QLoRA: Fine-Tuning a 7B Model on a 16GB GPU (It Shrank to 5.4GB in Front of Me)

In Part 2 , LoRA let me fine-tune a 1.5B model by freezing it and training tiny adapters. But the frozen base still sat in memory in 16-bit (~3GB). Now I wanted to go to Qwen2.5-7B — and hit a wall that LoRA alone doesn't solve. The problem A 7B model is ~15GB in 16-bit precision. A free-tier T4 GPU has 16GB. It would barely load, with no room left to actually train. The QLoRA insight QLoRA asks the question that naturally follows from LoRA: the base is frozen and only ever read — so why store it in full precision? So you quantize the frozen base to 4-bit (NF4, a format tuned for how neural-net weights are distributed) and run the LoRA adapters on top in normal precision. The base shrinks dramatically; the trainable part stays small and precise. from transformers import BitsAndBytesConfig bnb_config = BitsAndBytesConfig ( load_in_4bit = True , bnb_4bit_quant_type = " nf4 " , # NormalFloat4 bnb_4bit_use_double_quant = True , # quantize the quant constants too bnb_4bit_compute_dtype = torch . float16 , # dequantize to fp16 for the matmuls ) model = AutoModelForCausalLM . from_pretrained ( MODEL_ID , quantization_config = bnb_config , device_map = " auto " ) Each flag earns its place: load_in_4bit — store frozen weights in 4 bits instead of 16. nf4 — a 4-bit type matched to the bell-curve distribution of neural-net weights (better than plain int4). double_quant — quantize the quantization constants too, for a bit more savings. compute_dtype — dequantize to fp16 for the actual matmuls, so storage is 4-bit but compute stays precise. The moment it clicked One line of output: loaded in 4-bit. footprint: 5.44 GB I downloaded 15.2GB of weights and they sat in memory as 5.44GB. A model that couldn't be loaded for full fine-tuning was now training on a single consumer GPU — with room to spare. (The download is still 15GB; bitsandbytes quantizes on the fly during load.) The QLoRA-standard recipe Two more pieces beyond Part 2's LoRA setup: prepare the quantized model for trainin