Why time, not a blood number?
For cannabis, a single blood concentration predicts impairment very poorly (published correlations are near zero, R²≈0.03). So Clearhead does not report a meaningless number — it models recovery over time: how long a substance is likely to affect you, and when it clears, for your pattern of use and your body.
This is the core design choice. A concentration on its own tells you little about how someone actually functions; the same level means different things for an occasional and a daily user. Time — shaped by frequency, dose, route and body composition — is the more honest axis.
What model does it use?
Estimates are generated by a population pharmacokinetic model grounded in peer-reviewed research (notably a three-compartment population model for THC), rather than a single average copied from another site. The model is applied per substance family and adjusted to your inputs — frequency of use, typical dose, route of administration and body composition.
We publish what the model is built on and why. We do not publish the exact internal constants, calibration factors or formulas — those are the part of the product that took the real work, and they stay in the app. The point here is transparency about the evidence and the approach, not a build recipe.
How are the detection windows built?
For each medium — blood, saliva and urine — the window is driven mainly by two things: your pattern of use (single, occasional, regular, heavy) and, for the fat-stored metabolite that urine tests detect, your body-fat level. Because THC is lipophilic, it is stored in fatty tissue and released slowly, which is why a daily user can remain detectable far longer than the effects last. The windows shown across the site come from this same model.
The evidence base
Clearhead is calibrated on published, peer-reviewed pharmacology and real driving-impairment studies — not on marketing claims. The primary sources behind the cannabis estimates include:
- Alvarez Y et al., Br J Clin Pharmacol 2021;87(8):3139–3149 — population pharmacokinetic model of THC.
- McCartney D et al., Neurosci Biobehav Rev 2021;126:175–193 (PMID 33497784) — duration of cannabis impairment.
- Huestis MA, 1992 & Hunault CC et al., 2008 (PMID 18695931) — blood peak concentrations.
- Karschner EL et al., J Anal Toxicol 2009;33:469 — residual blood levels in chronic users.
- Lowe RH et al., J Anal Toxicol 2009 — elimination half-life during abstinence.
- Bergamaschi MM et al., Clin Chem 2013;59(3):519–526 (PMID 23449702) — prolonged excretion in chronic users.
- Spindle TR et al., 2019 (oral & vaporised dosing); Toennes SW et al., 2008 (occasional-user kinetics).
Legal limits are compiled separately from primary legislation, each cited on the THC limits page.
What it cannot tell you
Clearhead is honest about its limits — this is an estimate, not a measurement:
- It is not a lab or forensic result. Blood values are modelled in plasma as a physiological estimate; a laboratory measures serum or whole blood under a controlled protocol.
- Individual variation is real. Metabolism, genetics, health and body composition move the true value; the estimate is a likely range, not a guarantee.
- It is never permission. Being below a limit — or past an estimated window — is not permission to drive or to use any substance. The safest choice is always none.
- Safety guidance stays conservative. Recovery is never reported faster than the evidence supports.
How this differs from generic calculators
Most THC calculators output one number with no model behind it, ignore your pattern of use and body, and quietly repeat detox myths. Clearhead does the opposite: an explicit model, per-profile estimates across three media, cited sources, and a plain statement of what it cannot do. That is the whole point — and it is why the estimate is worth trusting only as far as it is honest about itself.
Authored by Rudolf Prokeš on behalf of Culture s.r.o. Last reviewed 30 July 2026. See the Impressum for company and contact details.