Computers are cool, and our brain is probably the coolest computer of all. When you’re shopping around for a new computer, one of the things you look for is the storage size of the hard drive—how much data your computer can hold. So it’s fun to pose the same question about ourselves. If our brains were computers, how much data could they store? It’s not an easy question to answer— in part because any comparison between a computer and our brain is far from exact. They don’t really work the same way—but that doesn’t stop scientists from trying.
In computer science, the smallest unit of data is a bit, short for “binary digit.” A bit can have one of two binary states: 0 or 1, similar to on and off. 8 bits make a byte. By combining bits, you can store more information. For example, if you have 2 bits, you can store 4 different states: 00, 01, 10, or 11. But not everyone agrees on what the neurological equivalent of a bit would be. Since a bit is the smallest unit of storage in a computer, one possible approach is to compare a single bit to a single synapse in the brain. That’s the area where two neurons meet and exchange information.
Synapses are the workhorse of memory—the smallest functional unit, much like a bit. And like a bit, they were originally thought to have two states: on and off. There are approximately 250 trillion synapses in our brains, so if one synapse is one bit, that would be about 30 terabytes of data in our brain or 30 trillion bytes.
However, it gets trickier. Signals from neurons at the synapse can vary in size. Instead of just being on or off, a single synapse can carry a signal with varying degrees of strength. These differences in synaptic strength are associated with how strong a memory is. If the synaptic strength is small, you will be less likely to remember something than if the strength is large. For a while, it was believed that a synaptic signal could only be small, medium, or large. With three possible states, this would make a synapse roughly equivalent to 1 to 2 bits—which boosts the overall estimate a little.
It gets even more complex. A study published in 2015 found that instead of a single synapse being able to have only small, medium, and large signals, there were actually 26 possible states. The researchers measured the synaptic strength of two synapses coming from the same neuron. They predicted that each would have the same strength. Instead, there was an 8% difference. Remember—these two synapses originated from the same neuron. When the researchers crunched the numbers, that 8% difference translated into a minimum of 26 different synaptic states. This increase in potential states increases the amount of synaptic strength that a single neuron can exhibit, making a single synapse capable of storing almost 5 bits.
Our brains are thought to have around 86 billion neurons, and a single neuron can actually have thousands of synapses. That works out to roughly 250 trillion synapses in the brain. If we do some back of the envelope math, at 5 bits per synapse, that gets us in the neighborhood of 150 terabytes.
It should be noted that these theoretical estimates are just approximations. We can’t really know for sure what our brain can store—this is more of an educated wild guess. Plus, what our brains do and what computers do are fairly different—so this is more of a thought experiment than actually saying anything about your brain being a hard drive. Even so, if you figure a DVD can hold about 4.7 gigs, that’s in the neighborhood of 30 thousand DVDs. So give your brain some credit. It’s probably pretty spacious.