Fresh cannabis contains surprisingly little THC. What the plant makes is mostly an acid called THCA, and THC appears only after that acid loses a small piece of itself. Lab reports list both, and labels combine them into one “total THC” figure using a multiplier of 0.877. This lesson explains the chemistry behind all three: the acid, the reaction and the number.
Everything here is chemistry and label arithmetic. It describes a reaction, not a method for carrying it out.
The plant builds cannabinoids as acids
Cannabinoids start from two building blocks. One is olivetolic acid, a ring-shaped molecule with a five-carbon tail. The other is geranyl pyrophosphate (GPP), a ten-carbon unit from the same pathway that supplies terpenes (Lesson 6). An enzyme joins them to make cannabigerolic acid, or CBGA.1 Researchers confirmed this route by rebuilding the whole pathway in yeast: with the cannabis genes added, yeast made CBGA, THCA and CBDA from sugar.1
CBGA is the branching point. Three different enzymes each reshape it in a different way:2
- THCA synthase turns CBGA into tetrahydrocannabinolic acid (THCA). It is a flavoprotein: it carries a bound FAD cofactor, and it stops working if the amino acid that holds that cofactor is changed.3
- CBDA synthase turns CBGA into cannabidiolic acid (CBDA) by an oxidative cyclization. It is closely related to THCA synthase in structure and function.4
- CBCA synthase turns CBGA into cannabichromenic acid (CBCA).5
Which of these enzymes a plant carries largely decides whether it ends up THCA-rich or CBDA-rich; the paper describing CBDA synthase calls it “the chemotype-determining enzyme”.4 Lesson 8 picks up that thread.
Scroll sideways to see the whole diagram.
What the “A” stands for
The A in THCA, CBDA and CBGA stands for acid. Each acid carries a carboxyl group, one carbon atom bonded to two oxygens and a hydrogen (–COOH), attached to its ring. The neutral forms, Δ9-THC, CBD and CBG, have a plain hydrogen in that spot instead. The chemical formulas show the difference:67
| Molecule | Formula | Molar mass (g/mol) | PubChem CID |
|---|---|---|---|
| THCA (Δ9-tetrahydrocannabinolic acid) | C₂₂H₃₀O₄ | 358.5 | 98523 |
| Δ9-THC (tetrahydrocannabinol) | C₂₁H₃₀O₂ | 314.5 | 16078 |
| Difference | CO₂ | 44.0 |
The acid has one more carbon and two more oxygens. That is exactly one molecule of carbon dioxide.
Decarboxylation: losing one CO₂
Decarboxylation is the reaction in which the carboxyl group leaves as carbon dioxide. One molecule of THCA becomes one molecule of Δ9-THC plus one molecule of CO₂. The same reaction turns CBDA into CBD, CBGA into CBG and CBCA into CBC; each acid’s formula is its neutral form’s plus one CO₂.8910
In cannabis it happens with heat, and slowly with age.11 A laboratory study that heated cannabis extracts at several temperatures and measured the acids over time found that:12
- the acids declined exponentially over time, and the rate rose with temperature;
- THCA converted about twice as fast as CBDA or CBGA under the same conditions;
- THCA converted cleanly to THC, while CBDA and CBGA showed losses the authors could not fully account for.
That last point matters for reading labels: a real sample doesn’t always convert completely or cleanly. But as the next section shows, the 0.877 factor isn’t trying to account for that.
Where 0.877 comes from
Because one THCA molecule becomes exactly one THC molecule, the mass of THC you could get from a given mass of THCA is set by their molar masses:
314.5 ÷ 358.5 = 0.877
So a gram of THCA holds the makings of 0.877 grams of THC. The other 0.123 grams is the carbon dioxide that leaves.67
| Starting acid | Becomes | Mass kept | Mass lost as CO₂ |
|---|---|---|---|
| 1.000 g THCA | Δ9-THC | 0.877 g | 0.123 g |
The US Department of Agriculture’s regulations (7 CFR Part 990) describe the factor in the same terms, as “a molecular mass conversion ratio”.13
A common misreading. You will find explanations online saying the 0.877 is there “because not all THCA converts to THC”. That isn’t what it does. It is a mass conversion that assumes complete conversion. Total THC is a potential figure: the most THC the measured acid could become, on paper.13
The factor depends on the molecule, because each acid and its neutral form have their own molar masses:891415
| Acid → neutral | Molar masses (g/mol) | Factor |
|---|---|---|
| THCA → Δ9-THC | 358.5 → 314.5 | 0.877 |
| CBDA → CBD | 358.5 → 314.5 | 0.877 |
| CBGA → CBG | 360.5 → 316.5 | 0.878 |
| THCVA → THCV | 330.4 → 286.4 | 0.867 |
CBDA and THCA share a formula, so CBD gets the same 0.877. CBG has two extra hydrogens, which nudges its factor to 0.878. THCV, with a shorter side chain (Lesson 5), loses the same 44 grams of CO₂ from a lighter molecule, so its factor is lower.
Total THC on a label
The same USDA regulations define total THC as Δ9-THC plus THCA converted at that ratio, reported on a dry-weight basis:13
Total THC = Δ9-THC + (0.877 × THCA)
They also note why two lab methods can arrive at the same figure. Gas chromatography heats the sample, so the THCA converts during the run and the instrument reports the total directly. Liquid chromatography keeps THCA intact, so the lab measures the two forms separately and applies the formula.13 Lesson 9 shows where each figure sits on a certificate of analysis.
A worked example (an invented label, for arithmetic only):
- The label lists THCA 24.0% and Δ9-THC 0.8%.
- Convert the acid: 0.877 × 24.0 = 21.05.
- Add the neutral THC: 21.05 + 0.8 = 21.8% total THC.
Simply adding 24.0 and 0.8 would give 24.8%, three points too high, because it counts the CO₂ as if it were THC. The same steps give total CBD from CBDA and CBD.
Δ9-THC 0.8% and THCA 24%: total THC = 0.8 + 0.877 × 24 = 21.8%
Every THC figure on our strain pages is total THC, computed by us from each lab result with this formula rather than copied from the lab’s own total.16
What “delta-9” means
The Δ9 (delta-9) in Δ9-THC names the position of one carbon–carbon double bond in the molecule’s ring system. Δ8-THC has the same formula, C₂₁H₃₀O₂, and the same molar mass, but its double bond sits one position over.177 Same atoms, different arrangement: chemists call these isomers.
Older papers sometimes call the same molecule Δ1-THC, using an earlier numbering system for the ring. The 2004 paper that first cloned THCA synthase uses that name in its title.3 It is the same compound, so an old “Δ1” and a modern “Δ9” can refer to one molecule.
Check yourself
What is the difference between THCA and Δ9-THC, in atoms?
One carbon and two oxygens: a molecule of CO₂. THCA is C₂₂H₃₀O₄ and Δ9-THC is C₂₁H₃₀O₂. Decarboxylation removes that CO₂.
A friend says 0.877 is there because about 12% of THCA “doesn’t convert”. Is that right?
No. 0.877 is the ratio of the molar masses (314.5 ÷ 358.5). It assumes complete conversion; the missing 12.3% of the mass is the carbon dioxide that leaves.
A label lists THCA 18.0% and Δ9-THC 0.5%. What is the total THC?
0.877 × 18.0 = 15.79, plus 0.5 = about 16.3% total THC.
Sources
- Luo X, Reiter MA, d’Espaux L, et al. Complete biosynthesis of cannabinoids and their unnatural analogues in yeast. Nature 567: 123–126, 2019. https://doi.org/10.1038/s41586-019-0978-9
- Gülck T, Møller BL. Phytocannabinoids: origins and biosynthesis. Trends in Plant Science 25(10): 985–1004, 2020. https://doi.org/10.1016/j.tplants.2020.05.005
- Sirikantaramas S, Morimoto S, Shoyama Y, et al. The gene controlling marijuana psychoactivity: molecular cloning and heterologous expression of Δ1-tetrahydrocannabinolic acid synthase from Cannabis sativa L. Journal of Biological Chemistry 279(38): 39767–39774, 2004. https://doi.org/10.1074/jbc.M403693200
- Taura F, Sirikantaramas S, Shoyama Y, et al. Cannabidiolic-acid synthase, the chemotype-determining enzyme in the fiber-type Cannabis sativa. FEBS Letters 581(16): 2929–2934, 2007. https://doi.org/10.1016/j.febslet.2007.05.043
- Morimoto S, Komatsu K, Taura F, Shoyama Y. Purification and characterization of cannabichromenic acid synthase from Cannabis sativa. Phytochemistry 49(6): 1525–1529, 1998. https://doi.org/10.1016/S0031-9422(98)00278-7
- PubChem, delta9-Tetrahydrocannabinolic acid (CID 98523). https://pubchem.ncbi.nlm.nih.gov/compound/98523
- PubChem, Tetrahydrocannabinol (CID 16078). National Center for Biotechnology Information. https://pubchem.ncbi.nlm.nih.gov/compound/16078
- PubChem, Cannabigerolic acid (CID 6449999). https://pubchem.ncbi.nlm.nih.gov/compound/6449999
- PubChem, Cannabigerol (CID 5315659). https://pubchem.ncbi.nlm.nih.gov/compound/5315659
- PubChem, Cannabichromenic acid, (+)- (CID 3084339). https://pubchem.ncbi.nlm.nih.gov/compound/3084339
- Radwan MM, Chandra S, Gul S, ElSohly MA. Cannabinoids, phenolics, terpenes and alkaloids of Cannabis. Molecules 26(9): 2774, 2021. https://doi.org/10.3390/molecules26092774
- Wang M, Wang YH, Avula B, et al. Decarboxylation study of acidic cannabinoids: a novel approach using ultra-high-performance supercritical fluid chromatography/photodiode array-mass spectrometry. Cannabis and Cannabinoid Research 1(1): 262–271, 2016. https://doi.org/10.1089/can.2016.0020
- US Department of Agriculture, Agricultural Marketing Service. 7 CFR § 990.1, Definitions (Total THC; Decarboxylated). Text via the Legal Information Institute, Cornell University. https://www.law.cornell.edu/cfr/text/7/990.1
- PubChem, Tetrahydrocannabivarinic acid (CID 59444416). https://pubchem.ncbi.nlm.nih.gov/compound/59444416
- PubChem, Tetrahydrocannabivarin (CID 93147). https://pubchem.ncbi.nlm.nih.gov/compound/93147
- CannabisStrains.com. Methodology and data pipeline, release 2026-10-21, method version 1.0.0. https://cannabisstrains.com/sources/
- PubChem, Delta-8-Tetrahydrocannabinol (CID 638026). https://pubchem.ncbi.nlm.nih.gov/compound/638026