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RTX 3090 vs RTX 4090 for Local LLMs: Real Numbers
Both cards hold 24GB — the same models fit both (3090 picks, 4090 picks). The difference is speed and price.
Head to head
| RTX 3090 | RTX 4090 | |
|---|---|---|
| VRAM | 24GB GDDR6X | 24GB GDDR6X |
| Memory bandwidth | 936 GB/s | 1008 GB/s |
| Decode speed (LLM) | ~8% apart — decode is bandwidth-bound | |
| Prompt processing | slower (FP16 TFLOPs ~36 vs ~83) | ~2× faster |
| Power | 350W | 450W |
| Price (used market) | ~$600-800 | ~$1500-1800 |
The verdict
For pure token generation the 3090 is the value king: within ~8% of a 4090 at half the used price. The 4090 wins on prompt processing — long system prompts, RAG, big documents feel 2× snappier — and on efficiency (same work, less wall power).
Two 3090s (~$1400, 48GB pooled) beat one 4090 for models that need 25-48GB, if your board has two x8+ slots and you accept NVLink-less splitting.
What fits 24GB, measured
See the live ranked list: best LLMs for 24GB VRAM — updated daily from measured GGUF file sizes.
Doesn't fit your machine? Rent a GPU by the hour instead — a 24GB RTX 4090 starts around $0.30-0.50/h (Vast.ai, RunPod), or use the model via a hosted API. Buying instead? Which GPU should I buy? →
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