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How big are ticks?
Smaller than most people expect. The stage that spreads the most disease is about the size of a poppy seed.
By life stage
| Stage | Size | Compare to |
|---|---|---|
| Larva | ~0.5 mm | A grain of sand (six legs) |
| Nymph | 1–2 mm | A poppy seed |
| Adult | 3–5 mm | A sesame or apple seed |
| Engorged adult | up to 10 mm | A small grape, grey and swollen |
Nymphs are active in late spring and summer, and are small enough to be mistaken for a freckle or a speck of dirt. Most Lyme cases come from a nymph bite.
Why size matters
A swollen tick has been feeding for a while, which raises the risk of transmission. A flat tick has probably attached recently. See how long a tick must be attached.
Why the smallest stage causes the most bites
It seems backwards that the tiniest ticks are the biggest problem, but the size is exactly why. A nymph is small enough to go unnoticed for days, especially in hairy or hard-to-see areas, which gives it far more time to feed undisturbed than an adult usually gets. Adults are large enough that most people spot and remove them sooner, simply because they are easier to see and feel.
Nymphs also tend to be active earlier in the season than adults, and in warmer weather when people wear less clothing and cover less skin. That combination (small, active, and hard to see) is why a poppy-seed-sized nymph is treated as more of a concern than its size alone would suggest.
How size changes with feeding
Size is not fixed once a tick attaches. An unfed tick of any stage is flat and roughly the dimensions in the table above. As it draws blood over several days, the body stretches, and by the time it is fully engorged it can be many times its original size and a noticeably different colour, usually shifting from brown towards a dull grey. This is a gradual change, not a sudden one, so a partly-fed tick can sit anywhere between the “unfed” and “engorged” figures.
Because of this, size alone is a rough guide to feeding time rather than a precise measurement. A tick that already looks swollen has almost certainly been attached longer than one that is still flat, but exactly how long varies by tick, temperature and how much blood it has managed to take.
Judging size without a ruler
Few people carry a ruler when they find a tick, so comparing it to something familiar is usually the fastest way to get a rough idea of the stage:
- Speck of pepper or a freckle: likely a larva or small nymph.
- Poppy seed or sesame seed: a nymph, or a small unfed adult.
- Apple seed: an unfed adult.
- Small grape or raisin, pale grey: an engorged adult that has been feeding for some time.
These comparisons are only a starting point. Species vary somewhat in size at every stage, and a partly fed tick can fall between two of these descriptions.
Size and species
Not every species is the same size at the same stage. Some adult ticks are noticeably larger than others even before feeding, and this is one of several visual cues used alongside shape and colour when trying to work out what species you are looking at; see what does a tick look like for the rest of that picture. Size on its own is rarely enough to identify a species with confidence.
Why small size makes checks worth doing properly
Because the highest-risk stage is also the hardest to see, a careful, methodical check matters more than a quick glance. Running your fingers over the skin to feel for a small bump can catch what your eyes miss, particularly in hair, skin folds and other areas covered in where ticks bite humans. If you have a photo of something you are unsure about, the tick identifier can help narrow down what stage and species it might be, and Live Scan can help highlight a small dark shape against skin in real time using contrast: it flags something worth a closer look, not a confirmed identification.