How Long Does Weed Stay in Your Hair?

Signs of weed use may be detectable in scalp hair for roughly 7–90 days after use, depending on hair length, growth rate, frequency of use, and the laboratory’s method. The THC detox calculator can provide a general estimate, but it cannot predict a hair test result.

A standard hair test usually examines a segment close to the scalp that represents about 90 days of growth. For a broader overview of testing, see how long weed stays in your system.

How long can a hair test detect weed?

The table describes the approximate period a sample may cover, not a guarantee that weed use will be detected. A single use may not produce enough detectable material for a positive result, while repeated use generally has a greater chance of detection.

Hair sample and use pattern	Approximate lookback or limitation
Standard scalp sample; single or occasional use	About 90 days of hair growth may be sampled, but a single use may not be detected.
Standard scalp sample; frequent use	About 90 days of hair growth may be sampled; detection is generally more likely than after a single use.
Longer scalp sample; any frequency	The lookback can be longer than 90 days if the laboratory tests a longer segment.
Body-hair sample; any frequency	The lookback is less predictable because body hair grows differently from scalp hair.

A standard scalp sample is often approximately 1.5 inches (3.8 cm) long, measured from near the scalp. Hair grows at different rates, so that length does not represent exactly the same period for everyone. A laboratory may use a different sample length or testing method.

Very recent use may not appear immediately: hair containing detectable material needs time to grow above the scalp. The 7–90-day estimate is therefore a rough guide, not a precise start or end date for detection.

What affects how long weed is detectable in hair?

Sample length and growth rate affect how much history a scalp sample covers. Longer hair can contain an older record, but only the portion the laboratory actually tests matters. Cutting the ends of longer hair does not necessarily change the segment collected near the scalp.

Frequency and timing of use also matter. Repeated use can leave more detectable material than a single use, but neither pattern guarantees a particular result. A hair test cannot reliably establish the exact day of use or whether someone is impaired when the sample is collected.

The laboratory’s methods affect what a test can detect. Laboratories may use different screening thresholds and confirmation methods. Hair treatments and external exposure can also affect results, and laboratories may wash samples and use confirmatory testing to help interpret them.

There is no way to turn these factors into a reliable personal detection date from the information here. An estimate cannot substitute for the collection and testing procedures used by a particular laboratory.

What is the half-life of weed in hair?

There is no single, reliable half-life that tells you when weed will disappear from hair. Detectable substances can remain in the hair shaft as it grows, until that section is cut off, shed, or otherwise no longer included in the tested sample.

A THC half-life measured elsewhere in the body does not predict when a hair test will become negative. Hair testing depends on the sample segment and laboratory method, not just on how quickly the body processes THC.

How can you estimate a hair test’s lookback window?

For a rough scalp-hair estimate, use an average growth rate of about 0.5 inches (1.3 cm) per month:

Find the length of the scalp-hair segment the laboratory plans to test.
Divide that length by approximately 0.5 inches (1.3 cm) per month.
Treat the result as an approximate lookback, not a prediction of a positive or negative result.

For example, 1.5 inches (3.8 cm) divided by about 0.5 inches (1.3 cm) per month gives approximately 3 months, or 90 days. This calculation does not account for individual growth differences, recent use that has not emerged above the scalp, or the laboratory’s detection threshold. It should not be used to date a body-hair sample.

Who should be careful, and when should you get help?

If you are pregnant or breastfeeding, discuss weed use with a healthcare professional. Tell your clinician or pharmacist about weed use if you take medicines that may interact with it, including a blood thinner such as warfarin; do not stop prescribed medication without medical advice.

If a child may have consumed weed, contact a poison control center promptly. Seek emergency help for trouble breathing, a seizure, loss of consciousness, severe confusion, or chest pain.