Walk into any hemp shop and you’ll see THCa flower labeled with impressive numbers: 23%, 27%, even 30% THCa. But what do those percentages actually mean once you light up? If you’ve ever compared two COAs and still felt unsure which batch was truly more potent, this guide is for you.
Below, you’ll learn exactly how to convert a THCa percentage into an estimate of the THC you’ll experience after heating, what “Total THC” really measures, how moisture basis changes the math, why some labels look stronger than they are, and how to use this knowledge to buy smarter—every time.
Why There Are Multiple “THC” Numbers on a COA
Raw flower contains cannabinoids mostly in their acid forms. For psychoactivity, the important pair is:
- THCa (tetrahydrocannabinolic acid): the predominant cannabinoid in raw, unheated flower. THCa is not psychoactive.
- Δ9-THC (delta-9 tetrahydrocannabinol): the psychoactive form that appears after heat removes a carboxyl group from THCa (a process called decarboxylation).
A Certificate of Analysis (COA) often shows low Δ9-THC and high THCa. That’s normal. The moment you smoke or vape, a portion of that THCa becomes Δ9-THC. The potency question is: how much Δ9-THC will be available after heating? That’s exactly what “Total THC” attempts to estimate.
The Core Formula You Need
When THCa decarboxylates, it loses mass (a CO₂ group). The ratio of molecular weight of Δ9-THC to THCa is about 0.877. This gives us the industry-standard formula:
Total THC (decarbed estimate) = Δ9-THC + (THCa × 0.877)
Notes:
- On a flower COA, Δ9-THC is often near zero. The driver is THCa × 0.877.
- 0.877 is chemistry-based; it’s not “loss from burning.” Combustion/vaping efficiency is separate.
- This formula assumes full decarb—your actual experience depends on method and cure.
Walk-Through Examples (Percentages)
Example A: THCa = 26.0%, Δ9-THC = 0.10% → Total THC ≈ 22.90%
Example B: THCa = 24.5%, Δ9-THC = 0.05% → Total THC ≈ 21.53%
Example C: THCa = 28.4%, Δ9-THC = 0.12% → Total THC ≈ 25.03%
Even a couple points of THCa make a noticeable difference after conversion.
Percent vs. mg/g (Don’t Let Formatting Trip You)
COAs may list cannabinoids as percent (%) or milligrams per gram (mg/g). They’re functionally equivalent:
1% ≈ 10 mg/g
So 25% THCa ≈ 250 mg/g. If your COA uses mg/g, divide by 10 to see percent, then apply the 0.877 factor.
Dry Weight vs. As-Received (Moisture Basis Matters)
- As-received: includes natural water content at test time.
- Dry weight: normalizes out moisture for apples-to-apples comparison.
If you’re comparing batches, look for “% by dry weight.” Brands listing only as-received numbers may show lower potency—or use it to manipulate comparisons. Transparent reports show both.
What About Real-World Efficiency?
The 0.877 factor estimates chemistry, but the real world introduces variability:
- Combustion losses: not all THC generated is inhaled.
- Vaping technique: lower temps preserve terpenes; higher temps convert more THCa but may feel harsher.
- Device & airflow: vape design, pack density, and draw style affect delivery.
- Cure & dryness: over-dry burns hotter and sheds aroma; too wet tastes grassy.
Think of Total THC as a ceiling. Your experience reflects that ceiling filtered through technique, device, and cure quality.
“Total THC” vs. “Total Potential THC”
Labs and states use different names, but in flower potency context they usually mean the same thing:
- Total THC
- Total Potential THC
- Total THC (calculated)
All refer to Δ9-THC + (THCa × 0.877). If a report uses a different factor or includes extras, check the footnotes.
Why “Compliant Δ9” Doesn’t Tell You Potency
Some hemp labels lean on lines like “Δ9-THC: 0.19% (compliant).” That speaks to legality, not strength. If the THCa % isn’t shown, you can’t estimate potency once smoked or vaped. For buying decisions, THCa % × 0.877 is what matters.
How Moisture and Cure Impact the Numbers You Feel
- Water Activity: target ~0.55–0.65 for cured flower. Higher = microbe risk/uneven burn; lower = muted flavor and hot burn.
- Moisture %: aim for 8–12%. Too wet = grassy; too dry = brittle and terpene loss.
- Total Terpenes: 1–3%+ usually means louder aroma and fuller profile.
These cure metrics don’t change the math, but they change the experience.
The Most Common Label Tricks
- No THCa % shown: you can’t compute Total THC. Ask for the COA.
- As-received only: may understate potency. Ask for dry-weight values.
- Perfectly round numbers: “30.00%” everywhere looks suspicious.
- Overemphasis on Δ9 compliance: legal, but not potency truth.
- Cropped COAs: missing footers/QR code. Always ask for the full report.
Choosing Between Two Jars in the Shop
- Find THCa % and Δ9-THC % on the COA/label.
- Compute Total THC = Δ9 + (THCa × 0.877).
- Prefer dry-weight basis; confirm fresh test date.
- Scan cure metrics (Water Activity + moisture) if available.
- Check total terps for aroma intensity and top terps for family (gas, citrus, dessert, pine).
- Verify lot number matches COA via QR code.
If two jars are close on Total THC, let terpene profile and cure decide—you can’t smoke a number, you smoke a profile.
Vaping vs. Smoking: Does Method Change the Math?
The formula doesn’t change, but delivery efficiency does:
- Vaping: can decarb more completely with less sidestream loss. Preserves volatile terpenes at moderate temps (170–200°C / 338–392°F).
- Smoking: instant decarb but sacrifices cannabinoids/terpenes to combustion. Technique—
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