Cannabis Potency Guide for Medical Patients
This guide helps medical patients understand cannabis potency and choose the right strain for their needs.
What potency means
Potency is the concentration of cannabinoids in a sample, reported as a percent of dry weight. The main number is total THC. A flower label that reads 22% THC means 22 grams of THC per 100 grams of dried plant material. Labs measure THC, THCA, CBD, CBDA, CBG, and other cannabinoids in the same run.
Cannabis Potency Guide for Medical Patients
Total THC math
Raw cannabis holds most of its THC as THCA, the acid form. THCA does not get you high. Heat removes a carboxyl group and turns THCA into THC. Labs report both, then calculate total THC:
Total THC = (THCA x 0.877) + THC
The 0.877 is the ratio of molecular weights. THCA weighs 358.47 g/mol. THC weighs 314.46 g/mol. 314.46 divided by 358.47 equals 0.877. A sample with 20% THCA and 0.5% THC works out to 18.04% total THC.
Lab workflow, step by step
- Collect a representative sample. Take buds from several points in the batch, not just the top cola.
- Dry and grind the material to a uniform powder.
- Weigh a fixed mass, often 100 mg to 500 mg.
- Extract with methanol, ethanol, or acetonitrile, with shaking or sonication.
- Filter the extract and dilute it to fit the instrument range.
- Inject into the instrument and compare peaks against certified reference standards.
- Report each cannabinoid as a percent of dry weight, plus total THC.
HPLC versus GC
High performance liquid chromatography (HPLC) runs at room temperature. THCA stays intact, so the lab sees THCA and THC as separate peaks. This is the standard method for flower and concentrate testing.
Gas chromatography (GC) heats the sample in the injector port. That heat decarboxylates THCA into THC. A GC result shows one combined THC peak, so it cannot separate the acid form from the active form. GC also drives off volatile terpenes.
Detection methods
- UV or diode array detection: common, low cost, good for major cannabinoids.
- Mass spectrometry: higher sensitivity, needed for low-level compounds and pesticide screens.
- GC-MS: used for terpene profiles and residual solvents.
Reading a certificate of analysis
A COA should list the lab name and license number, the batch ID, the collection date, the test method, the limit of quantification, and the result for each analyte. Check the batch ID against the package. Check the test date against the harvest date. A potency number without a batch ID is not verifiable.
Home testing
Consumer kits use thin layer chromatography or a handheld spectrometer. They give a rough range, not a lab-grade figure. Expect error of several percentage points. Sample size is small, and calibration is limited. A home result does not match a COA.
What potency does not show
Percent THC does not describe terpene content, moisture, mold, pesticides, or heavy metals. Two samples at 20% THC can differ in effect. Flower moisture matters: a wet sample reads lower because water adds mass. Labs report on a dry-weight basis to remove that variable.