the ideal gas law
One tidy equation linking the four things you can measure about a gas: its pressure, its volume, its temperature, and how much gas there is. Everyday intuition: squeeze a balloon and its volume drops while its pressure rises; warm it and it swells; blow more air in and there is more gas.
Precisely, P V = n R T. Here P is pressure (Pa), V is volume (m^3), n is the amount in moles, T is the absolute temperature in kelvin, and R is the universal gas constant, 8.314 J/(mol·K). Equivalently it can be written P V = N k_B T, where N is the number of molecules and k_B is the Boltzmann constant. It merges the laws of Boyle, Charles, Gay-Lussac, and Avogadro into a single statement.
A crucial caveat: T must be in kelvin (absolute temperature), never Celsius, or the proportions come out wrong. The law holds for ideal gases; real gases deviate at high pressure or low temperature.
Pumping air into a tyre keeps the volume nearly fixed, so forcing in more gas (a larger n) raises the pressure. At standard conditions each mole of gas occupies about 22.4 litres.
One equation binding P, V, n, and T.
Always convert the temperature to kelvin before using this law; forgetting to do so is the single most common beginner mistake.