachusetts, we can provide a simple recommendation.  Please reach out to

About Cannabis

While vaporization of flower (again, not oil or “vape” pens) are clearly safest, I recognize that some people are going to smoke regardless of my begging them not to.  At least, if we figure out how much THC you’re getting we can be safe in that regard.  

As you’ve heard me say, THC is the main medicine.  It does appear to work best in conjunction with some “helper chemicals” that we get from cannabis.  THC is also the part that poses the greatest known risk in terms of tolerance, dependence, and addiction.  Hence, knowing how to calculate your dose and keeping it as low as possible is crucial. 

Not All Smoking Approaches Are the Same

Among smoking options there are joints, pipes, and bongs.  Joints and pipes present further complications.  There is a myth that bongs are better because the water filters out some of the junk.  This has been proven to be false. 

Bongs provide just as much junk to your lungs as pipes and joints.  Nonetheless, bongs do have one advantage which is that most if not all the smoke is contained in the water chamber, so less is lost to the atmosphere.  This is important when we try to calculate how much THC you’re getting.  

As a side note, please do not ever mix cannabis with tobacco.  This is just adding poison to a perfectly good medicine. 

Few people ever measure their cannabis.  Studies have shown that cannabis users are terrible at estimating how much they’re using.  On the one hand, they typically vastly overestimate how much is in their smoking apparatus, yet on the other hand they vastly underestimate how much THC they’re taking.  

Being precise is actually pretty easy. 

How To Calculate Your Dose

First, you need to get a milligram scale.  Here’s one from Amazon.  Not stupid cheap, but less than that new piece of glass you’ve been eyeing.  

Next you need to know the percent THC in your weed.  Sometimes the label says THC, or d9-THC, or THC-A.  This can range from 15-20% on up to 30-35%.  As you may recall, I always recommend using 15-20% THC strains to leave room for those helper chemicals that make cannabis do what we need (beside getting high). 

Finally we get to the math part.  This isn’t that hard, don’t give up now!  The basic equation is:

X mg cannabis x Y percent THC (expressed as a decimal) = Z mg THC

So if we start with 100mg cannabis at 20% THC, that’s 100 x 0.2 (that’s 20% as a decimal) = 40mg THC (way too much). 

But, most of the time we want to run this calculation backward, in other words we want to start with the dose and figure out how much marijuana to put in the bowl.  That equation is:

X mg cannabis = Z mg THC / Y percent THC (as a decimal)

Now let’s start with a more reasonable dose like 5mg.

X mg cannabis = 5 mg THC / 0.2, X= 25 mg of cannabis.  So, to get 5mg THC, you put 25mg cannabis in the bowl.  25mg of cannabis is a tiny, tiny pinch.  I see you holding up your fingers – it’s even less than that.  This is where that scale comes in.  You’ve got to actually weigh the weed or you’ll get it wrong.  

By the way, 25 mg on the milligram scale will look like 0.025 grams.

What’s Wrong With Joints and Pipes?

When it comes to joints and pipes this all goes wonky because it’s hard to put that little into a joint or to estimate how much of a joint equals 25 mg.  The pipe is easier to put in the 25 mg but both methods lose 50-60% of the dose to the atmosphere, so we really can’t tell how much you’re getting.

Vaporization is Best

Of course, the flower vaporizer does all this work for you.  If you put that 15-20% THC strain in the vaporizer and take a puff that’s a full deep breath, you will get about 5mg per puff.  Also, vaporizers are much more efficient, so that your cannabis will last about 2.5 times longer and saves you a lot of money on weed.  

Now you know how to calculate your dose of cannabis when smoking a bong.  However, you also know that flower vaporization is safer, less calculation, and saves you so much money it’s a no-brainer investment. 

Consult with a Qualified Boston Medical Marijuana Expert Today

Those considering using THC, CBD, or any type of medicine found in cannabis to help manage their condition should consider speaking to a trained medical expert who is knowledgeable about using cannabis therapeutically. Massachusetts medical marijuana doctor Jordan Tishler, M.D. sits on the faculty of Harvard Medical School and has years of experience helping patients treat pain and other ailments using cannabis. He and the team at InhaleMD stand ready to assist patients in determining whether medical marijuana is right for them.

For more information, or to set up a virtual consultation with the team at InhaleMD, call us at (617) 477-8886 today.

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