Knots are a speed measurement that is nautical miles per hour. This unit is typically used in maritime and aviation. There have historically been different nautical miles used and, thus, different variations of knots. However, we have based our knots calculator on the international nautical mile which is commonly used thoughout the world today.
Mach is a speed measurement based on the speed of sound. Since sound travels at different speeds in different conditions, the calculations here are given for 20°C in dry air at sea level. Mach is typically used in aviation and space exploration.