text model · gpt-oss · iOS

Can I run gpt-oss 20B on iPhone 15 Pro?

Compatibility verdict VRAM threshold engine

No, not enough memory would not load

No. gpt-oss 20B needs ~13.2 GB even at Q4_K_M, but iPhone 15 Pro only has ~4.5 GB usable.

Needs ~13.2 GB Device usable ~4.5 GB

Needs ~13.2 GB even at Q4_K_M, but only ~4.5 GB is usable.

That figure is at a 4k context and moves about ±15% as context length changes.

The gap is about 8.7 GB: gpt-oss 20B needs roughly 13.2 GB at Q4_K_M and iPhone 15 Pro leaves only about 4.5 GB usable for a model. The lightest tracked hardware that runs gpt-oss 20B is the Nvidia GeForce RTX 4060 Ti (16GB) at 16 GB. See gpt-oss 20B on Nvidia GeForce RTX 4060 Ti (16GB).

Q4_K_M needed: ~13.2 GB
Usable on device: ~4.5 GB
Device memory: 8 GB

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Which quant fits

Quant ladder vs ~4.5 GB usable

Q2_K

~10.7 GB

Q3_K_M

~12.2 GB

Q4_K_M

~13.2 GB

Q5_K_M

~16.9 GB

Q6_K

~19.1 GB

Q8_0

~2.8 GB

FP16

~43.9 GB

The line marks iPhone 15 Pro's ~4.5 GB budget; rungs past it are too large.

How to run it

On iOS use Apple Foundation Models (Built into iOS 26, ~3B on-device model, zero download, fully private.).

Model gpt-oss

Parameters: 21B (MoE, 3.6B active)
Q4_K_M size: 11.28 GB
Q8_0 size: 0.86 GB
Context: 128k
Ollama tag: gpt-oss:20b

Full gpt-oss 20B requirements →

Device iOS

Memory: 8 GB unified
Usable for weights: ~4.5 GB
Power draw: ~14 W
Best runtime: llama.cpp + Metal (via PocketPal or Off Grid app)

Best models for iPhone 15 Pro →

What you can run instead

Run gpt-oss 20B on other hardware

Nvidia GeForce RTX 4060 Ti (16GB)Nvidia GeForce RTX 4080 (16GB)Apple M4 (24GB)Apple M4 Pro (24GB)Nvidia GeForce RTX 4090 (24GB)Nvidia GeForce RTX 3090 (24GB)

FAQ

Can iPhone 15 Pro run gpt-oss 20B?

No. gpt-oss 20B needs ~13.2 GB even at Q4_K_M, but iPhone 15 Pro only has ~4.5 GB usable.

How much memory does gpt-oss 20B need?

iPhone 15 Pro does not have enough memory. At Q4_K_M the weights are ~11.28 GB; with KV cache and runtime overhead, budget ~13.2 GB at a 4k context. It is a Mixture-of-Experts model (21B total / 3.6B active), so all experts must stay in memory; memory tracks total params, not active params.

What is the best tool to run gpt-oss 20B on iOS?

On iPhone and iPad, Apple Foundation Models (Built into iOS 26, ~3B on-device model, zero download, fully private.) is the standard choice. Phones realistically run 1B-4B class models. Anything larger thermally throttles or OOMs.

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Markdown

[![gpt-oss 20B on iPhone 15 Pro](https://localmodel.run/badge/gpt-oss-20b/iphone-15-pro.svg)](https://localmodel.run/can-i-run/gpt-oss-20b/iphone-15-pro)

Sources

Weights are measured from GGUF files; KV cache and overhead are computed, so totals can vary ~15% with context and runtime. Any tok/s is a bandwidth estimate. See methodology.