RTX 3090 vs RTX 4090 for Local LLMs in 2026

This is now a used-versus-legacy comparison. The RTX 3090 is relevant as a 24GB used-value card. The RTX 4090 is a faster legacy 24GB card. Neither is the default current new purchase.

Capacity is tied

Both cards expose 24GB of VRAM, which fits 32B models at Q4_K_M with about an 8K context. A 70B model at Q4_K_M needs about 43GB for its weights, so neither card holds it. The 4090’s additional compute and memory bandwidth can improve throughput for workloads that fit, but it does not make a model that needs materially more than 24GB fit.

The bandwidth gap is small: 936 GB/s for the 3090 versus 1,008 GB/s for the 4090. Token generation on models that fit is largely limited by memory bandwidth, so the 4090’s much larger compute advantage shows up mainly in compute-heavy work such as prompt processing. NVIDIA recommends a 750W system power supply for the 3090 and 850W for the 4090.

That makes the purchase decision different from a gaming comparison. Buy from the model, quantization, context, and concurrency requirement first.

The 3090 is a risk-adjusted used purchase

Inspect physical condition, connector history, fan behavior, memory thermals, and seller return terms. Add a capable host, PSU, and case to the price. The card’s used-market value disappears if the rest of the system must be replaced.

The 4090 is not the current flagship

Remaining new stock can carry scarcity pricing unrelated to current value. Compare it with the RTX 5090 and current lower-power 16GB cards. If 24GB is the requirement, the used 3090 may be the rational compromise; if more than 24GB is required, neither card solves it.

Bottom line

Choose the used RTX 3090 when 24GB CUDA capacity is worth used-card and power risk. Choose the RTX 4090 only when its higher throughput inside the same memory ceiling has a specific, measured value and the price survives comparison with current hardware.