Total THC
Definition: Total THC is a lab measurement that adds a sample's delta-9 THC to the THC its THCA would yield once heated, usually calculated as delta-9 THC + (0.877 × THCA). It reflects a flower's potential potency and, under Section 781 of H.R. 5371, becomes the federal yardstick for the 0.3% hemp limit on December 11, 2026.
How Total THC works
Fresh and dried cannabis contains mostly THCA, with only a small amount of delta-9 THC. Measuring delta-9 alone understates what the flower delivers once heated. Total THC fixes that by counting both.
The formula
Total THC = delta-9 THC + (0.877 × THCA)
The 0.877 factor reflects the weight lost when THCA sheds carbon dioxide during decarboxylation. A sample testing 0.3% delta-9 THC and 22% THCA has a total THC of about 0.3 + 19.3 = 19.6%.
Why it matters legally
The 2018 Farm Bill defined hemp by delta-9 THC alone, at 0.3% or less on a dry-weight basis. USDA’s hemp production rules already required pre-harvest crop tests to account for THCA, but the statutory definition did not, which let high-THCA flower be marketed as hemp.
Section 781 of H.R. 5371 (Public Law 119-37), signed November 2025, writes total THC, including THCA, into the federal hemp definition. It also excludes viable seeds from plants above 0.3% total THC from that definition. H.R. 6500 (Public Law 119-103), enacted September 2, 2026, made December 11, 2026 the effective date for most of these provisions, including the seed exclusion.
What it means for seed buyers
Seeds contain only trace cannabinoids. A strain listed at 20% THC describes the potential of its finished flower, not the seed. Under the new standard, a seed’s federal status is tied to the total THC of the plant it came from, while state law continues to govern home growing. Our seed legality guide covers the details. It is educational content, not legal advice.