How it works
Cannabis plants make THC mostly in an acid form, THCA. A lab measures the two separately, so a label can show a small Δ9-THC figure next to a much larger THCA figure.
When THCA loses its carboxyl group as carbon dioxide (CO₂), a process called decarboxylation, each molecule of THCA becomes one molecule of Δ9-THC. The THC molecule is lighter than the THCA molecule it came from, because the CO₂ has gone. So to count THCA as THC, a lab multiplies it by the ratio of the two weights: 0.877.
Worked example: a label shows Δ9-THC 0.6% and THCA 24.3%.
Total THC = 0.6 + 0.877 × 24.3 = 0.6 + 21.3 = 21.9%
The same sum works in mg/g: 6 mg/g + 0.877 × 243 mg/g = 219 mg/g. CBD works the same way, with CBDA × 0.877. CBG uses 0.878, because the CBG and CBGA molecules are slightly heavier.
Where 0.877 comes from
0.877 is the ratio of the two molecules’ masses (THC 314.47 ÷ THCA 358.48). This is arithmetic on a lab result. It is not a measure of experience and not a guide to use.
In full: Δ9-THC (C₂₁H₃₀O₂) has a molar mass of 314.47 g/mol, and THCA (C₂₂H₃₀O₄) has 358.48 g/mol, worked out from their formulas with standard atomic weights (PubChem lists them rounded, as 314.5 and 358.5). The difference, 44.01 g/mol, is one molecule of CO₂. Dividing 314.47 by 358.48 gives 0.8772, which rounds to 0.877. For CBG and CBGA it is 316.48 ÷ 360.49 = 0.8779, which rounds to 0.878. The USDA’s testing rule describes 0.877 the same way, as “a molecular mass conversion ratio”.
So 0.877 is not a “conversion efficiency” and not an estimate of how much THCA actually turns into THC in any process. It is the THC weight that THCA represents on paper. Some websites explain it as allowing for THCA that doesn’t convert; that isn’t what the number means.
Questions about total THC
What is total THC?
Total THC is Δ9-THC plus THCA counted by its THC weight: Δ9-THC + 0.877 × THCA. It puts both forms on one scale, so results from different products and labs can be compared.
Why do labels show THCA at all?
Because the plant makes THC mostly as THCA, and a lab measures what is in the sample. Labs that use liquid chromatography (HPLC) can measure THCA and Δ9-THC separately, and the label reports both.
Does total THC assume all the THCA becomes THC?
It is a weight calculation: the amount of THC the THCA represents, molecule for molecule. It doesn’t estimate what happens to any product after testing, and this calculator doesn’t either.
My label's total doesn't match. Is the label wrong?
Not necessarily. Labels round their figures, some report on a dry-weight basis and others as received, and some reports total the figures a different way. The label check tells you which calculation the printed total matches, if any. Labels can read higher or lower than typical for many reasons: the batch, the grower, the lab, and how the sample was taken.
Does every state calculate total THC the same way?
Most use the same core formula, Δ9-THC + 0.877 × THCA. Some add other forms: Ohio’s total THC also includes Δ8-THC. This calculator uses the core formula.
What's the difference between % and mg/g?
They are the same measure on different scales: 1% is 10 mg per gram. A THCA result of 24.3% is 243 mg/g.
What do "dry weight" and "as received" mean?
“Dry weight” results are calculated as if the sample’s water were removed; “as received” results include the water. The same flower reads higher on a dry-weight basis. States differ: Vermont asks labs to report flower potency on a dry-weight basis, while Ohio and Washington report results as received. Check which one your label uses before comparing two labels.
Does a higher total THC mean a better strain?
No. Total THC measures one compound in a sample. It does not predict how a strain will feel, and labels often read higher than independent tests.
Is anything I enter saved?
No. The sums run in your browser, and nothing you type is sent or stored.
Sources
- Tetrahydrocannabinol (CID 16078). PubChem, National Center for Biotechnology Information (NIH).
- delta9-Tetrahydrocannabinolic acid (CID 98523). PubChem, National Center for Biotechnology Information (NIH).
- Cannabigerol (CID 5315659). PubChem, National Center for Biotechnology Information (NIH).
- Cannabigerolic acid (CID 6449999). PubChem, National Center for Biotechnology Information (NIH).
- 7 CFR § 990.1 Definitions (Total THC; Decarboxylated; Decarboxylation). US Department of Agriculture, Agricultural Marketing Service (text via Legal Information Institute, Cornell University).
- DCC Guidance: Total THC Content and Total CBD Content. Ohio Division of Cannabis Control.
- How is total THC calculated?. Connecticut Department of Consumer Protection.
- WAC 314-55-102 Quality assurance and quality control. Washington State Legislature (Liquor and Cannabis Board rule).
- Cannabis Lab Potency Reporting Metrics. Vermont Cannabis Control Board.
- Updated guidance: potency testing and reporting. Ohio Department of Commerce, Division of Cannabis Control.
- Decarboxylation study of acidic cannabinoids: a novel approach using ultra-high-performance supercritical fluid chromatography/photodiode array-mass spectrometry. Cannabis and Cannabinoid Research (Wang M, Wang YH, Avula B, et al.).