What calculation formed the basis for Watt's standardized unit of horsepower?

Answer

A horse could perform about 33,000 foot-pounds of work per minute

Watt established horsepower as a measurable metric by calculating that a horse could accomplish approximately 33,000 foot-pounds of work per minute, turning mechanical energy into a quantifiable commodity for commerce.

What calculation formed the basis for Watt's standardized unit of horsepower?
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