Pick up two jars of flower at our Hastings-on-Hudson counter and the first number most people read is the biggest one, usually labeled THCa. That number is real, but it is not the amount of THC you actually get when you light a bowl or hit a vape. The figure that reflects active potency is the smaller one: Total THC. Understanding the gap between the two is the single most useful skill for comparing flower.
Here is the short version. THCa is the raw, acidic form of the cannabinoid that exists in unheated flower. It is not intoxicating on its own. When you apply heat by smoking, vaping, or dabbing, THCa loses part of its structure and converts to THC. That conversion is why a label can read THCa 28% and Total THC around 24%. Both numbers describe the same jar. One is before heat, one is after.
What THCa is, and why it dominates the label
In a living, drying, or cured cannabis plant, almost all of the THC-family content sits in its acidic form, THCa (tetrahydrocannabinolic acid). Raw flower is mostly THCa, not THC, which is why the THCa figure is the largest cannabinoid number on a flower jar. THCa carries an extra carboxyl group (a COOH cluster) on the molecule. That group is what comes off during heating.
Because THCa is the dominant compound before combustion, growers and labs lead with it. It is an honest measurement of what is in the flower. It just is not the measurement of what reaches you after the lighter does its job.
The conversion math: 0.877
When THCa is heated it undergoes decarboxylation, dropping that carboxyl group as carbon dioxide. The molecule that remains, THC, is lighter. THCa has a molecular weight of about 358.5 and THC about 314.5. Divide them (314.5 / 358.5) and you get roughly 0.877. So a maximum of about 87.7% of the THCa mass can become THC, and about 12.3% is lost as CO2 in the process.
New York's Office of Cannabis Management uses this exact factor in the Total THC formula that appears on regulated labels: Total THC = THC + (THCa × 0.877). The THCa is multiplied down to account for the mass lost when it activates, then added to any free THC already present in the flower.
How do you calculate Total THC from THCa?
Multiply the THCa percentage by 0.877, then add any THC the lab already measured separately. So flower testing at 28% THCa and 0.5% THC works out to (28 × 0.877) + 0.5, which is about 25.1% Total THC. The 0.877 factor reflects mass lost as CO2 during heating.
How to compare two jars on our shelf
The mistake is comparing one jar's THCa number against another jar's Total THC. Labs and brands do not always format labels the same way, so always line up the same metric. For predicting how a flower will hit, Total THC is the apples-to-apples figure because it estimates the activated content after you heat it.
- Find the same line on both labels, ideally Total THC, before comparing.
- If only THCa is listed, multiply it by 0.877 yourself for a close estimate.
- Remember 0.877 is the ceiling, not a promise. Real-world combustion never converts at 100%, so actual delivered THC runs a bit lower.
- Potency is one variable. Terpene content, cultivar, and freshness shape the experience as much as the headline number.
Is higher THCa always better flower?
Not necessarily. A high THCa number tells you potential potency, but it says nothing about terpenes, smoothness, or how a cultivar suits you. Many experienced shoppers report that a balanced terpene profile shapes the session more than chasing the highest percentage. Pick for the whole label, not one figure.
THCa is what is in the jar. Total THC is what reaches you after heat. Compare the same one on every label.
Once you can read past the big number, the whole wall of flower gets easier to shop. Our ~40 craft cultivars from independent upstate growers list both THCa and Total THC, and our budtenders are happy to walk you through any label in person or over the phone. Browse current flower and the full lab breakdowns at /order?category=flower.