How many cc is 21 hp?

The conversion from horsepower (hp) to cubic centimeters (cc) isn't direct, as it depends on engine efficiency, design, and other factors. However, a rough estimate is that 1 hp is approximately equivalent to 15-20 cc in a small engine. Thus, 21 hp would roughly translate to around 315 to 420 cc, depending on the specific engine characteristics. For precise calculations, more context about the engine type is needed.