RTX 3090 vs RTX 4090
Both hold the same 24 GB, so the split is speed: the RTX 4090 has about 1.1× the memory bandwidth. Decode speed on a memory-bound workload scales with bandwidth, so that ratio is the number that matters once the model fits at all.
Specs side by side
| Spec | RTX 3090 | RTX 4090 |
|---|---|---|
| VRAM | 24 GB | 24 GB |
| Memory bandwidth | 936 GB/s | 1008 GB/s |
| TDP | 350 W | 450 W |
| Models that fit (Q4_K_M) | 417 of 506 | 417 of 506 |
What the extra VRAM actually buys
Nothing, in model-fit terms: every model in our catalog that fits the RTX 4090 at Q4_K_M also fits the RTX 3090. The extra capacity goes to longer context, bigger batches and higher-precision quantization — not to a larger model tier.
Largest model each can run
At Q4_K_M the biggest fit in our catalog is Seed-OSS 36B Instruct (22.1 GB) on the RTX 3090, and Seed-OSS 36B Instruct (22.1 GB) on the RTX 4090. Both figures are weights plus runtime overhead; KV cache for your context is extra.
Power and running cost
At 8 hours a day the RTX 3090 draws about 84 kWh a month against 108 kWh for the RTX 4090 — a difference of 24 kWh. Multiply by your own tariff; we do not assume one. Board TDP is the rated figure, not measured draw.
FAQ
RTX 3090 vs RTX 4090: which should I buy for local AI?
The RTX 4090 fits 417 of the 506 models in our catalog at Q4_K_M against 417 for the RTX 3090, and has about 1.1× the memory bandwidth, which is what sets decode speed once a model fits. If the models you run already fit the RTX 3090, the extra spend buys context length and speed, not a new model tier.
Is the RTX 4090 faster than the RTX 3090?
On memory-bound decoding, yes — roughly 1.1× the throughput, since token generation scales with memory bandwidth (1008 GB/s vs 936 GB/s). This is an upper bound from bandwidth alone, not a measured benchmark.
How much more VRAM does the RTX 4090 have?
Neither has more — both carry 24 GB. No model in our catalog needs the difference at Q4_K_M.
Need a third card in the mix? The interactive compare tool lines up any three. To size KV cache for your context length, use the GPU Memory Calculator.
Related matchups
Other head-to-heads involving these cards:
RTX 4090 vs RTX 5090RTX 3090 vs RTX 5090RTX 4070 Ti SUPER vs RTX 4090RTX 4090 vs RTX A6000RTX 4090 vs H100 80GB
New to sizing? What VRAM is and why bandwidth sets decode speed cover the basics, and quantization explains the formats in the table above. Common mistakes when sizing VRAM lists the traps.