Morgan Fox at Cato Institute sees great profit potential from legalizing marijuana because “experts” tell him that the market in this commodity may be worth as much as $120 Billion a year, which is more than wheat and corn combined. An impressive claim indeed, but is it true?

You are going to hear many numbers like this in the coming year. In my continuing quest to make our democracy work better, I am going to lay out some DIY math that will help you discern in a few easy steps which estimates are believable and which are not. Given the ideological nature of marijuana legalization debates, within 5 minutes of my posting this, someone somewhere on the Internet will write “I don’t care about evidence, I believe X”. If that’s you, skip down to the comments section now and start bloviating, what follows is data, nuance, the encouragement of independent thinking and a lot of other stuff you will find upsetting.

But for the blessed and admirable rest of you….

Step 1: Convert the estimate of the financial value of the U.S. marijuana market into something more understandable, for example number of ounces or joints.

Start with your market number. For the sake of argument, in this case let’s take Fox’s upper estimate of $120 Billion/year. That’s too big a number for any of us to relate to or understand, so Step 1 is to convert it into ounces or joints of marijuana.

The price of an ounce varies based on where you are, whether you buy in bulk, and what grade you purchase. But $120/ounce isn’t a bad estimate for a national average in any case, and here it makes the math easy so let’s use it. If marijuana costs $120/ounce, Americans would have to purchase 1 billion ounces a year to reach the $120 Billion dollar estimate. Already you will be suspicious of Fox’s market number because this works out to over 3 ounces per American (1 billion ounces divided by slightly over 300 million people), assuming that everyone uses marijuana, including newborns, people in comas and emphysema patients.

For many people knowing ounces gives them a sense of how much pot we are talking about. For others, an estimate of joints is more helpful. Drug policy wonks estimate the size of a joint at .454 grams because that is not a bad estimate and it makes the math easy (1 pound = 1000 joints). So using this “standard joint” metric, an ounce of marijuana yields 1000/16 = 62.5 joints. Data from dispensaries and users indicate that most experienced users actually roll slightly bigger joints than “standard size”. To acknowledge that and again make the math easy, bump the joints/ounce ratio up to 50 and you get in the example we are working with 3 ounces X 50 joints = 150 joints per American (again, counting every man Jack and girl Jill one of the them) per year if the $120 Billion figure is correct.

Step 2: Divide your ounce and joint estimates by the number of people who are current pot smokers.

We can’t stop at Step 1 because not all Americans smoke pot, indeed the vast majority don’t and never have. The federal government National Survey of Drug Use and Health gives somewhat suspect estimates of the use of heroin, methamphetamine and the like, but it’s marijuana numbers are pretty good so take your estimate of the number of current pot smokers from there. The survey’s current reported number of Americans who have smoked pot in the last month is 22.6 million. You can add a bit to that depending on how much you think people lie. For the sake of our example let’s make the math simple again and assume that another 2.4 million Americans fibbed about not using marijuana, giving a total of 25 million current users.

When you divide 25 million marijuana users into one billion ounces, you get 40 ounces per user per year. If you were not doubtful of the $120 Billion figure yet, the implication that the average U.S. pot smoker is sucking down two and a half pounds of marijuana a year should set off alarm bells for you. Again using our simple math above, this works out to 40 ounces X 50 joints/ounce = 2000 joints per user per year.

Step 3: Divide your estimate of amount of marijuana use per user by days of use.

Current pot smokers report that they use marijuana an average of 60 days a year. Using our current example, 40 ounces/60 days of use means that the average user would have to go through 2/3 of an ounce of marijuana on each day that they used marijuana. That’s .67 X 50 or 33.5 joints per day of use. And there’s a terrific bridge for sale in Brooklyn too.

Step 4 (Optional): Do some sensitivity analyses.

I have made the math above transparent so that you can fiddle with the numbers as you see fit and see what happens. You could for example alter joint size or number of users up or down see how that affects the validity of whatever market estimate you are looking at. There is no amount of tweaking of assumptions that will make the “experts” whom Morgan Fox says he believes sound credible, but such sensitivity analyses may help you determine whether other estimates you hear are believable to you.

[Cross-posted at The Reality-Based Community]

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Keith Humphreys is a Professor of Psychiatry at Stanford University and served as Senior Policy Advisor in the White House Office of National Drug Control Policy in the Obama Administration. @KeithNHumphreys