How is THC measured in hemp and cannabis? Lab testing explained

Laboratories measure THC in hemp and cannabis with chromatography: gas chromatography (GC) or high-performance liquid chromatography (HPLC), sometimes combined with mass spectrometry. The result is given as a percentage of the dried sample. Most legal limits are now checked as total THC, which adds the THC precursor THCA to the THC already present.

How is THC measured in a laboratory?

Every method follows the same basic steps. The EU's official method for hemp shows them clearly (Regulation (EU) 2022/126, Annex I):

1. Sampling. Inspectors cut a 30 cm section containing at least one female flower head from each of 50 plants across the field, avoiding the edges.
2. Drying. Drying starts within 48 hours, below 70 °C, until the sample holds 8–13% moisture.
3. Grinding. Stems and seeds larger than 2 mm are removed, and the rest is ground to pass a 1 mm sieve.
4. Extraction. 100 mg of powder is shaken in hexane in an ultrasound bath, so the cannabinoids dissolve into the solvent.
5. Separation and detection. The extract goes into a gas chromatograph, which separates the compounds. A detector measures how much THC comes out, compared against a reference standard.
6. Reporting. The result is given in grams of THC per 100 g of dry sample (that is, %) to two decimal places.

Product labs follow the same logic for oils, flowers or cosmetics, with sample preparation adapted to each material. An oil, for example, needs no drying or grinding and can be analysed directly (UNODC).

GC or HPLC: which method is better for THC?

Both are accepted. The main difference is heat. A gas chromatograph vaporises the sample at around 300 °C, and at that temperature THCA loses its acid group and turns into THC. So a plain GC reading already shows something close to total THC. To measure THCA and THC separately, labs first convert the acids into a heat-stable form (derivatisation). HPLC works at room temperature, so it measures THC and THCA separately without that extra step.

Heat can also distort results in CBD-rich samples. In a 2022 study, a brief pass through the hot GC inlet broke down about 20% of the CBD, forming Δ9-THC and CBN as by-products, and 17.2% of the Δ9-THC itself degraded. The authors recommend derivatisation to prevent this (García-Valverde 2022).

What is total THC?

A fresh cannabis plant makes very little THC. It makes THCA, which turns into THC when it is heated or stored for a long time (what is THCA?). Total THC adds both together. Because THCA is a heavier molecule (358 g/mol, against 314 g/mol for THC), labs convert it with a factor of about 0.877:

Total THC = THC + (0.877 × THCA)

The UN's manual for drug laboratories says that, where the law doesn't specify, it is common practice to measure total THC, because that best represents the material's activity (UNODC). US hemp rules already require total THC testing (7 CFR 990.25), and from 12 November 2026 the federal definition of hemp itself counts "total tetrahydrocannabinols concentration (including tetrahydrocannabinolic acid)" (7 U.S.C. 1639o).

How is hemp tested for the legal THC limit?

• European Union: farm payments for hemp require certified seed of varieties listed in the EU Common Catalogue, with no more than 0.3% THC (Regulation (EU) 2021/2115, Art. 4(4)). National authorities sample fields during flowering and analyse them with the official GC method. A variety that exceeds the limit is checked more strictly the following year.
• Switzerland: cannabis with 1% THC or more is a narcotic. Hemp below 1% is outside the Narcotics Act and can be used for products such as cosmetics (FOPH).
• United States: labs must use post-decarboxylation or similarly reliable methods, such as gas or liquid chromatography. They must report the measurement uncertainty with each result. Since 2023, only DEA-registered laboratories may test hemp for USDA licence holders (7 CFR 990.25).

For how these limits separate hemp from marijuana, see what is hemp?

Why can two labs give different THC results?

An analysis of Washington State's seed-to-sale data found systematic differences in the cannabinoid content reported by different laboratories, even after controlling for other explanations. The authors called for standardised lab methods (Jikomes 2018). The usual reasons for differences are:

• Sampling: flower tops hold far more THC than leaves or stems, so which part was cut matters.
• Method: GC heat converts THCA and some CBD; HPLC does not.
• Calculation: some reports show Δ9-THC only, others total THC.
• Uncertainty: every result has a margin. The EU method allows 0.03 g per 100 g, and US rules require the uncertainty to be reported.
• Storage: THC degrades over time when exposed to air, light and humidity, and CBN is one of its breakdown products.

Is a THC test on a person the same as testing a product?

No. Drug tests look for THC or its breakdown products in a person's urine, saliva or blood. In US federal workplace testing, a urine screen counts as positive at 50 ng/mL of THC metabolite. A positive screen must then be confirmed at 15 ng/mL by a more specific laboratory test (HHS 2025). In saliva, THC was generally measurable for about 48 hours in chronic daily cannabis smokers who then stopped (Lee 2011). For how this affects CBD users, read CBD, drug tests and driving.

How can I check the THC content of a CBD product?

Ask for the certificate of analysis (COA) for the batch you bought. A useful COA names the lab, the sample and the batch, the method used, and both Δ9-THC and THCA (or total THC), with units and the limit of quantification. Our guide to reading a CBD certificate of analysis explains each line. You can see the reports for our products on our test results page.

Every batch of our CBD oils is tested in Switzerland: CBD oils.