Oct 8 - OpenAI has told investors its annualized revenue for September was almost $50 billion, a drop from what it signaled earlier, a person familiar with the matter told Reuters on Thursday.

The company previously indicated to investors at a separate event that its revenue run rate for last month was approaching $70 billion, Reuters and other media reported.

The discrepancy mainly arose from an attempt to produce a direct comparison with figures from rival Anthropic, the person said on condition of anonymity. OpenAI did not respond to a request for comment. The latest revenue figure was first reported by the Financial Times.

OpenAI, unlike Anthropic, does not include, for instance, revenue from sales via cloud partners such as Amazon's (AMZN.O) AWS and Alphabet's (GOOGL.O) Google Cloud.

Anthropic pays cloud partners about 16% of every dollar earned through them, which last year accounted for half of revenue, a Reuters analysis showed.

Both OpenAI and Anthropic are preparing to go public, a process through which Wall Street is likely to gain a clearer view of finances, including the sustainability of rapid revenue growth brought about by a boom in demand for AI applications.

OpenAI started this year with $20 billion in annualized revenue versus $6 billion in 2024. Its quarterly revenue was eclipsed by Anthropic for the first time in the second quarter, with OpenAI reporting $6.7 billion compared to Anthropic's $11.5 billion.

Anthropic's annualized revenue crossed $65 billion in July, and is set to reach $100 billion by year-end, sources previously told Reuters.

Analysts consider annualized revenue run rate a sometimes misleading sales metric, which often involves multiplying one month's revenue by 12. It has nonetheless become a popular metric among fast-growing Silicon Valley startups. Adds own sourcing