How is total THC calculated?

Total THC = Δ9-THC + 0.877 × THCA. The 0.877 factor is the ratio of the molecular weights of THC and THCA, so it converts the acid to the amount of THC it would give if it decarboxylated completely.

Why does GC under-report total THC?

A GC inlet heats the sample, and THCA and CBDA decarboxylate there to THC and CBD, but not completely. A 2025 study found about 50–60% conversion in GC-MS without derivatisation, at every concentration tested. Applying the total-THC formula to a result that assumes full conversion therefore reads low when the acid forms are present.

Is HPLC better than GC for cannabis potency?

For potency, HPLC or LC-MS measures the acid and neutral forms separately without heating them, so the total can be calculated correctly. GC-MS gives correct totals too if the acids are derivatised (silylated) before injection. GC still has a role for terpenes, which LC-MS cannot see.

Does the extraction step affect cannabis potency results?

Yes. In a validated NRC Canada LC-MS/MS method, the first extraction recovered 90.9–94.3% of the cannabinoids tested and a second extraction recovered another 5.5–7.8%. A method with one extraction can read several percent low before the instrument is involved.

How accurate are cannabis oil labels in Ontario?

A 2024 study tested 30 legal oral cannabis oils from the Ontario Cannabis Store. 40% were outside the ±15% variability allowed for THC and 10% for CBD, and all but one of these were labelled higher than the tested amount.

What is the allowed variability for THC on a cannabis extract label in Canada?

The Ontario study describes the federal allowance for extracts as 15% above or below the labelled amount.

Total THC by GC or HPLC: why the number on a cannabis oil can be wrong

A cannabis oil label gives a THC number. Behind it is a calculation, an extraction, and an instrument, and each of them can move the number.

This piece is about three of those places. The instrument choice, the extraction, and the result when someone tested legal products against their labels.

Total THC is a calculation

The cannabis plant makes mostly the acid forms, THCA and CBDA. Heat converts them to THC and CBD by removing a carboxyl group, which is called decarboxylation. That is what happens when cannabis is smoked, vaped or baked.

So potency is usually reported as total THC:

Total THC = Δ9-THC + 0.877 × THCA

The factor 0.877 is the ratio of the molecular weights of THC and THCA. It converts the acid into the amount of THC it would give if every molecule decarboxylated (Franzin et al. 2025, p. 6). The same factor works for CBD and CBDA.

The important words are “if every molecule decarboxylated.” The formula assumes complete conversion.

Why GC can under-report total THC

A gas chromatograph injects the sample into a hot inlet. In that inlet, THCA and CBDA decarboxylate to THC and CBD. So a GC run without derivatisation sees mostly the neutral forms, and some labs report that GC result as the total.

That works only if the conversion in the inlet is complete. Franzin and co-workers, at a hospital laboratory in Trieste, measured it. Without derivatisation, GC-MS converted THCA and CBDA to THC and CBD at about 50–60%, and the rate was similar across all the concentrations they tested (pp. 1, 13).

So a GC-MS method without derivatisation can under-report total THC and total CBD in a product that still contains the acid forms. The authors’ conclusion is that labs using GC-MS without derivatisation for total THC and CBD should be aware of this underestimation.

Two methods gave correct totals in the same study (p. 1):

LC-MS, which measures THCA and THC separately at room temperature, and the total is then calculated with 0.877
GC-MS after silylation (BSTFA with TMCS), which protects the acid group so it does not decarboxylate in the inlet

One limit, which the authors state. In the 6 pharmacy-prepared oils they tested, the difference was under 10%, because those oils had already been heated and contained little acid (pp. 1, 13). The error is largest in products that are rich in THCA or CBDA: raw flower, and extracts made without a decarboxylation step.

GC still has a job here. LC-MS cannot measure terpenes, and GC can (p. 12).

A single extraction leaves some behind

Before any instrument, the cannabinoids have to come out of the sample.

The National Research Council of Canada validated an LC-MS/MS method for 17 cannabinoids in cannabis and hemp plant material, and tested serial extraction of the same sample. The first extraction recovered 90.9–94.3%. A second extraction recovered another 5.5–7.8%, and a third added up to 1.1% (McRae et al. 2020, p. 7).

So a method with one extraction can read about 6–9% low before the chromatography begins. The practical step is to measure your own extraction recovery during validation, and to know how much of your label tolerance it uses.

On seven real cannabis samples, the same method’s intermediate precision was 2.2–12.8% RSD (p. 1). Put that next to a ±15% label allowance and the room for method error is small.

What happened when someone checked the labels

In 2024, Doggett and co-workers bought 30 legal oral cannabis oils from the Ontario Cannabis Store and had them tested by a licensed laboratory (Doggett et al. 2024, JAMA Network Open).

The federal allowance they used is 15% above or below the labelled amount for extracts. Against that:

40.0% of products were outside the allowance for THC
10.0% were outside for CBD
all but one of those were over-labelled: the label said more than the lab found
among the 16 products labelled at 2.5 mg/g THC or more, 7 (43.8%) tested more than 15% below the label

The paper does not say why. Degradation in the bottle, a manufacturer’s own method, the testing lab’s method: any of these could contribute, and the methods are only in a supplement I have not seen. I would be careful with any single cause.

What it does show is that the label number and the tested number disagree often enough that the method behind each of them matters. A GC method without derivatisation, used on a product that still contains THCA, would push results in the same direction the study found. I cannot say from this paper that it did.

What I would check in a potency method

If you run potency testing, or you buy it from a lab:

How are the acid forms measured? By HPLC or LC-MS, or by GC after derivatisation. If the method is GC without derivatisation, ask how the inlet conversion was measured.
Is the 0.877 factor applied to a measured acid, or to a neutral result that already assumes conversion?
What is the extraction recovery? Measured by re-extracting the residue, not assumed.
What is the method’s precision on real samples, compared with the ±15% label allowance?
Is the sample decarboxylated before the test? If so, the conversion step needs its own check.

Sources

Franzin M et al. Incomplete decarboxylation of acidic cannabinoids in GC-MS leads to underestimation of the total cannabinoid content in cannabis oils without derivatization. Pharmaceutics 2025; 17(3):334. doi:10.3390/pharmaceutics17030334. A small study, 6 real oils.
McRae G, Melanson JE. Quantitative determination and validation of 17 cannabinoids in cannabis and hemp using liquid chromatography-tandem mass spectrometry. Anal Bioanal Chem 2020; 412:7381–7393. doi:10.1007/s00216-020-02862-8. National Research Council Canada.
Doggett A et al. Label accuracy of legal oral cannabis oil products in Ontario, Canada. JAMA Netw Open 2024. doi:10.1001/jamanetworkopen.2024.14922.

I worked in natural health product QC with LC-MS, and the questions above are the ones I would ask of any potency method. If you have a potency result you do not trust, tell me the product type and the technique on LinkedIn. The validation-readiness checklist covers the rest.

Common questions

How is total THC calculated?: Total THC = Δ9-THC + 0.877 × THCA. The 0.877 factor is the ratio of the molecular weights of THC and THCA, so it converts the acid to the amount of THC it would give if it decarboxylated completely.
Why does GC under-report total THC?: A GC inlet heats the sample, and THCA and CBDA decarboxylate there to THC and CBD, but not completely. A 2025 study found about 50–60% conversion in GC-MS without derivatisation, at every concentration tested. Applying the total-THC formula to a result that assumes full conversion therefore reads low when the acid forms are present.
Is HPLC better than GC for cannabis potency?: For potency, HPLC or LC-MS measures the acid and neutral forms separately without heating them, so the total can be calculated correctly. GC-MS gives correct totals too if the acids are derivatised (silylated) before injection. GC still has a role for terpenes, which LC-MS cannot see.
Does the extraction step affect cannabis potency results?: Yes. In a validated NRC Canada LC-MS/MS method, the first extraction recovered 90.9–94.3% of the cannabinoids tested and a second extraction recovered another 5.5–7.8%. A method with one extraction can read several percent low before the instrument is involved.
How accurate are cannabis oil labels in Ontario?: A 2024 study tested 30 legal oral cannabis oils from the Ontario Cannabis Store. 40% were outside the ±15% variability allowed for THC and 10% for CBD, and all but one of these were labelled higher than the tested amount.
What is the allowed variability for THC on a cannabis extract label in Canada?: The Ontario study describes the federal allowance for extracts as 15% above or below the labelled amount.