Our Methodology
How the THC Detection Calculator works, what it's based on, and where its numbers come from.
What This Calculator Estimates
The calculator gives a personalized estimate of how many days after your last use THC is likely to clear your system, at two urine testing thresholds:
- 50 ng/mL - Initial Screening. The standard cutoff used for most workplace, pre-employment, and general drug screening.
- 15 ng/mL - Laboratory Confirmation. A more sensitive cutoff used when a lab confirms an initial screening result. Both thresholds are standard federal screening and confirmation cutoffs under 49 CFR §40.85.
It does not estimate detection windows for blood, saliva, or hair samples, and it is not a substitute for an actual test. It’s a population-average estimate. Your own results may vary based on physiology, age, lifestyle, hydration, dietary habits, and metabolism.
The Science Behind THC Detection
THC and its main metabolite (THC-COOH) are fat-soluble, meaning they get stored in the body's fat tissue rather than being cleared quickly like most substances. This is the core reason why detection windows vary so widely between a single use and long-term heavy use. With repeated use, THC-COOH accumulates faster than the body can eliminate it, extending the time it stays detectable.
This calculator's base timelines are anchored to five usage categories, each with a documented detection range at the 50 ng/mL threshold:
| Usage Pattern | Typical Detection Window (50 ng/mL) |
|---|---|
| Single use / first time | 1–3 days |
| Occasional (weekly or less) | 6–12 days |
| Moderate (several times a week) | 10–14 days |
| Heavy (daily) | 10–15 days |
| Chronic (multiple times a day) | 21–30+ days |
Confirmatory (15 ng/mL) windows are 50–100% longer than the standard 50 ng/mL window, scaled by usage tier, reflecting how much more slowly the most sensitive cutoff clears at higher usage levels.
What Else Affects Your Result
Beyond usage frequency, several other factors adjust your personalized estimate up or down:
- Body composition. Since THC is stored in fat tissue, higher body fat is associated with longer retention. The calculator estimates this from your height and weight as BMI, which is a rough proxy for body-fat percentage rather than a direct measure of it.
- Duration of use. Longer-term use allows more THC-COOH to accumulate, extending clearance. However, this effect is scaled by how often you use, since infrequent use allows your body to largely clear between sessions.
- Age. Metabolism generally slows with age, modestly extending detection time.
- Activity level. More active lifestyles are associated with somewhat faster clearance.
- Product type and dosage. Different consumption methods and amounts affect how much THC enters your system per use.
- Alcohol, nicotine, and diet. General lifestyle factors that can modestly influence clearance, though the evidence for their specific effect on THC clearance is weaker than for the factors above.
Sources & References
- Huestis, M.A. (2007). Human cannabinoid pharmacokinetics. Chemistry & Biodiversity, 4, 1770–1804. View source →
- Goodwin, R.S., et al. (2008). Urinary elimination of THC-COOH in chronic users, Journal of Analytical Toxicology, 32(8), 562–569. View on PubMed →
- Bergamaschi, M.M., et al. (2013). Impact of prolonged cannabinoid excretion in chronic daily cannabis smokers' blood, Clinical Chemistry, 59(3), 519–526. View on PubMed →
- Substance Abuse and Mental Health Services Administration (SAMHSA) - Federal Workplace Drug Testing Program guidelines. View on SAMHSA.gov →
- U.S. Department of Transportation, 49 CFR §40.85 - cutoff concentrations for urine drug tests. View official regulation →
- Moeller, K.E., Lee, K.C., Kissack, J.C. (2008). Urine drug screening: practical guide for clinicians, Mayo Clinic Proceedings, 83(1), 66–76. View on PubMed →
Limitations
This calculator is an informational tool, not medical or legal advice, and not a substitute for an actual test. It does not estimate detection windows for blood, saliva, or hair samples. Individual results can vary meaningfully from the estimate based on physiology, age, lifestyle, hydration, dietary habits, metabolism, and factors the calculator cannot account for. Documented extreme cases of prolonged retention in long-term heavy users have exceeded even this calculator's upper estimates. If your result matters for an official test, we recommend testing yourself with an at-home kit rather than relying on the estimate alone.
