Pick a big open-source model, and we'll show the memory math and the minimum real hardware you'd need to buy to run it.
A model's size is its parameter count — the "B" in a name like Llama 3.1 405B means 405 billion parameters. The rule of thumb the whole industry uses: each billion parameters needs about 1GB of GPU memory, plus about 20% extra working room for the running conversation itself (this is called the "context" — the text being read and generated). So: minimum GB = parameters (billions) × 1GB × 1.2
One DGX H100 supplies 640GB — well past the 486GB minimum, leaving ~154GB of headroom for longer conversations or future growth.
4 A100 80GB cards, wired together, supply 320GB — past the 282GB minimum, at a lower price and lower power draw than H100-based alternatives.
One DGX H100 (640GB) is not enough on its own. Two DGX H100 systems wired together supply 1,280GB — comfortably past the 805GB minimum.