How long does weed stay in your system?
It depends on the test and on how often you use. THC is typically detectable for 4 to 48 hours in saliva, up to 7 days in blood and from a few days to more than 30 days in urine for daily users. The calculator below estimates your own window for all three from what you took, how often you use and your body — not from an average.
Free THC calculator: your personal estimate
This is the real Clearhead app running in your browser, not a form with a lookup table. Pick THC, enter what you took — joint, bong, vape, dab or edible — plus your usage pattern and body, and get a time-based estimate: how long the effects are likely to last, when blood THC is likely to fall below common cut-offs, and how long saliva and urine tests are likely to stay positive. It also covers five other substance families.
Free · one estimate per browser · nothing you enter leaves your device. The calculator runs in English. The app on your phone has no limit, live multi-dose tracking and history.
THC detection windows by test and frequency
| Test | Single use | Occasional | Daily | Heavy daily |
|---|---|---|---|---|
| Saliva | 4–12 h | 6–24 h | 12–36 h | 18–48 h |
| Blood | 4–12 h | 6–24 h | 1–3 days | up to 7 days |
| Urine | 1–4 days | 3–7 days | 7–15 days | 14–30 days + |
Ranges compiled from published studies at common laboratory cut-offs (urine 50 ng/mL THC-COOH screening; saliva and blood THC). Hair testing can reach back about 90 days; Clearhead does not model hair.
What the calculator takes into account
- How you used it. Smoking and vaping put THC into the blood within minutes and it peaks fast; edibles are absorbed slowly through the gut, peak after 1–3 hours and keep working for 6–8 hours or longer. The two need different kinetics, and the calculator uses both.
- How much. Grams and potency for flower, puffs for a vape, milligrams for an edible. The app converts everything to the THC that actually reached your body, so a 10 mg gummy and a 0.3 g joint of strong flower are compared honestly.
- How often, and for how long. First time, occasional, regular or daily — and, for regular users, years of use. This is the single strongest predictor of the urine window.
- Your body. Weight, height, sex and age give an estimate of body fat, where THC is stored between uses. Higher body fat means a slower release and a longer urine window; the calculator scales for it instead of assuming an average adult.
- The test. Blood, saliva and urine each get their own curve, because each answers a different question.
Which drug test are you facing?
- Saliva — the roadside test in most of Europe and Australia. It reflects recent use in hours, not days; that is what makes it the police’s tool of choice.
- Blood — the confirmation after a roadside screen and the matrix most countries write their legal driving limits in. Closest to actual impairment, though still a weak predictor of it.
- Urine — the standard for workplace, clinical and probation testing. It does not measure THC itself but its metabolite THC-COOH, which is why the window is by far the longest.
- Hair — looks back roughly three months and is not affected by anything you do this week. Clearhead does not model it.
The number one factor: frequency, not dose
Most THC calculators give you a single number. That is misleading, because THC is fat-soluble: it moves into body fat and leaks back into the blood slowly. Two people who smoked the same amount on the same evening do not have the same window — the one who uses every day has a loaded reservoir that takes weeks to drain. Frequency, body fat and history weigh more than the amount from one night. That is also why a first-time user can be urine-negative in two days while a heavy daily user can still test positive after a month of abstinence.
Why most THC calculators get it wrong
Many online calculators are lead magnets for detox products, and the “clean date” they print is designed to sell a kit, not to be right. Others use one half-life for everyone and ignore the test entirely. Clearhead was built the other way round: it starts from a population pharmacokinetic model (Alvarez et al., 2021) and driving-impairment data (McCartney et al., 2021), computes a separate curve for blood, saliva and urine, and adjusts each for your pattern and body. It shows ranges and probabilities, cites its sources and tells you what it cannot know. Read the full methodology.
Detox myths the calculator ignores on purpose
Water, cranberry juice, saunas, niacin and “detox” drinks do not shorten the window. Water dilutes urine, and labs check creatinine and specific gravity to catch exactly that. Exercise mobilises fat and can briefly raise blood THC. A 2026 randomised trial found no effect of exercise on the urine window at all. The only variable that clears THC is time — so the calculator gives you time, not a shopping list.
THC and driving: the honest position
Legal limits differ enormously: some countries apply zero tolerance, some set a blood THC threshold, and blood versus serum values are not directly comparable. Being under a limit is not proof of being fit to drive, and a saliva screen can stay positive long after the effects have faded. Clearhead shows you when blood THC is likely to fall below a cut-off and how long impairment typically lasts — it never tells you it is safe to drive. See the THC driving limits by country.
How Clearhead calculates your window
The estimate does not come from an average but from a three-compartment population pharmacokinetic model, calibrated against published controlled studies and adjusted for your frequency, usual dose, route and body composition. Impairment duration is modelled separately, on a time-based recovery curve rather than a blood number, because blood concentration barely predicts impairment for cannabis. Everything runs on your device: no account, no data collection, nothing sent anywhere.
Sources: Alvarez Y et al., Br J Clin Pharmacol 2021;87(8):3139–3149 · McCartney D et al., Neurosci Biobehav Rev 2021;126:175–193, PMID 33497784 · Huestis MA, Chem Biodivers 2007;4(8):1770–1804 · Karschner EL et al., J Anal Toxicol 2009;33:469 · Vandrey R et al., J Anal Toxicol 2017;41(2):83–99. Reviewed 9 September 2026.
Frequently asked questions
How long does weed stay in your system?
It depends on the test and on how often you use. THC is typically detectable for 4 to 48 hours in saliva, up to 7 days in blood, and from a few days to more than 30 days in urine for daily users. A single use clears far faster than a daily habit.
Is this THC calculator free?
Yes. The calculator on this page is the real Clearhead app running in your browser, with one free estimate per browser. Nothing you enter leaves your device. The app on your phone has no limit and adds live tracking of several doses.
What matters most for the detection window: dose or frequency?
Frequency. THC is fat-soluble and builds up in body fat, so a daily user carries a loaded reservoir that takes weeks to empty. How often you use, your body fat and your history weigh more than the amount from one evening.
Do edibles stay in your system longer than smoking?
The effects last longer — often 6 to 8 hours or more — because THC from edibles is absorbed slowly through the gut and liver. Detection windows are driven mainly by frequency, but a large edible dose can extend them somewhat because more THC ends up stored.
Can water, exercise or a detox drink shorten the window?
No. Drinking water only dilutes urine; labs check for that. Exercise can briefly raise blood THC by releasing it from fat. Controlled studies have found no method that reliably speeds up THC clearance; only time does.
Does the calculator tell me when I can drive?
No. It estimates when THC is likely to fall below common detection cut-offs and how long impairment typically lasts. Legal limits differ by country, some have zero tolerance, and being under a limit is not proof of being fit to drive. The safest choice after using is not to drive.