We are massively far away from modeling the human brain. First of all, no one can agree what level is necessary to model the brain, and that varies tremendously by scientific question. Personally, my lower limit would be something like the computational package Neuron which models voltages across axon compartments and distribution of ion channels, My upper limit confidence bound is we don’t care about anything subatomic.

At the upper bound: In molecular dynamics, which is used extensively in modern day neuroscience to understand the function of ion channels and GPCRs, a single H100 can model 70ns/day of compute for 1M atoms. There are 8.64e+13 nanoseconds per day. There are ~10^26 atoms in a human brain. Therefore, an upper limit back of envelope is you need fewer than 10e+26 atoms / 10e+9 atoms * 8.64e+13 ns / 70 ns = 1.23e+29 H100 GPUs.

Calculating the lower bound is more difficult, but let’s start by saying you can get away with a fp16 for each synapse. Storing the weights of that model for 100 trillion synapses is 200 Terabytes, and if you figure weight size * 4 or so to do anything useful then this is in spitting distance. Note that this example lower bound is massively less complex than the Neuron model I suggested, as the entire field of neuromodulators, homeostatic mechanisms, glia, and more are thrown out, which are all important for modeling how the brain works under certain computational regimes.