How to Determine THC and CBD Levels in Cannabis Plants

The reliable way to determine a cannabis plant’s THC and CBD levels is to test a representative sample with a validated laboratory method. A strain name, seed-catalog percentage, smell or photograph cannot establish the cannabinoid concentration of your particular batch. The report also needs to distinguish THC from THCA, and CBD from CBDA.

What Is Cannabis Potency?

In a laboratory report, potency usually means cannabinoid concentration. Flower may be reported as a percentage by mass or in milligrams per gram. These describe how much of a substance is in the tested material, not how much reaches someone’s bloodstream or how effective it will be for a medical condition.

For example, 1% by mass equals 10 mg/g; 20% equals 200 mg/g. Compare results on the same moisture basis. A dry-weight figure excludes water and is not directly interchangeable with a figure reported for the material as received.

Why the Sample Matters

A result belongs to the submitted sample and the batch it properly represents. Ask the laboratory which plant parts to collect, how many samples are needed, and how to package and identify them. A single selected flower should not be assumed to represent an entire crop.

USDA’s hemp laboratory guidelines require preparation of the composite sample, including grinding to make the material more uniform, and reporting total THC on a dry-weight basis with measurement uncertainty. A grower’s informal monitoring sample is distinct from an official compliance sample.

THC, THCA and Total THC

THCA is the acidic precursor of THC. Heat can convert it into THC through decarboxylation. Laboratories may therefore list the substances separately and also calculate potential total THC.

The NIST cannabinoid laboratory report uses this calculation:

Total THC = THC + (0.877 × THCA)

Use matching units throughout. If a sample has 1% THC and 20% THCA, its calculated total THC is 18.54%, not 21%. The conversion factor accounts for the mass lost during decarboxylation. This is a potential amount in the material, not a prediction that heating, extraction or consumption will deliver all of it.

CBD and CBDA should likewise be identified separately. Check how the laboratory defines any “total CBD” field rather than adding a reported total to its components and counting them twice.

Which Laboratory Methods Measure Potency?

Liquid Chromatography

High-performance liquid chromatography, commonly called HPLC, separates cannabinoids for measurement with an appropriate detector. A validated method can measure acidic and neutral cannabinoids separately. NIST’s liquid-chromatography method, for example, uses photodiode-array detection to quantify THC and THCA in prepared botanical samples.

Gas Chromatography and Mass Spectrometry

Gas chromatography can also be used for cannabinoid testing. USDA accepts suitably validated liquid- and gas-chromatography methods for hemp compliance testing when they account for potential THCA conversion. Ask how the chosen method handles acidic cannabinoids and calculates totals.

Mass spectrometry is a detection technique often combined with chromatography, such as GC-MS or LC-MS/MS. It should not be presented as a guarantee of accuracy by itself: sample preparation, calibration, separation and quality controls remain essential. NIST’s cannabis laboratory program supports multiple analytical platforms and reference materials for this work.

Why a Drug-Screening Strip Is Different

A urine drug screen does not measure the THC percentage of a plant. The FDA describes home urine drug tests as qualitative: they indicate whether a drug may be present, not how much is there. Do not substitute one for a plant-potency assay. For any consumer plant-testing device, check its intended sample type and independently validated accuracy before relying on its number.

How to Read a Certificate of Analysis

• Match the sample: Check the product or lot identifier, collection information and test date.
• Check the units: Distinguish percentages, mg/g and any serving or package quantities.
• Read individual cannabinoids: Identify THC, THCA, CBD and CBDA rather than relying only on a headline total.
• Check the reporting basis: Look for moisture correction, method details, detection or quantification limits and uncertainty where relevant.
• Separate potency from other testing: A cannabinoid result alone does not show that pesticides, microbes, heavy metals or residual solvents were tested.

Laboratory quality matters even when a report looks precise. A 2025 NIST study showed how compounds interfering with THC measurement can inflate results. The researchers described ways to improve separation and detection. If a result is disputed or near a legal threshold, ask the laboratory about its method and the applicable confirmation or retesting process.

Can Genetics or Harvest Timing Predict the Result?

They can inform a testing plan, but they do not replace measurement. Avoid treating a cultivar’s advertised range as a batch certificate or assuming that later harvest always raises both THC and CBD. Compare properly collected samples over time if monitoring a crop, and keep that work separate from required regulatory sampling.

Potency and Hemp Compliance

Under USDA’s production-testing framework, the 0.3% threshold is assessed using total THC, including potential THCA conversion, with the required treatment of measurement uncertainty. “Less than 0.3% THC” on its own is an incomplete explanation of that process. Follow the specific federal, state or tribal program’s sampling and reporting requirements.

A crop test also does not settle whether every finished product can legally be sold. Product composition, processing and jurisdiction can introduce additional rules. Do not use a potency certificate as a substitute for checking those requirements.

What the Number Cannot Tell You

A higher cannabinoid percentage does not establish superior quality, a fair price or suitability for treating a condition. Use a trustworthy batch report to understand composition, then evaluate the product’s other relevant information separately. For plant potency, the useful answer is a measured result with a clear sample identity, method and reporting basis.