Physical quantities such as length, mass and time are measured in SI units. SI stands for the International System of Units, and using it means scientists everywhere measure in the same way.
The base units you need are the metre (m) for length, the kilogram (kg) for mass, the second (s) for time, the ampere (A) for current, the kelvin (K) for temperature and the mole (mol) for amount of substance.
Other quantities have derived units with special names: hertz (Hz) for frequency, newton (N) for force, joule (J) for energy, watt (W) for power, pascal (Pa) for pressure, coulomb (C) for charge, volt (V) for potential difference, ohm (Ω) for resistance and tesla (T) for magnetic flux density.
Equations only work if every quantity is in its SI unit. Convert to SI units before calculating: lengths in metres, masses in kilograms and times in seconds.