Start with the potency panel. Find total THC and total CBD. Check the serving size and package weight. Then read the contaminant panel for pass or fail results. Match the batch ID on the label to the lab report. Confirm the lab holds accreditation for cannabis testing.
understanding thc percentage on weed labels
Read potency first
Potency appears as percent by weight. THC and THCA are separate rows. Total THC combines THC and THCA after decarboxylation. Use this formula: total THC = THC + (THCA x 0.877). The 0.877 factor accounts for mass loss when THCA converts to THC. A label that lists 20% THCA and 1% THC has about 18.5% total THC. 1 + (20 x 0.877) = 18.54.
Check serving size and package total
Edibles list milligrams per serving and per package. A 100 mg package with 10 servings holds 10 mg per serving. A 5 mg serving is half that. Compare the package total to the label. State limits vary. Some states cap a single package at 100 mg THC. Some states cap serving size at 5 mg or 10 mg. If the report lists mg/g, multiply by package grams to get total mg. Example: 150 mg/g x 3.5 g = 525 mg total THC.
Read the contaminant panel
Contaminant tests cover pesticides, heavy metals, microbials, mycotoxins, and residual solvents. Reports show pass or fail against state action limits. A pass means the batch fell below the limit for each analyte. A fail means the batch exceeded at least one limit. Look for the limit and the result. A result below the limit of quantitation (LOQ) may appear as <LOQ or ND. That does not always mean zero. It means the lab could not measure a number below that threshold. Limits differ by state and product type. Check the rule for your state.
Confirm the batch and lab
Lab reports tie to a batch, not to a brand. Find the batch ID or harvest ID on the package. Match it to the report. Check the test date, harvest date, and expiration date. A report for a different batch does not apply. Look for the lab name and accreditation. ISO/IEC 17025 is a common lab standard. Some states require certification. A report without a batch ID or lab name gives you little to verify.
Note variance and method limits
Potency results vary. Sampling, moisture, and analytical method affect the number. Flower tests can differ by several percent between labs. Treat one result as an estimate. For concentrates and edibles, check the method and the sample size. Small samples can skew results.
Questions to ask at the store
- What is the total THC, not just THCA?
- What is the mg per serving and mg per package?
- Does the batch ID on the label match the report?
- Which contaminants did the lab test?
- What is the lab accreditation?