# Tokenizer Evaluation: Fertility, Compression Ratio, Downstream Impact, and Information-Theoretic Metrics

> Source: https://sukruyusufkaya.com/en/learn/llm-muhendisligi/tokenizer-evaluation-fertility-compression-downstream
> Updated: 2026-05-13T13:00:26.742Z
> Category: LLM Mühendisliği
> Module: Module 6: Tokenization Microsurgery
**TLDR:** Deep anatomy of all metrics that measure tokenizer quality: fertility (tokens/word), compression ratio (bytes/token), OOV rate, bits-per-character (BPC), impact on perplexity, cross-lingual fertility, downstream task impact, vocab coverage, A/B testing protocols, Turkish-specific metrics, cost 'tax' analysis, capstone evaluation framework.