Total THC Formula and Calculator
A lab result reporting separate THC and THCa percentages needs a molecular-mass correction before the values can be combined.
A reported THCa percentage and a reported delta-9 THC percentage refer to different molecules. Adding the raw percentages overstates the THC-equivalent contribution from THCa because the carboxyl group leaves as CO2.
For percentages reported on the same sample-mass basis, potential total THC is calculated as existing THC plus the THC-equivalent mass of THCa.
The total THC calculation
Potential total THC (%) = measured delta-9 THC (%) + [measured THCa (%) × (314.46 / 358.47)]. The molar-mass ratio is approximately 0.87723. This formula converts the reported acid fraction into an equivalent neutral THC mass fraction.
For a report listing 20% THCa and 1% delta-9 THC on the same basis, potential total THC is about 18.545%. It is not 21% and it is not an assurance that processing can recover 18.545% THC.
Make sure the percentages share a basis
Use THC and THCa values from the same sample and the same basis. A dry-weight THCa percentage combined with an as-received THC percentage does not produce a meaningful total. Check whether a laboratory report lists wet-weight, as-received or dry-weight values.
The formula expresses a potential mass equivalent before process losses. If you need estimated milligrams in a specific batch, multiply the calculated percentage by the starting batch mass. If you need a formulation yield after heating, conversion completion and recovery are separate inputs.
Potential THC versus a finished batch
The total THC number is useful for comparing report values and describing the THC-equivalent content of starting material. It does not say how quickly THCa reacts, what fraction degrades, or how much THC survives handling and formulation.
For a physical batch, the batch-yield model accounts for CO2 leaving the material. For edibles, the formulation tool also lets you choose completion and recovery assumptions instead of treating the report number as the delivered dose.
References
Questions about this calculation
Can I add THCa percent directly to THC percent?
No. THCa has a greater molecular mass. Potential total THC on a common percentage basis is THC + THCa × (314.46 / 358.47).
Does total THC predict how much THC I will recover?
No. It is a theoretical mass-equivalence value. Reaction completion, degradation and processing losses require separate assumptions or measurements.