The question arrives with urgency. A job offer contingent on a drug screen. A custody hearing. A probation check. A DOT-regulated position. For millions of Americans each year, the question “how long does THC stay in my system?” is not academic — it is the difference between employment and unemployment, custody and separation, freedom and incarceration.
The answer is more complex than most sources acknowledge, because it depends on a web of biological variables that no simple chart can fully capture. But the pharmacology is well-understood, and the data is clearer than most internet sources suggest.
Why THC Detection Is Complicated
When you consume cannabis, your liver converts delta-9-THC — the psychoactive molecule — into a metabolite called 11-hydroxy-THC, which is then converted into THC-COOH (11-nor-9-carboxy-THC). It is THC-COOH that most drug tests detect, not THC itself.
This matters enormously because THC-COOH is fat-soluble. It dissolves into adipose tissue and is released slowly over days or weeks. The dose you consume is one of the biggest variables in how long metabolites persist — our THC dosing guide explains what each milligram level delivers. This means that unlike alcohol, which is water-soluble and clears the body within hours, cannabis metabolites accumulate with repeated use and release gradually. A heavy daily user carries a significantly larger reservoir of stored THC-COOH than an occasional user, and this reservoir takes much longer to deplete.
Detection Windows by Test Type
Urine testing is the most common workplace drug test. Standard immunoassay screens use a cutoff of 50 ng/mL, with confirmation testing at 15 ng/mL via mass spectrometry. Detection windows depend heavily on usage patterns:
Single use (one session after a long abstinence period): 3 to 5 days. Moderate use (3 to 4 times per week): 7 to 14 days. Daily use: 15 to 30 days. Heavy daily use (multiple sessions per day for months): 30 to 90+ days. The longest documented detection window in peer-reviewed literature is 90 days for a very heavy user with high body fat percentage.
Blood testing detects active THC rather than metabolites, making it a better indicator of recent use but a poor indicator of past use. THC is detectable in blood within seconds of inhalation and typically falls below detectable levels within 24 to 48 hours for occasional users. Heavy users may show detectable blood THC for up to 7 days due to slow release from fat tissue.
Saliva testing is increasingly used in roadside impairment checks. Detection windows are short: 12 to 72 hours for most users. The cutoff varies by jurisdiction — some use 5 ng/mL, others use 25 ng/mL, which dramatically affects the detection window.
Hair testing has the longest detection window: up to 90 days as a standard, with some laboratories testing longer segments. Hair tests detect THC-COOH deposited in the hair follicle via the bloodstream as hair grows. The standard test analyzes the 1.5 inches of hair closest to the scalp, representing approximately 90 days of growth. Hair tests are the most difficult to beat and are most commonly used in federal employment and custody cases.
The Variables That Matter Most
Usage frequency is the dominant variable. A single session produces detectable metabolites for days. Daily use for months produces detectable metabolites for weeks to months. This is not linear — the relationship is exponential because fat tissue accumulation compounds over time. This accumulation is directly tied to CB1 receptor downregulation and tolerance development.
Body fat percentage matters because THC-COOH is stored in fat. Higher body fat means a larger reservoir and longer detection windows. Two people with identical consumption patterns but different body compositions will have meaningfully different clearance times.
Metabolism and exercise play secondary roles. Higher metabolic rates clear THC-COOH faster. Exercise can temporarily increase THC-COOH release from fat into blood and urine — which is why some experts recommend avoiding intense exercise in the days immediately before a drug test, as it can temporarily spike metabolite levels.
Hydration affects concentration but not total metabolite presence. Drinking large amounts of water dilutes urine, potentially pushing THC-COOH below the immunoassay cutoff — but laboratories check for dilution using creatinine levels and specific gravity, and excessively dilute samples are flagged as invalid.
What the Science Cannot Tell You
No calculator, chart, or formula can predict with certainty whether you will pass a specific drug test. The variables interact in ways that are unique to each individual, and laboratory cutoffs, testing methodologies, and interpretation standards vary.
What the science can tell you is the range of likely detection windows based on your usage pattern and test type. Use the interactive detection calculator below to estimate your personal detection window based on your consumption frequency, body type, test type, and time since last use.
The most reliable strategy remains the simplest: abstinence for a period exceeding the maximum detection window for your test type and usage pattern. Everything else — detox drinks, synthetic urine, excessive water consumption — carries risk, and the consequences of a failed test often outweigh the inconvenience of waiting. For practical strategies, see our guide on how to pass a drug test.