RTX 3090 vs RTX 4090 for Local AI: Same 24GB, So What's the Difference?

RTX 3090 vs RTX 4090: the specs side by side

What each one fits: the same list

The hardware calculator only checks memory, and both cards have 24 GB, so every verdict is the same on both. In short:

• Comfortable at Q4: everything from Llama 3.1 8B up to Mistral Small 24B and Gemma 3 27B, with a short chat.
• Tight: Qwen and DeepSeek-R1-Distill 32B at Q4 with a short chat, at about 22.9 GB.
• Won't fit: 32B models at Q4 with a 32K context (about 29.9 GB), and any 70B model.

The full table, with Q5 and Q8 columns, is on the RTX 3090 page, and it applies to the 4090 line for line. One practical note for both: Ollama's docs list a 32K default context for cards with 24 to 48 GiB, which pushes 32B models past these cards. The 32K context catch explains how to check and fix it.

Speed and compute

Text generation speed is mostly limited by memory bandwidth, and the gap there is small: the 4090's 1,008 GB/s is about 8% more than the 3090's 936 GB/s. The bigger difference is compute. The 4090 has about 56% more CUDA cores on a newer architecture, which helps most when the card is processing a long prompt, like a pasted document or a large code file, before it starts answering.

So the 4090's advantage grows with long inputs and shrinks with short chats. We haven't benchmarked either card, so this page doesn't put a number on it. If you also game or generate images on the same card, the 4090's compute lead counts for more there.

Two RTX 3090s vs one RTX 4090

Because the 3090 is the older card, two of them is a common alternative to one 4090. Two 3090s give 48 GB, Ollama spreads a model across all available GPUs when it doesn't fit on one, and the 3090 supports NVLink if you want to bridge them. By the calculator's single-pool math, this is what 48 GB adds over a single 24GB card:

That's 12 model and setting combinations that change, mostly the 24B to 32B range at higher quality or longer context, and 70B at Q4 moving from won't-fit to tight. The calculator doesn't model the overhead of splitting a model across two cards, so treat "Tight" on 48 GB as a maybe. The costs are real too: 700 W of graphics cards, a motherboard and case with room for two three-slot cards, and a power supply sized for both.

Power and running cost

The 4090 is rated 100 W higher, and NVIDIA's recommended power supply goes from 750 W to 850 W. The connectors differ as well: the 3090 Founders Edition uses a 12-pin plug with a two 8-pin adapter, and the 4090 uses the 16-pin connector with a three 8-pin adapter. NVIDIA's own investigation of melted 4090 adapters in 2022 pointed to plugs that weren't fully seated, so push that connector all the way in.

The calculator uses each card's rated power, so treat the results as an upper limit. A card generating text often draws less than its rating, and an idle card draws far less.

Which one to buy for local AI

Pick the RTX 3090 if fitting models is the goal, since you get the same 24 GB. It's also the pick if you might add a second card later with NVLink, or your power supply is around 750 W. Its used-buying checklist covers what to inspect.

Pick the RTX 4090 if you often feed models long inputs, you use the card for gaming or image generation too, or you want the faster single card and the price gap is small enough. Its buying checks cover the 16-pin connector and power.

Both are previous-generation cards, and the 4090 reportedly wound down production in late 2024, so you'll mostly be comparing used or renewed listings. Compare real prices, condition and return terms, not launch prices. If neither card's 24 GB is enough, the RTX 5090 is the single-card step up to 32 GB.