Legality

What is total THC and why does THCa count?

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Answer

Total THC is delta-9 THC plus the delta-9 that THCa becomes when heated. USDA's hemp rule writes it as Total THC = (0.877 x THCA) + THC (7 CFR 990.1). From December 11, 2026, the federal hemp definition uses this total, so THCa-rich flower no longer qualifies as hemp.

"Total THC" is the number that decides whether a plant or a product is hemp after December 11, 2026. It is not new. USDA has applied it to hemp crops for years. What is new is applying it to finished consumer products under the federal definition. The broader context is at /learn/2026-federal-hemp-rule.

What is the difference between THC and THCa?

Answer

Cannabis plants do not make much delta-9 THC directly. They make tetrahydrocannabinolic acid, THCa, which converts to delta-9 THC when heated. USDA's hemp regulations describe decarboxylation as the completion of the chemical reaction that converts THCa into delta-9 THC, and they define the decarboxylated value as delta-9 THC plus 87.7 percent of THCa (7 CFR 990.1). The 0.877 factor is a molecular-mass correction: THCa loses a carboxyl group as CO2 when it converts, so one milligram of THCa yields about 0.877 milligrams of THC.

That is why "THCa flower" could test under 0.3 percent delta-9 while containing 20 percent or more THCa. On a delta-9-only test it looked like hemp. On a total THC test it is well above the line.

How is total THC calculated?

Answer

The rule states it in one line: "Total THC = (0.877 x THCA) + THC" (7 CFR 990.1). Worked example for a lab report showing 0.05 percent delta-9 THC and 0.40 percent THCa:

  • 0.877 x 0.40 = 0.351 percent
  • 0.351 + 0.05 = 0.401 percent total THC

That sample exceeds 0.3 percent and is not hemp under the total THC standard, even though its delta-9 reading alone is a tenth of the limit.

USDA also builds in measurement uncertainty. A crop sample is at an "acceptable hemp THC level" when applying the lab's measurement of uncertainty to the reported total THC produces a range that includes 0.3 percent or less (7 CFR 990.1). That tolerance applies to the crop compliance test, not to the new per-container product cap.

When does total THC apply to products?

Answer

The 2018 Farm Bill's hemp definition referenced delta-9 THC only. The CRS notes that reading let products with less than 0.3 percent delta-9 and other psychoactive compounds be sold as hemp (CRS IN12620). Section 781 of P.L. 119-37 rewrote the definition to a total THC level, including THC acid, of not more than 0.3 percent on a dry weight basis (7 U.S.C. 1639o).

Section 2019 of P.L. 119-103 delayed that total THC standard, along with the 0.4 mg per container cap, to December 11, 2026 (GovInfo). Carriers moved first: UPS states that effective December 11, 2026, raw hemp shipments must contain a certificate of analysis demonstrating "a total THC concentration (including tetrahydrocannabinolic acid) of not more than 0.3%" on a dry weight basis (UPS).

What should I look for on a lab report?

Answer

Three lines: delta-9 THC, THCa, and a total THC line that applies the 0.877 factor. If the report lists only delta-9, do the math yourself. For a finished CBD product, the relevant question after December 11 is whether total THC across the whole container stays under 0.4 mg, which usually means both delta-9 and THCa read ND at a low LOQ. See How do I read a CBD lab report? and What is the 0.4 mg total THC per container cap?.

General information, not personalized guidance.

Sources

  1. 7 CFR 990.1, Definitions Cornell Law School, Legal Information Institute (accessed 2026-10-13)
  2. 7 U.S.C. 1639o, Definitions (hemp) Cornell Law School, Legal Information Institute (accessed 2026-10-13)
  3. Public Law 119-103, Continuing Appropriations and Extensions Act, 2027, section 2019 U.S. Government Publishing Office (accessed 2026-10-13)
  4. Change to Federal Definition of Hemp and Implications for Federal Enforcement (IN12620) Congressional Research Service (accessed 2026-10-13)
  5. Shipping Hemp and Hemp-Derivatives UPS (accessed 2026-10-13)

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