A 1.58-bit Large Language Model (1.58-bit LLM, also ternary LLM) is a version of a transformer large language model with weights using only three values: -1, 0, and +1. This restriction theoretically allows the model to replace costly multiplications with additions and reduce the storage memory. Since the end-task performance and perplexity of the 1.58-bit LLMs, at least for smaller model sizes (up to 3-4B parameters), are close to their "full precision" (16-bit FP16 or BF16) counterparts, this design allows reaching the same artificial intelligence goals with much lower hardware requirements, latency, and training effort.
The name comes from a fact that a single trit, a ternary arithmetic equivalent of a bit that can take the {-1, 0, 1} values, carries formula_1 bits of information. The 1.58-bit LLM models are also called 1-bit LLMs (the true 1-bit models also exist).
BitNet.
In 2024, Ma et al., researchers at Microsoft, declared that their 1.58-bit model, "BitNet b1.58" is comparable in performance to the 16-bit Llama 2 and opens the era of 1-bit LLM. BitNet creators did not use the post-training quantization of weights but instead relied on the new "BitLinear" transform that replaced the "nn.Linear" layer of the traditional transformer design.
In 2025, Microsoft researchers had released an open-weights and open inference code model "BitNet b1.58 2B4T" demonstrating performance competitive with the full precision models at 2B parameters and 4T training tokens.
Critique.
Some researchers point out that the scaling laws of large language models favor the low-bit weights only in case of undertrained models. As the number of training tokens increases, the deficiencies of low-bit quantization surface.