A few drops, but there is no audited measurement. In a June 2025 blog post, Sam Altman, OpenAI's chief executive, cited ~0.3 mL for an average query (one fifteenth of a teaspoon) without publishing a method. The figure is consistent with the only measurement made in production, Google's for Gemini: 0.26 mL for a median query, which counts only the data centre's cooling and leaves out the water tied to electricity.
Read the source article: How much water does ChatGPT use? A few drops per query