Today’s computers depend on billions of tiny switches known as CMOS transistors (CMOS logic) to process information using Boolean logic. Each transistor, smaller than a virus, flips billions of times a second without error—a feat that would amaze the early inventors of the transistor, whose first devices were famously unreliable.
But just how reliable must these transistors be? Our intuitions can fail us badly here — do they have an error rate of one part in a billion? Maybe even one part in a trillion? Amazingly, such an ostensibly-reliable CPU, whose transistors have only a one-in-a-trillion chance of malfunctioning, wouldn’t even get through the bootup process. It would fail in milliseconds. You might be lucky enough to see a few characters on the screen before it halted.
So, just how reliable do the transistors in modern CPUs need to be? Let’s estimate.
Imagine a university computer lab with 100 computers (or, 100 students in a lecture using their smartphones, which are roughly as complex). Each device has a CPU with about one billion transistors (this is conservative for 2025), and we’ll assume that about 10% switch states every clock cycle (sounds reasonable to conservative, since transitions happening where there shouldn’t be any is a problem, too) at 4 GHz (typical for modern CPUs). That’s around 100 million transistors switching 4 billion times per second, for 8 hours every school day. And yet, crashes due to transistor errors are almost unheard of. (That’s what software is for.)
To quantify this, if we assume there’s a 10% chance a single transistor error causes a computer crash, for the lab to have only a 50% chance of experiencing at least one crash in an 8-hour day, each transistor switching event must be about 99.9999999999999999999994% reliable —that’s less than one error in every 10²³ switches!
…And one computer failing each day would be a big reliability problem for the lab. You shouldn’t have anywhere near that many hardware problems. Not in 2025. Maybe a few per semester or something if your PCs are old. And most of those will be related to bad power supplies, failed motherboard capacitors, or just good old PEBKAC.
In practical terms, this reliability means each transistor would likely switch continuously at 4 GHz for millions of years without making a single mistake. Engineers achieve this remarkable reliability through precise manufacturing, rigorous testing, and built-in error correction mechanisms. (The very nature of digital electronics corrects for noise.)
Real-world studies from companies like Intel and research institutions like CERN confirm this high reliability, noting that rare errors, often caused by cosmic radiation, are usually managed through error-correcting systems.
Next time your computer runs flawlessly for days or months on end, consider the silent marvel happening billions of times every second—tiny transistors working almost perfectly, making modern digital life possible.
This article was coauthored by ChatGPT 4.5 in Deep Research mode, and then abridged and modified upon request and edited and expanded by the author.
