Marijuana Math: Figuring Potency by Fill Weight

Figuring potency based in a complicated formula for something like an infused topical or edible can be daunting. Often there are several different ingredients with different densities that make the calculations difficult and confusing. There’s a much easier way: Fill Weight.

The Fill Weight is the actual weight of the product. For topicals that would be the weight of the product in the container. For a cookie that would be the weight of a single cookie.

To find the weight in a container simply weigh the container empty, fill it with a known amount of product, and weigh it again. Subtract the weight of the empty container from the weight of the filled container and you have the fill weight. Pretty simple, huh? As long as you put the same weight of product in each container, your potency should be the same each item.

Here’s an example:

An empty 60ml jar weighs 5 grams. The total weight of a filled 60ml jar is 45 grams. Subtract 5 from 45 to get the fill weight: 40 grams. That means the weight of the product inside the jar is 40 grams.

Let’s say the product tested at 2.0% CBD.
First, convert grams to milligrams. 40 grams = 40,000mg
Multiply milligrams by percentage. 40,000 X 2.0% = 800
There’s 800mg of CBD in the 60ml (40g) jar of product.

Here’s another example:

A cookie weighs 4 grams and tests at 0.15% CBD
4 grams = 4,000mg
4000 X 0.15% = 6
The whole cookie has 6mg of CBD.

So how much does your product weigh?

That’s not always an easy answer, at least not if you’re in the United States. Unlike most of the world, the U.S. uses the imperial system of weights and measures rather than the metric system. Where this gets in the way (or weigh) is when an ounce isn’t really an ounce.

The imperial system uses “ounce” to indicate both weight and volume. But a one-ounce jar of lead doesn’t weigh the same as a one-ounce jar of water, and ten ounces of feathers will take up more space (volume) than ten ounces of water, so you have to know what you’re talking about.

The weight of a tincture in a one-ounce bottle will vary depending on its formula. This gets particularly complicated when it comes to topicals where ingredients are often whipped together with air taking up space without adding weight.

We work in the metric system, and if you’re producing a cannabis product, you should too. It’s so much easier to keep weight and volume straight during formulation when you’re talking milligrams and milliliters.

If you need to convert ounces to milligrams, there are some great calculators online that make it easy.

Get a good scale

The real trick is having an accurate metric scale that measures in milligrams (.001 gram). The accuracy of your scale can really skew your results. If you want us to figure your Fill Weight, simply send an empty jar along with the full jar to be tested and we’ll weigh, compare and make the calculations for you.

Now you know how to figure potency by Fill Weight. Pretty simple and pretty useful.

Go forth and formulate good products. And remember to get them tested at The Good Lab.