Llama 3.1 405B Instruct
Llama 3.1 405B Instruct loads in 243 GB at Q4_K_M, comfortably from 256 GB of memory. Below is the memory math we ran for it, the cheapest card that fits, and the fastest machine we track.
How much memory does Llama 3.1 405B Instruct need?
Llama 3.1 405B Instruct is a 405B dense model built for quality, reasoning, coding. The weights alone take 243 GB at Q4_K_M; add context and the OS, and it belongs in the 256 GB memory class. Every number below comes from the ModelFit engine — speeds are estimates, labeled est., because we do not benchmark hardware ourselves.
KV = fp16 estimate (q8_0 cache roughly halves it). "Comfortable" = weights + KV within the engine's tiered budget (~70-85% of memory).
Which machines run Llama 3.1 405B Instruct?
Verdicts come from the same engine that powers our fit checker — memory capacity first, bandwidth for speed. How we compute these numbers · Hugging Face model card
More Llama models
Frequently asked questions
How much memory does Llama 3.1 405B Instruct need?
243 GB for the Q4_K_M weights, plus ~6.0 GB of KV-cache at 16k context — about 249.0 GB total. Comfortable from 256 GB of VRAM or unified memory.
Does Llama 3.1 405B Instruct run on a Mac?
Not on the current Mac configs we track; Llama 3.1 405B Instruct needs more memory than they offer.
What is the cheapest GPU for Llama 3.1 405B Instruct?
No tracked consumer GPU runs Llama 3.1 405B Instruct comfortably.
Cite this page
ModelFit: Llama 3.1 405B Instruct — specs, memory math and hardware verdicts. https://modelfit.io/models/llama-3.1-405b/ (dataset updated 2026-09-03, CC BY 4.0).