Reviewed by Jeff Zorn, founder of Cannabis Training University (since 2009, 50,000+ students) · LinkedIn
Jeff Zorn is the founder of Cannabis Training University (CTU), an online cannabis school with 50,000+ students since 2009. CTU is an IACET Accredited Provider, and its courses cover cannabis cultivation, business, medical cannabis, budtending, cooking, extraction and industry regulations.
Cannabis science · chemistry and law
What is the difference between THC and CBD?
They have the same atoms, C21H30O2. THC’s atoms close into a third ring; CBD’s stay open.
Both are cannabinoids with a molar mass of 314.5 g/mol. In THC, an oxygen atom links the two halves of the molecule into a ring. In CBD that ring is open, so CBD has a second OH group and an extra carbon–carbon double bond instead. Federal law also treats them differently: hemp limits count THC, not CBD.
Checked October 2026 · Sources below
The molecules
Ring closed: the O–C bond makes a third ring
Ring open: an OH group and a C=C double bond
Skeletal formulas drawn from PubChem’s 2D structures. Each corner or line end is a carbon atom; hydrogens on carbon are not drawn. The dotted line in CBD marks where the bond would form to make THC’s ring. Two molecules with the same formula but a different structure are called isomers.
Source: PubChem, National Library of Medicine, compounds 16078 and 644019 (formula C21H30O2, 314.5 g/mol). Checked October 2026.
Side by side
| THC | CBD | |
|---|---|---|
| Molecular formula | C21H30O2 | C21H30O2 |
| Molar mass | 314.5 g/mol | 314.5 g/mol |
| Rings | Three: the middle ring is closed by an oxygen atom | Two, not joined into a third |
| OH groups | One | Two |
| Acid form in the raw plant | THCA (C22H30O4, 358.5 g/mol) | CBDA (C22H30O4, 358.5 g/mol) |
| Made in the plant from | CBGA, by the enzyme THCA synthase | CBGA, by the enzyme CBDA synthase |
| Acid → neutral form | Heat removes CO2 (decarboxylation) | Heat removes CO2 (decarboxylation) |
| Counted in the federal hemp limit | Yes: 0.3% delta-9 THC today; total THC including THCA from Dec 11, 2026 | No |
| Federal status (Oct 2026) | Cannabis above the hemp limit is marijuana, a controlled substance | Not controlled when it comes from hemp; CBD from marijuana is controlled |
| Federal workplace drug-test panel | Yes: urine tests target the THC metabolite Δ9THCC (50 ng/mL screen, 15 ng/mL confirm) | Not on the panel |
| FDA: food and dietary supplements | Not allowed in either | Not allowed in either |
Sources: PubChem (formulas, masses); Gülck and Møller 2020 (biosynthesis); 7 U.S.C. 1639o, P.L. 119-37 and P.L. 119-103 via CRS IF13136 (hemp definition); DEA, 85 FR 51639 (CBD from hemp); HHS FR Doc. 2026-04981 (drug-test panel); FDA (food and supplements). Full list below. Checked October 2026.
Decarboxylation
The raw plant makes mostly the acid forms, THCA and CBDA. Heat breaks off a carboxyl group as carbon dioxide and leaves THC or CBD.
THCA C22H30O4 · 358.5 g/mol
CO2 · 44.0 g/mol · 12.3%
314.5 ÷ 358.5 = 0.877: each gram of THCA can become at most 0.877 g of THC. CBDA → CBD works the same way.
Enter the THCA and delta-9 THC results from a certificate of analysis (COA), as percent of dry weight.
Formula from USDA’s hemp rule, 7 CFR 990.1: Total THC = (0.877 × THCA) + delta-9 THC. This is arithmetic only, not a compliance finding; official hemp compliance testing follows USDA and state sampling rules.
Federal law
Dec 20, 2018
2018 Farm Bill
Hemp is defined as cannabis with no more than 0.3% delta-9 THC on a dry weight basis and is removed from the federal definition of marijuana (P.L. 115-334 §10113; 7 U.S.C. 1639o).
Aug 21, 2020
DEA rule
DEA confirms that hemp-derived products within the 0.3% delta-9 THC limit, including CBD, are not controlled substances (85 FR 51639).
Jan 19, 2021
USDA hemp rule
Crops must be tested for total THC, counting THCA with the 0.877 factor (7 CFR 990).
Nov 12, 2025
New hemp definition enacted
The FY2026 agriculture appropriations law (P.L. 119-37, Div. B §781) redefines hemp by total THC, including THCA, and caps final products at 0.4 mg total THC per container. Set to start one year later.
Today
Oct 28, 2026
Nov 12, 2026
First part takes effect
Products containing cannabinoids the cannabis plant cannot naturally produce are no longer hemp.
Dec 11, 2026
The rest takes effect
Delayed from Nov 12 by P.L. 119-103 (Sept 2, 2026): the total-THC definition, the 0.4 mg per container cap and the exclusion of cannabinoids synthesized outside the plant.
Status checked October 26, 2026. Bills to delay the change by two more years or to repeal it have been introduced but not passed (CRS IF13136, Sept 28, 2026). State hemp laws can be stricter. This page is not legal advice. State-by-state detail: federal hemp definition change 2026.
Testing and regulation
More cannabinoids, side by side: cannabinoids chart.
Sources: PubChem, National Library of Medicine: Tetrahydrocannabinol, CID 16078, Cannabidiol, CID 644019, THCA CID 98523, CBDA CID 160570. Gülck, T. and Møller, B.L. (2020), “Phytocannabinoids: Origins and Biosynthesis”, Trends in Plant Science 25(10): 985–1004. 7 U.S.C. 1639o (2018 Farm Bill, P.L. 115-334 §10113). Congressional Research Service, IF13136, “Changes to the Statutory Definition of Hemp and Implications for Agricultural Policy”, updated September 28, 2026 (P.L. 119-37 Div. B §781; P.L. 119-103). USDA, 7 CFR 990.1, definitions of “Decarboxylated” and “Total THC” (86 FR 5680, Jan. 19, 2021, as amended at 88 FR 82236, Nov. 24, 2023). DEA, “Implementation of the Agriculture Improvement Act of 2018”, 85 FR 51639 (Aug. 21, 2020). HHS/SAMHSA, “Mandatory Guidelines for Federal Workplace Drug Testing Programs: Authorized Testing Panels”, FR Doc. 2026-04981 (Mar. 13, 2026). FDA, “FDA Regulation of Cannabis and Cannabis-Derived Products, Including Cannabidiol (CBD)” (content current as of July 16, 2024) and statement of January 26, 2023. Checked October 2026.
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