A standard hair drug test needs about 100 milligrams of hair, which comes out to roughly 90 to 120 strands cut to 1.5 inches long. That small clump is clipped from the crown or back of your head, as close to the scalp as the collector can manage, and sealed in a tamper-evident envelope for the lab.1Labcorp. Hair Follicle Drug Testing: Process & Benefits The spot is small enough to be virtually unnoticeable afterward, but the sample has to meet those weight and length requirements or the lab may reject it.
The Standard Sample Size in Detail
Three numbers define the sample: about 100 milligrams by weight, roughly 90 to 120 individual strands, and 1.5 inches measured from the root end.1Labcorp. Hair Follicle Drug Testing: Process & Benefits2Quest Diagnostics. Hair Drug Testing FAQ A trained collector cuts the hair close to the scalp, labels the sample at the root end so the lab knows which direction represents the most recent growth, and seals it for shipment.
The volume is deliberately modest. You are not losing a visible patch. Collectors typically take small clippings from a few spots at the crown or back of the head and combine them to hit the required weight, which spreads the collection out enough that it does not leave a bare spot.
If Your Head Hair Is Shorter Than 1.5 Inches
A shorter head hair sample can still be tested. What changes is the detection window, not the ability to run the test at all. Head hair grows about half an inch per month, so 1.5 inches captures roughly 90 days of growth.2Quest Diagnostics. Hair Drug Testing FAQ Half an inch of hair covers about 30 days instead. The lab still needs enough total mass to work with, though, so if the hair is extremely short or thin, the lab may flag the specimen as insufficient.
If You Have No Head Hair
When head hair is unavailable, collectors turn to body hair from the chest, arms, legs, or underarms. Body hair is collected by weight rather than length. The trade-off is precision. Body hair grows on different cycles than head hair, so it cannot be segmented month by month, and a body hair sample may reflect drug use over a broader and less defined period, potentially up to about a year.3Drugs.com. How Far Back Does a Hair Follicle Test Detect Drugs Quest Diagnostics notes that growth rates and drug incorporation rates for body hair have not been studied as extensively as for head hair, which makes the detection window less reliable to pin down.2Quest Diagnostics. Hair Drug Testing FAQ
Shaving your head does not get you out of a hair test. Collectors will simply use body hair instead. Being hairless everywhere is the only physical way to make the test impossible to perform, and in an employment context, arriving freshly shaved raises its own red flags. Employers often treat an inability to provide a specimen the same as a refusal, which can carry the same consequences as a positive result.
What That 1.5 Inches Actually Covers
The reason labs ask for 1.5 inches specifically is that head hair grows about half an inch per month, so the standard length maps to roughly the last 90 days.2Quest Diagnostics. Hair Drug Testing FAQ That 90-day window is what distinguishes hair testing from urine or oral fluid testing, which typically cover only a few days to a couple of weeks.1Labcorp. Hair Follicle Drug Testing: Process & Benefits Because the lab can segment the length, it can also estimate roughly when within that window drug use occurred.
There is a blind spot at the recent end. It takes roughly three to seven days after drug use for metabolites to become embedded in the hair shaft and grow above the scalp where they can be collected. Very recent use within the last week often will not show up on a hair test. This is why some employers pair a hair test with a urine test: the urine catches recent use, and the hair catches the longer pattern.
Getting the Sample Right
A few practical points follow from the numbers above. The collector, not the person being tested, cuts the hair, and the sample is taken close to the scalp so the growth closest to the root is captured. Root-end labeling matters because the lab reads the hair like a timeline, with the oldest growth at the tip and the most recent at the root. If a sample arrives without clear orientation, or if it falls short of the weight or length threshold, the lab may return it as insufficient and the collection has to be repeated. For anyone testing for employment, court, or personal reasons, the safest approach is to arrive with normal hair, not freshly cut and not freshly shaved, and let the collector work from there.