How Many Smells Can the Human Nose Detect? What the Science Really Says

Not 10,000, and maybe not a trillion either. What the research really says about how many smells we can tell apart, and how to notice more of them.

The short answer is that nobody knows for certain, but it is far more than the "10,000 smells" you may have read in a textbook. A widely reported 2014 study estimated that people can tell apart more than a trillion different odour mixtures. Other scientists showed that the trillion figure depends heavily on the maths behind it, and could be much higher or much lower. Both camps agree on one thing: the human nose is a far finer instrument than its reputation suggests.

Where the "10,000 smells" number came from

For most of the twentieth century, the figure quoted in textbooks and popular articles was 10,000. It did not come from testing what people could actually smell. As the olfaction researcher Avery Gilbert explained to Big Think, it goes back to a 1927 classification scheme by two American chemists. They proposed that every odour could be described by four "primary" qualities (fragrant, acid, burnt and caprylic, a goaty note) and rated each one on a nine-step scale. Nine times nine times nine times nine gives 6,561 combinations, which was later rounded up to 10,000.

In other words, the number described a filing system, not the nose. Gilbert's verdict was that it "says more about bean counting than about the biology of olfaction." Yet it was repeated for decades, partly because it fitted an older belief that humans are poor smellers.

The study that said "a trillion"

In 2014, researchers at The Rockefeller University tried to measure the question directly. As the university describes the work, they started with 128 odour molecules (smells like orange, anise and spearmint) and blended them into mixtures of 10, 20 or 30 components. Volunteers were given three vials at a time, two identical and one different, and asked to pick the odd one out.

On average, people could tell two mixtures apart until they shared more than about half of their ingredients. From that threshold, the team calculated how many distinguishable mixtures could exist across the whole set of molecules, and arrived at more than one trillion. Their own view was that the real figure was probably higher still, because the world contains far more than 128 odour molecules.

Why scientists still argue about it

The trillion headline travelled fast, but it was quickly challenged. In 2015, Richard Gerkin and Jason Castro published a reanalysis in the journal eLife with a pointed title: "The number of olfactory stimuli that humans can discriminate is still unknown." They showed that the calculation was extremely sensitive to its assumptions. Small changes in how the threshold was set, or how many people were tested, could swing the estimate by many orders of magnitude.

So the fair summary is this:

The old 10,000 figure is not a measurement and should be retired.
The trillion figure is an estimate built on one experimental design, not a count.
The true number, if one even exists, is still an open question.

Part of the difficulty is philosophical. Colours sit on a few neat dimensions (hue, brightness, saturation), so they can be counted. Smells do not. There are hundreds of receptor types, millions of possible molecules, and endless ways to mix them. Asking "how many smells are there?" is a little like asking how many melodies there are.

How your nose tells smells apart

Smell begins high inside the nose, where a small patch of tissue holds millions of sensory neurons. Each neuron carries one type of odour receptor, and humans have roughly 400 working receptor types. Most smells activate a pattern of many receptors at once, and the brain reads that pattern rather than any single signal. (For the full journey from molecule to memory, see our guide to how your sense of smell works.)

That combinatorial code is why the possibilities are so large. A typical everyday smell, such as coffee or a rose, is made of dozens or even hundreds of molecules, and our primer on fragrance chemistry explains how a handful of them can do most of the work.

There is a twist, though. Being able to tell two smells apart is not the same as being able to pick out what is in them. Research by David Laing and colleagues, summarised in Chimia, found that people can identify only up to about three individual odours in a mixture, even with training. Beyond that, the nose hears the chord, not the separate notes. Researchers at the Weizmann Institute even found an "olfactory white": blends of 30 or more evenly balanced molecules, drawn from across the range of smells, begin to smell almost the same as one another.

Is the human sense of smell actually weak?

No. In a 2017 review in Science, neuroscientist John McGann of Rutgers argued that the idea is a nineteenth-century myth. As ScienceDaily reported, much of it traces back to the anatomist Paul Broca, who noted the relatively small olfactory area of the human brain and tied it to a theory about free will rather than to any test of smelling. McGann points out that the human olfactory bulb has a number of neurons similar to that of other mammals, and that people are more sensitive than rodents and dogs to some odours.

Dogs are still better at many scent tasks. But humans are not the poor relations we were told we were. We have mostly stopped paying attention.

Did you know?

People can track a scent trail like a dog. In a study published in Nature Neuroscience, researchers at the University of California, Berkeley laid a ten-metre string soaked in chocolate essence through a grassy field, then blindfolded volunteers and blocked their ears and hands. Many could follow the trail on hands and knees using smell alone. According to the Berkeley news release, a small group who practised over a few days doubled their speed, and people did better with both nostrils open than with one.

Six ways to notice more of what you smell

The study above has a useful lesson: smelling is partly a skill, and skills improve with practice. These habits cost nothing and make a real difference to how much you notice.

Sniff naturally, don't force it. A normal, gentle sniff is all the nose needs. Take a few easy sniffs, pause, then come back, rather than one long, forced inhale that only tires you.
Use both nostrils, then compare. Each nostril samples air slightly differently. Smelling with one side, then the other, often reveals a note you missed.
Reset between smells. Step outside, smell the inside of your elbow or sip water. A short break clears what scientists call olfactory adaptation, the reason you stop noticing your own home (we explain this in why you can't smell your own home).
Name what you smell, out loud. Saying "toasted, a bit like bread crust" trains the link between nose and language, which is where most people struggle.
Smell ingredients on their own. Before cooking, smell the fresh herbs, the citrus peel and the spices separately. Learning single notes makes it easier to recognise them later inside a blend.
Keep it simple at home. Because we can only pick out about three notes in a mixture, a room scented with one well-chosen fragrance usually smells clearer and more refined than three competing ones in the same space.

Frequently asked questions

How many smells can humans smell?

No one knows exactly. A 2014 study estimated more than a trillion distinguishable odour mixtures, but later analysis showed that figure depends heavily on assumptions, so the true number is still debated.

Is it true that humans can only smell 10,000 scents?

No. The 10,000 figure came from a 1927 classification scheme, not from testing people, and modern research suggests the real range is vastly larger.

How many odour receptors do humans have?

Humans have roughly 400 working types of odour receptor. Each smell triggers a pattern across many of them, which the brain reads like a code.

Do dogs smell better than humans?

For many tasks, yes, but the gap is smaller than people assume. Humans are more sensitive to some odours than dogs and can even follow a scent trail with practice.

Can you improve your sense of smell?

Practice helps you notice and name more. In one study, people who trained at scent-tracking for a few days doubled their speed.

If you'd like a simple way to practise at home, our Thé Blanc Reed Diffuser is the one we reach for: a clear, quiet scent that is easy to learn note by note.

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