For decades, if you cracked open a biology textbook, you’d see the same number staring back at you. 100 billion. It was the gold standard, the "settled" fact about the number of neurons in the brain. It sounded perfect. Clean. Massive.
But it was basically an educated guess.
Scientists just sort of repeated it until everyone believed it. Honestly, for a long time, nobody actually sat down to count them because, well, how do you count something that small and that packed together? You can't just poke a brain with a pair of tweezers and keep a tally. It wasn't until Dr. Suzana Herculano-Houzel came along with what is essentially a "brain soup" method that we got the real answer.
The actual number? It's closer to 86 billion.
Where did the 100 billion myth come from anyway?
It’s kind of funny how science works sometimes. We like round numbers. 100 billion feels right. It’s like saying there are as many stars in the galaxy as there are neurons in your head—which, by the way, is also a common comparison that isn't quite right depending on which astronomer you ask.
The 100 billion figure was a placeholder. It was an estimate based on scaling up smaller mammals. But humans aren't just "big rats." Our brain structure is uniquely dense. When Herculano-Houzel developed the isotropic fractionator method, she turned brain tissue into a liquid soup. By counting the nuclei in a homogenized sample, she could calculate the total with incredible accuracy.
Losing 14 billion neurons might not sound like a big deal until you realize that 14 billion is roughly the entire brain of a baboon. So, we basically "lost" a whole monkey's worth of gray matter once we actually did the math.
It's not just about the neurons
We talk about neurons like they’re the only thing that matters. They’re the "stars" of the show, the ones sending the electrical zaps that let you think, move, and obsess over why you said that awkward thing in 2012. But they aren't alone in there.
Non-neuronal cells, called glia, are just as important. For a long time, people thought glia outnumbered neurons 10-to-1. You've probably heard that "we only use 10% of our brain" myth? Yeah, that’s partly where it came from. People assumed 90% of the brain was just "glue" (which is what glia means in Greek) and therefore useless.
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Wrong.
The ratio is actually closer to 1-to-1. In the human cerebral cortex, there are about 16 billion neurons and about 60 billion non-neuronal cells. In the cerebellum, it’s the opposite—it's packed with tiny neurons.
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If you want to talk about density, you have to look at the back of the head. The cerebellum is small. It’s only about 10% of the brain’s mass. But here’s the kicker: it holds about 80% of your total neurons.
- Cerebral Cortex: ~16 billion neurons
- Cerebellum: ~69 billion neurons
- Rest of the brain: ~1 billion neurons
Why does a part of the brain mainly responsible for motor control and "fine-tuning" need so many more cells than the part that handles language, logic, and consciousness? It’s one of the great mysteries of neuroscience. It suggests that moving your body smoothly is actually a way more computationally expensive task than thinking about philosophy.
Does a higher number of neurons in the brain mean you're smarter?
Not necessarily. If total numbers were everything, elephants would be the undisputed geniuses of Earth.
An African elephant’s brain has about 257 billion neurons. That’s three times as many as you. However, 98% of those neurons are located in their cerebellum to manage that massive trunk and body. When you look at the cerebral cortex—the area associated with "higher intelligence"—humans have about 16 billion, while elephants have about 5.6 billion.
That’s the "human advantage." We have the most neurons packed into the cortex of any species on the planet. It’s not about the total size; it’s about where the hardware is allocated.
The energy cost of being a human
Your brain is an energy hog.
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Even though it’s only 2% of your body weight, it consumes 20% of your daily calories. Every one of those 86 billion neurons needs power. This is actually why other primates didn't get as "smart" as us. A gorilla, for example, would have to spend over 8 hours a day eating raw plants just to support the caloric needs of a brain with our neuron count. They literally don't have enough hours in the day to eat that much.
We cheated the system. We discovered cooking.
By cooking our food, we made calories much easier to digest and absorb. This "predigestion" outside the body allowed our ancestors to get the massive amount of energy needed to fuel an 86-billion-neuron brain without spending all day chewing.
Neuroplasticity: The number isn't fixed for life
You aren't born with 86 billion and then it's just a slow slide to zero.
While it's true we lose neurons as we age (and especially if we drink too much or don't sleep), the brain is also capable of neurogenesis. Specifically in the hippocampus—the part of the brain involved in memory—we continue to produce new neurons throughout our lives.
But more importantly than the number of cells is the connections between them.
Each neuron can have thousands of synapses. You have roughly 100 trillion connections in your head. Learning a new skill doesn't usually mean you grew a bunch of new cells; it means you rewired the 86 billion you already have. You're building infrastructure.
Common misconceptions that just won't die
- The 10% Myth: No, you don't use only 10% of your neurons. You use all of them. Even when you're sleeping, your brain is humming along at high capacity. If you only used 10%, brain damage to the "unused" 90% wouldn't matter. But we know even tiny amounts of damage can be catastrophic.
- Left Brain vs. Right Brain: You aren't "a math person" because your left brain has more neurons. The two halves are constantly talking to each other across a bridge called the corpus callosum.
- Brain Size = IQ: A bigger brain doesn't make you smarter. Einstein’s brain was actually slightly smaller than average. It's the density and the connections that count.
What this means for your daily life
Understanding the sheer scale of the 86 billion neurons in your brain should change how you treat your head. You are carrying the most complex object in the known universe.
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Because the brain is so metabolically expensive, it is very sensitive to what you put into your body. High-quality fats, consistent glucose levels, and oxygen are non-negotiable. When you feel "brain fog," it’s often because your neurons are struggling to maintain the electrical gradient they need to fire.
How to protect your 86 billion neurons
- Prioritize REM sleep: This is when your brain flushes out metabolic waste (via the glymphatic system). Without it, your neurons are essentially sitting in their own trash.
- Challenge the connections: Use it or lose it. Learning a new language or instrument forces those 86 billion cells to create new pathways, which builds "cognitive reserve."
- Watch the inflammation: Chronic inflammation from poor diet or stress can actually damage the glia, the support cells that keep your neurons healthy.
Moving forward: The future of counting
We are getting better at mapping this stuff. Projects like the Human Connectome Project are trying to map not just the cells, but every single "wire" between them.
It’s an enormous task. If we tried to map every connection in your brain at the same resolution we use for a computer chip, the data would take up more storage than the entire current internet.
We are still learning. Some recent studies suggest the 86 billion number might vary by several billion depending on the individual's history and health. We aren't static machines. We are biological systems that are constantly pruning and growing.
Your next steps for brain health:
Start by focusing on Sleep Hygiene. Since your neurons cannot physically "clean" themselves while you are awake, getting 7–9 hours is the single most effective way to maintain your 86 billion neurons. Second, incorporate Omega-3 fatty acids (found in fish or algae oil), which make up a significant portion of the neuronal membranes. Finally, engage in Cross-training your brain—if you are a numbers person, read poetry; if you are an artist, try basic coding. This forced novelty is what keeps the 86 billion neurons from becoming "static."
The 86 billion neurons you have right now are the only ones you're getting. Treat them like the finite, high-energy, world-shaping processors they are.
References:
- Herculano-Houzel, S. (2009). The human brain in numbers: a linearly scaled-up primate brain. Frontiers in Human Neuroscience.
- Azevedo, F. A., et al. (2009). Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. Journal of Comparative Neurology.
- The Human Connectome Project (NIH).