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Can the Apple M3 Ultra (256GB) run gpt-oss 20B?

Apple M3 Ultra (256GB) × gpt-oss 20B8K context
YesRuns fully on the GPU
Memory at FP1642.6 GB of 192 GB
Weights 41.8 GBKV cache 0.2 GBRuntime 0.6 GB
QuantFP16Full quality
Speed~68tokens per second
Max context128Kthe model's full window
  1. FP16
  2. Q8_0
  3. Q6_K
  4. Q5_K_M
  5. Q4_K_M
  6. Q3_K_M
  7. Q2_K

Yes. The Apple M3 Ultra (256GB) can run gpt-oss 20B fully in unified memory at FP16 (full quality), using about 42.6 GB of the 192 GB available, at roughly 68 tokens/second.

Try other context lengths

Run it with Ollama

With Ollama installed, one command downloads and starts gpt-oss 20B:

ollama run gpt-oss:20b

Ollama's default download is Q4_K_M (about 13.5 GB here). You have room for FP16, which is full quality: pick that GGUF in LM Studio or from Hugging Face.

New to this? Step-by-step setup for the Apple M3 Ultra (256GB)

Run it in the cloud

No Apple M3 Ultra (256GB)? Run gpt-oss 20B in the cloud.

Rent a cloud GPU by the hour. No upfront cost, stop any time.

Rent a GPU on RunPodRent a GPU on Vast.ai

Memory needed by quantization

At 8,192 tokens of context. The black line marks the 192 GB the Apple M3 Ultra (256GB) can use. macOS lets the GPU use about 192 GB of the 256 GB unified memory by default.

QuantQualityMemoryFits?Speed
FP16Full quality42.6 GBFits~68 tok/s
Q8_0Near-lossless23 GBFits~128 tok/s
Q6_KExcellent17.9 GBFits~166 tok/s
Q5_K_MVery good15.7 GBFits~192 tok/s
Q4_K_MGood, most popular13.5 GBFits~225 tok/s
Q3_K_MNoticeably degraded11 GBFits~279 tok/s
Q2_KHeavily degraded8.6 GBFits~364 tok/s

Measured speed from real Apple M3 Ultra (256GB) owners

Tokens per second that people measured on their own Apple M3 Ultra (256GB), next to our estimate.

No one has measured gpt-oss 20B on the Apple M3 Ultra (256GB) yet. The speeds above are estimates. Run the benchmark and yours becomes the first real number on this page.

Have an Apple M3 Ultra (256GB)? Add your real speed

One command times gpt-oss 20B in your own Ollama, shows you exactly what it would send, and asks before sending anything. No sign-up, no personal data.

macOS and Linux

curl -fsSL https://gpufit.dev/bench.sh | sh -s -- gpt-oss:20b

Windows (PowerShell)

$env:GPUFIT_MODEL="gpt-oss:20b"; irm https://gpufit.dev/bench.ps1 | iex

How the benchmark works

GPUs that run gpt-oss 20B comfortably

Models that run well on the Apple M3 Ultra (256GB)

Frequently asked questions

Can Apple M3 Ultra (256GB) run gpt-oss 20B?

Yes. The Apple M3 Ultra (256GB) can run gpt-oss 20B fully in unified memory at FP16 (full quality), using about 42.6 GB of the 192 GB available, at roughly 68 tokens/second.

How much VRAM does gpt-oss 20B need?

About 13.5 GB at Q4_K_M with 8,192 tokens of context, and about 42.6 GB at full FP16 precision.

How fast is gpt-oss 20B on the Apple M3 Ultra (256GB)?

Roughly 68 tokens per second at FP16, estimated from the Apple M3 Ultra (256GB)'s 819 GB/s memory bandwidth.