Marijuana Math: Calculating milligrams per milliliter in liquids

Accurately converting percentage to milligrams per milliliter can be confusing, and it’s easy to get it wrong if you don’t factor in the density of the liquid suspension.

You know how oils typically float to the top when mixed in water, while other substances like honey sink to the bottom? That’s because their density and molecular weight are different. One is lighter and less dense, while the other is heavier and more dense.

In order to accurately calculate milligrams per milliliter, you’ll need the following information: Potency percentage, Density of the suspension, and Volume of the liquid.

Dosing Infused Oils

Let’s say you want to know how many mg are in a 50 ml bottle of ethanol tincture at 2% potency:

Potency Percentage = 2%
Density of ethanol* = 0.789 g/ml
Volume of liquid = 50 ml

Step One: Convert Density from g/ml to mg/ml:
0.789 x 1000 = 789 mg

Step Two: Multiply Density in mg/ml by Potency Percentage:
789 x 2% = 15.78 mg/ml

Step Three: Multiply mg/ml by Volume of liquid:
15.78 x 50 = 789 mg in 50 ml

For this example, let’s assume you’re putting .5 ml of infused MCT (liquid coconut oil) into capsules:

Potency Percentage = 3%
Density of MCT* = 0.955 g/ml
Volume of liquid = 0.5 ml

Step One: Convert density from g/ml to mg/ml:
0.955 x 1000 = 955 mg

Step Two: Multiply Density in mg/ml by Potency Percentage:
955 x 3% = 28.65 mg/ml

Step Three: Multiply mg/ml by Volume of liquid:
28.65 x 0.5 = 14.33 mg in 0.50 ml

Let’s say you’re planning to bake some edibles and want to know how many milligrams are in a tablespoon of butter with a potency of 0.5%.

Potency Percentage = .5%
Density of butter* = 0.911 g/ml
Volume of liquid = 15 ml (approximately 1 tablespoon)

Step One: Convert density from g/ml to mg/ml:
0.911 x 1000 = 911 mg

Step Two: Multiply Density in mg/ml by Potency Percentage:
911 x 0.5% = 4.56 mg/ml

Step Three: Multiply mg/ml by Volume of liquid:
4.56 x 15 = 68.4 mg in 15 ml (1 tbsp)

*Each suspension will have a different density. Here are some common ones.
Ethanol: .789 g/mL
Vegetable glycerin = 1.26 g/mL
Coconut oil = .926 g/mL
Olive oil = .915 g/mL
Safflower oil = .921 g/mL
Butter = .911 g/mL
MCT Oil = .955 g/mL
Honey = 1.43 g/mL
(Most oils have a density between 0.90 to 0.95)

Bring your infused oils to The Good Lab for a Cannabinoid Potency Profile. We can help you figure out the milligrams per milliliter. Contact us to schedule a time to drop off your sample.

 

Testing for THC in hemp

hemp-field-625x410

We folks at The Good Lab were concerned about SB17-090, a bill regarding how THC is calculated in hemp, and how it might negatively impact hemp producers if certain adjustments in the calculations weren’t considered.

So we wrote this email to the bill’s sponsors:

After reading SB17-090 regarding how THC is measured in hemp, we’re concerned that there is a calculation error that could negatively and unfairly impact hemp farmers. Simply adding THC-A and delta-9-THC together will not give an accurate or fair result. Considering that Amendment 64 specifies delta-9-THC, factoring in a correction value to account for decarboxylation is important.

Decarboxylation is a chemical reaction that removes a carboxyl group and releases carbon dioxide (CO2). When THC-A is decarboxylated and converted to delta-9-THC, there is a reduction in the molecular weight that will affect the final percentage calculation. The molecular weight of THC is less than that of THC-A due to the loss of the carboxyl group.

At our lab, we calculate the delta-9 potential in hemp or cannabis using a factor of 0.877. I’ve attached a sample potency report so you can see how we make our calculations.

This article from High Times further explains the error in the current bill’s calculation and why you can’t simply add THC-A and delta-9-THC together to get the accurate number you’re looking for..
http://hightimes.com/grow/thca-vs-thc-how-to-read-a-lab-result/

We respectfully recommend an amendment to SB17-090 correcting this calculation error.

Apparently, the sponsors passed our concerns onto the Colorado Department of Agriculture. We were excited to get the following response from Mitch Yergert, Director, Division of Plant Industry:

This bill (SB17-090) only affects the testing being conducted by CDA at our lab. We have no desire to affect how the private labs conduct testing for hemp producers. We know most (if not all) of you use HPLC and will continue to do so. We were very specific in the bill to not require a certain piece of equipment or methodology to accommodate this and additionally our approach could change in future years to HPLC or even something else if a better type of machine comes along. We don’t believe the bill language would prevent this in the future.

Currently we use a GC for our analysis as it is more cost effective for the program and the hemp producers. Because of this we don’t have the issue with needing to calculate the THC-A conversion. We would recommend you use the 0.877 molecular weight value as the most conservative approach. We have seen some reports that actual yield from decarboxylation will be less than the exact .877 and that makes scientific sense. We have seen numbers as low as 0.700 in one study. But we don’t have sufficient data to select a specific number less than 0.877 that we would stand behind.

As your testing is not regulatory and we don’t base our regulatory decision on those numbers, by using the .877 number you are providing a conservative estimate to the grower which provides the highest potential THC for the crop. That seems to be the best number for the grower to consider. They can make the decision how to move forward with their crop based upon that. It is conceivable if they are minimally over in your testing using HPLC and the 0.877 conversion and we run a GC analysis, that the value could come in at 0.3 or slightly under, but that is good for everyone.

I believe the variability in sampling conducted by the grower versus CDA is probably a much bigger variable in the process than whether the private labs use HPLC and we use GC. So comparing the two numbers just based on the lab values may not be that productive.

What was really exciting was the validation we got from the CDA about the importance of private labs like ours.

The private labs perform a very valuable function for the hemp producers as the industry tries to get established. We appreciate you efforts to work with them and us on this issue.

We’re excited to work with the CDA, hemp farmers as well as other private cultivators to produce and develop high-quality hemp and cannabis products.

For more information on how The Good Lab might help you, please give us a call at (303) 455-3801.