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A percentage on a seed listing is a claim about a plant that does not exist yet. If it was measured at all, it was measured on flower the breeder grew under their own conditions, and what you end up with depends on your light, your feeding and when you cut. Treat the figure as a ceiling and a description of the variety, not as a specification.
What the figure describes
THC on a listing almost always means THC in dried flower as a percentage of dry weight, and usually the top end of a range: 24% is the median of the strains here that state one at all. Ranges like "18 to 22%" are more honest than a single figure, because the variation between two plants of the same variety, or between the top and the bottom of one plant, is genuinely that wide.
CBD is quoted less often and only really matters when it is high. A variety bred for CBD is a different thing from a THC variety with a trace of CBD in it, and the listings that mean it give you a ratio.
Why the numbers drift upward
Potency figures have crept up across the whole industry, and somebody has measured by how much. A 2023 study published in PLOS One tested 23 flower samples from ten Colorado dispensaries against their labels: the observed potency averaged 15%, while the labels claimed a low of 20% and a high of 24%. Seventy percent of the samples were at least 15% below what the packaging said.
That was retail flower under regulation, not seed listings, which are not tested by anyone at all. It is the closest measurement available of how far a potency figure can drift from what is in the jar. Seed listings are a different case again: they describe a plant nobody has grown yet, which is why they are best read as a range rather than a promise.
What a grower actually controls
Genetics set the ceiling; the grow decides how close you come to it. Light intensity through flowering, a stable environment and the timing of the harvest all move the result, and the timing moves it most. Cut early and the cannabinoids have not finished accumulating; cut late and THC breaks down into CBN. Ripeness is read on the trichomes under magnification, not on the calendar.
Drying and curing then decide how much of that survives, which is a subject of its own: see harvest, drying and curing.
Reading a listing sensibly
Read a stated percentage as a band rather than a figure, and trust a range more than a single number. Compare varieties in broad steps rather than by the decimal: the difference between a strain listed at 18% and one at 20% is inside the noise, while the difference between 8% and 20% is real.
In any case the effect you are after is not held in that one number. The terpene profile and the balance between cannabinoids shape an experience more than the THC figure does on its own, which is what the genetics research concluded about indica and sativa labels as well.
On this site
Where a shop states a THC figure, the strain page shows it exactly as given. Nothing here is tested independently, and a listing without a number is left blank rather than filled in with an estimate.