The decarb formula: why 0.877?

The formula

Total THC (%) = (THCA% × 0.877) + Δ9-THC%

0.877 = molar mass of THC (314.46 g/mol)
      ÷ molar mass of THCA (358.47 g/mol)

Limit: Total THC ≤ 0.3% dry weight

This is the conversion in USDA's hemp production rule (7 CFR 990.1, “Total THC”) and in Florida's rule 5K-4.034(2)(q), and Public Law 119-37 section 781 counts THCA in the federal definition of hemp. Our free THCA calculator applies this formula instantly to any COA values you enter.

Where 0.877 comes from

THCA (tetrahydrocannabinolic acid) and THC (delta-9 tetrahydrocannabinol) are the same core molecule, but THCA carries an extra carboxyl group (–COOH) attached to its ring structure. That carboxyl group adds mass — specifically the difference between the two molecular weights:

• THCA molar mass: 358.47 g/mol
• THC molar mass: 314.46 g/mol
• Ratio: 314.46 ÷ 358.47 = 0.8774…, rounded to 0.877

When you heat THCA — whether by smoking, vaping, or baking — a process called decarboxylation occurs: the carboxyl group leaves as CO₂ gas. The molecule loses that mass permanently. So one gram of THCA cannot produce one gram of THC; it produces 0.877 grams. Expressed as percentages, 24% THCA yields at most 24 × 0.877 = 21.05% THC after full decarboxylation.

This ratio comes from the molar masses, not from a regulatory choice. It appears in USDA's definition of “Total THC” (7 CFR 990.1) and in Florida's definition of “total delta-9 tetrahydrocannabinol concentration” (Rule 5K-4.034(2)(q)) for exactly this reason.

Why Δ9-THC is added separately

Cannabis flowers do not contain only THCA — they also carry a small amount of already-decarboxylated delta-9 THC that formed naturally during drying and curing. A COA will list this as "Δ9-THC" or "d9-THC". Because it is already in the active form, it counts at face value toward total THC without any conversion factor applied. Hence the additive term in the formula:

Total THC = (THCA × 0.877) + Δ9-THC
            ↑ converts THCA   ↑ already active, no factor needed

For typical flower, the Δ9 figure is small (often 0.1–0.3%). For high-potency THCA flower, it is almost irrelevant compared to the converted THCA contribution — but it still must be included for an accurate total.

The 0.3% total THC limit

The 2018 Farm Bill (Public Law 115-334) defined hemp as cannabis with no more than 0.3% delta-9 THC by dry weight, and that is still the text codified in 7 U.S.C. 1639o(1). Public Law 119-37 division B section 781 replaces it with a total tetrahydrocannabinols concentration (including tetrahydrocannabinolic acid) of not more than 0.3 percent on a dry weight basis, that is, the formula above rather than the raw delta-9 number alone. Section 781 takes effect 365 days after its November 12, 2025 enactment, on November 12, 2026. This matters for high-THCA products:

A COA that lists only the delta-9 column can hide this. Check the total-THC line, or calculate it yourself.

The 0.4 mg per-container cap and the dates

Section 781 also excludes final hemp-derived cannabinoid products with more than 0.4 milligrams combined total per container of total THC (including THCA) and other cannabinoids with similar effects. “Container” means the innermost wrapping, packaging, or vessel in direct contact with the final product. To compare a lab result with the cap, the calculator uses:

Total mg = Total THC % × 10 mg per gram × grams in the container
Compare with 0.4 mg

Two limits on that calculation: it counts THC and THCA only, while the statute also counts other cannabinoids with similar effects; and a COA percentage is on a dry weight basis, so the result for a container of material that is not dry is an estimate.

Public Law 119-103 section 2019 (signed September 2, 2026) says that until December 11, 2026 the section 781 amendments apply only to products described in 7 U.S.C. 1639o(1)(C)(ii)(I) and (iv)(I), the cannabinoids not capable of being naturally produced by the plant. The total-THC definition, the exclusion of synthesized or manufactured cannabinoids, and the per-container cap apply in full from December 11, 2026. The dates have moved once, so check the sources below. For the dated summary, see The 2026 hemp ban, explained.

How to read a COA for this calculation

A certificate of analysis (COA) from an ISO-accredited lab will have a potency panel, typically listing results as percentages of dry weight. Here is what to look for:

1. THCA% — the acidic precursor value. This is usually the largest number on a flower COA, often labelled "THCA" or "Δ9-THCA".
2. Δ9-THC% — the already-active delta-9 THC. Often very small on properly cured flower, labelled "d9-THC", "Δ9-THC", or "THC".
3. Total THC% — some labs include a pre-calculated "Total THC" row using the 0.877 formula. If it is present, you can read it directly. If it is absent, use our calculator to derive it.
4. Batch / lot number — verify the COA is batch-matched to the product in question, not a generic lab report for the strain.
5. Lab accreditation — look for ISO/IEC 17025 or a state-licensed hemp testing lab. Results from non-accredited labs carry higher uncertainty.

A batch-specific COA from an accredited lab is the reliable source for these numbers. A generic report, or a "typical" potency figure, is not a substitute for a batch-matched COA.

State vs. federal rules

The federal definition (section 781, in effect on the dates above) uses total THC with THCA, and USDA's hemp production rule already tests on that basis. Individual states can set stricter or different rules. Florida's Rule 5K-4.034 uses the same 0.877 formula and a 0.3% limit for hemp and hemp extract intended for human consumption. Other states may:

• Regulate THCA directly, independent of the total-THC formula.
• Apply a lower percentage threshold than 0.3%.
• Explicitly adopt the federal total-THC definition by statute.
• Have no specific hemp testing statute and default to federal rules.

This calculator implements the total-THC formula and the two comparisons above. It is information only, not legal advice; verify your state's rules, because cannabinoid law is jurisdiction-specific and changes frequently.