Last updated: October 13, 2026 · Formulas and examples checked against the amended federal hemp definition and USDA testing rules. We'll update this page as federal rules change.
Quick answer: what's the difference between total THC and delta-9 THC?
Delta-9 THC counts only the THC already active in a product. Total THC also counts THCA, which turns into delta-9 THC when heated. The standard formula is Total THC = delta-9 THC + (0.877 × THCA).
Until December 10, 2026, federal hemp law only limits delta-9 THC (0.3% by dry weight). From December 11, 2026, the limit applies to total THC instead. That one change is why high-THCA flower, which tests low in delta-9, stops counting as hemp.
This guide covers the percentage math. The new law also adds a separate 0.4 mg cap for each finished product's container, which we explain in our 0.4 mg THC per container guide.
Delta-9 THC vs Total THC: The Two Definitions
| Delta-9 THC standard | Total THC standard | |
|---|---|---|
| What it counts | Only delta-9 THC already present | Delta-9 THC plus the THC that THCA can become |
| Federal limit | 0.3% by dry weight | 0.3% by dry weight |
| Applies to hemp products | 2018 Farm Bill through December 10, 2026 | From December 11, 2026, under P.L. 119-37 |
| Effect on THCA products | THCA isn't counted, so high-THCA flower can pass | THCA is counted, so high-THCA flower fails |
The limit stays at 0.3%. What changes is what gets added up to reach it. The new statute defines hemp by its "total tetrahydrocannabinols concentration (including tetrahydrocannabinolic acid)," which writes THCA directly into the test.
Where the 0.877 in the Total THC Formula Comes From
THCA is the raw, acidic form of THC that cannabis plants make. When it's heated by smoking, vaping or baking, it loses a carboxyl group (released as carbon dioxide) and becomes delta-9 THC. This is called decarboxylation.
Because THCA loses that piece, the THC it becomes weighs less than the THCA it started as. The 0.877 factor is the ratio of their molecular weights:
THC molecular weight (about 314.47 g/mol) ÷ THCA molecular weight (about 358.47 g/mol) = 0.877
In plain terms, 1 gram of THCA can become at most about 0.877 grams of THC. Total THC is a "potential THC" figure: it assumes all the THCA converts.
Regulators use this formula already. Oklahoma's agriculture department, for example, defines total THC as "the measured THC plus 87.7 percent of THCA." For the bigger picture on what THCA is, see our THCA guide.
Total THC vs Delta-9 THC: 5 Worked Examples
These examples use round numbers to show how the math works. They are not results for any specific product.
Example 1: Typical THCA flower
Lab report: 0.2% delta-9 THC, 20% THCA.
Delta-9 test: 0.2% is under 0.3%, so it passes today.
Total THC test: 0.2 + (0.877 × 20) = 0.2 + 17.54 = 17.74% total THC, about 59 times the 0.3% limit. It fails from December 11.
Example 2: Hemp that's just over the line
Lab report: 0.1% delta-9 THC, 0.25% THCA.
Delta-9 test: 0.1% passes easily.
Total THC test: 0.1 + (0.877 × 0.25) = 0.1 + 0.219 = 0.319% total THC. That's just over 0.3%, so it fails, even though the THCA figure looked small.
Example 3: Passing the percentage but failing the container cap
Lab report: 0.05% delta-9 THC, 0.28% THCA, sold as a 1 g pre-roll.
Total THC test: 0.05 + (0.877 × 0.28) = 0.05 + 0.246 = 0.296%, just under 0.3%.
But: 0.296% of 1,000 mg is about 2.96 mg of total THC in the package, more than 7 times the 0.4 mg per-container cap for finished products. Passing one test doesn't mean passing both.
Example 4: A delta-9 gummy
A 5 g gummy with 10 mg delta-9 THC and no THCA.
Both percentage tests: 10 mg ÷ 5,000 mg = 0.2%. With no THCA, delta-9 THC and total THC are the same, so it passes the 0.3% test either way.
But: the gummy alone holds 10 mg, 25 times the 0.4 mg container cap. For edibles, the switch to total THC barely matters. The container cap is what changes the picture.
Example 5: Why "dry weight" matters
A plant sample tests at 0.28% total THC "as received," and the sample is 12% moisture.
Dry weight result: 0.28 ÷ (1 − 0.12) = 0.28 ÷ 0.88 = about 0.318%. Removing the water raises the percentage above 0.3%. Federal limits are measured on a dry weight basis, so check which basis a lab report uses.
How to Read Total THC on a Lab Report
- Look for a "Total THC" or "Total Potential THC" line. Many labs calculate it for you. If it's there, that's the number the new federal percentage test uses.
- If there's no total, calculate it. Find "Δ9-THC" and "THCA" (sometimes "THCA-A"), then add delta-9 THC + (0.877 × THCA).
- Check the units. Percent (%) and mg/g convert easily: 1% = 10 mg/g. Make sure you're comparing like with like.
- Check the basis. Look for "dry weight" versus "as received" or "wet weight," especially on flower.
- Watch for "ND." It means "none detected," below the lab's detection limit. That's good news for THC-free products, but on large containers it's worth checking the per-container result too.
Total THC Isn't New for Hemp Farmers
Hemp crops have been tested this way for years. The USDA's hemp production rule, finalized in January 2021, requires total THC testing of plants in the field using post-decarboxylation or similarly reliable methods. Samples can be taken up to 30 days before harvest, and a crop over 1.0% total THC counts as a negligent violation.
The gap was in processed products. THCA flower could be grown from a compliant crop, then sold on a delta-9-only lab report. The 2025 law closes that gap by applying total THC to hemp itself and to products made from it.
What Changes for Shoppers on December 11
- THCA flower, pre-rolls and THCA vapes stop counting as hemp at the federal level, because their THCA now counts.
- Delta-9 edibles and drinks mostly already passed the percentage test; they're affected by the 0.4 mg container cap instead.
- THC-free CBD products are unaffected by the switch, since they contain neither meaningful delta-9 THC nor THCA. See what hemp products will still be legal after the ban.
For every date and rule in one place, read our federal hemp ban 2026 guide.
Want your voice in the decision?
If you'd rather see hemp regulated with testing, labeling and a 21+ age limit than banned outright, tell your lawmakers. Find your U.S. senators and representative at congress.gov, and your North Carolina legislators at ncleg.gov.
Total THC vs Delta-9 THC FAQs
1. What is the difference between total THC and delta-9 THC?
Delta-9 THC measures only the THC already active in a product. Total THC adds the THC that THCA can turn into when heated, using the formula delta-9 THC + (0.877 × THCA). For products with little or no THCA, the two numbers are nearly the same.
2. How do you calculate total THC?
Multiply the THCA percentage by 0.877, then add the delta-9 THC percentage. For example, flower with 0.2% delta-9 THC and 20% THCA has 0.2 + 17.54 = 17.74% total THC.
3. Why is THCA multiplied by 0.877?
When THCA is heated, it loses a carboxyl group as carbon dioxide and becomes THC, which weighs less. The ratio of THC's molecular weight (about 314.47) to THCA's (about 358.47) is 0.877, so 1 gram of THCA can yield up to about 0.877 grams of THC.
4. When does the total THC rule take effect for hemp products?
December 11, 2026, unless Congress changes the law first. Until December 10, federal law still defines hemp products by delta-9 THC only. A stopgap funding law signed September 2, 2026 moved the date from November 12.
5. Does the total THC rule ban THCA flower?
At the federal level, yes, if the law takes effect as written. Typical THCA flower at 20% THCA works out to over 17% total THC, far above the 0.3% limit. It would no longer count as hemp, and products outside the hemp definition are treated as marijuana under federal law.
6. Is total THC the same as the 0.4 mg per container limit?
No, they're two separate tests. Total THC is a percentage limit of 0.3% by dry weight. The 0.4 mg cap limits the total milligrams of THC and THC-like cannabinoids in a finished product's package. A product can pass the percentage test and still fail the container cap.
7. What does "dry weight basis" mean on a lab report?
It means the THC percentage is calculated after accounting for the sample's water content. Removing moisture raises the percentage, so a sample at 0.28% as received with 12% moisture is about 0.318% on a dry weight basis.
8. Does North Carolina use total THC or delta-9 THC?
North Carolina currently defines hemp by delta-9 THC, at 0.3% by dry weight. The federal total THC standard applies in NC from December 11, 2026. NC's HB 328, which passed the state Senate in July 2026, would also switch state law to total THC if the House passes it.
Sources
- 7 U.S. Code § 1639o, definition of hemp, current and as amended by P.L. 119-37 (Cornell Legal Information Institute)
- 2026 Hemp Clarification Letter: total THC definition (Oklahoma Department of Agriculture, Food and Forestry, Jan 16, 2026)
- Industrial hemp regulations finalized (National Agricultural Law Center, Jan 2021)
- Total THC formula: molecular weights behind the 0.877 factor (GrowerIQ)
- Most of the federal hemp ban delayed until December 11 (Rod Kight, Cannabis Law Report, Sept 1, 2026)
- Changes to the Statutory Definition of Hemp and Issues for Congress (Congressional Research Service, Sept 28, 2026)
- North Carolina considers overhauling hemp product definition (Ward and Smith)
- NC Senate approves new hemp regulations (WFAE, July 2, 2026)
Update log
- October 13, 2026: Published. Federal total THC standard scheduled for December 11, 2026.
This article is for general information only and is not legal advice. Examples are illustrative calculations, not lab results for any specific product. Laws change quickly; check current federal and state law or consult an attorney for your situation. Hemp-derived products are for adults 21 and over.
