Can I Run GPT-OSS 20B on Apple M1 Ultra?
Written by Jakub Rusinowski · Last updated August 15, 2026
Yes. GPT-OSS 20B needs 12.3 GB at 8K context (11.1 GB weights + 1.2 GB KV cache/overhead) at MXFP4 and the Apple M1 Ultra has 96 GB usable, leaving about 83.7 GB of headroom, at roughly 128 tok/s (estimated).
See what else this hardware can run →
The numbers
| Model | GPT-OSS 20B |
| Parameters | 20B |
| Quantization | MXFP4 |
| VRAM needed (8K context) | 12.3 GB |
| Apple M1 Ultra usable memory | 96 GB |
| Verdict | Runs |
| Estimated speed | ~128 tok/s |
GPT-OSS 20B on the Apple M1 Ultra: VRAM by quantization
| Quant | VRAM needed | Fits 128 GB? | Max context | Whole PC |
|---|---|---|---|---|
| F16 | 43 GB | ✓ Yes | 128K | — |
| Q8_0 | 23.4 GB | ✓ Yes | 128K | — |
| Q6_K | 18.3 GB | ✓ Yes | 128K | — |
| Q5_K_M | 16 GB | ✓ Yes | 128K | — |
| Q4_K_M | 13.8 GB | ✓ Yes | 128K | — |
| Q3_K_M | 10.1 GB | ✓ Yes | 128K | — |
| Q2_K | 8.1 GB | ✓ Yes | 128K | — |
Assumes an 8K-token context with an f16 KV cache. A longer window needs more; a quantized KV cache needs less. “Max context” is the largest window that still fits in 128 GB of unified memory. Figures are estimates from parameter count, quantization and memory bandwidth — the analyzer lets you tune KV-cache quant and context.
Context costs VRAM too. At MXFP4 on the Apple M1 Ultra: 8K 12.3 GB · 32K 13.5 GB · 128K 18.3 GB. Every window this model publishes still fits 128 GB.
What to do next
Nothing to change — run it at MXFP4. If you want more capability, check whether the next size up also fits.
No compatible GPU? GPT-OSS 20B on 16 GB of system RAM, CPU only: runs.
Running GPT-OSS 20B on this card:
GPT-OSS 20B needs ~12 GB but this GPU has 128 GB. Rent a RTX 4090 (24 GB)-class GPU by the hour instead of buying one:
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Cloud rates verified 2026-07 — estimates, and marketplace prices vary. Buying price is GPU MSRP only, not a full PC.
Other GPT-OSS sizes on the Apple M1 Ultra
- GPT-OSS 120B — runs (71.9 GB)
GPT-OSS 20B on nearby hardware
- NVIDIA RTX PRO 6000 Blackwell (96 GB) — runs
All GPT-OSS sizes on the Apple M1 Ultra | GPT-OSS 20B model page | Apple M1 Ultra GPU page | VRAM calculator | Check your hardware