Boyle, Charles and Gay-Lussac in one law
How three simple gas laws fold into P1V1/T1 = P2V2/T2, why the Kelvin scale is required, and when the relation stops working.
The three simple gas laws
Each classic gas law fixes one variable and links the other two. Boyle's law says that at constant temperature pressure and volume trade off inversely, so squeezing a gas into half the space doubles its pressure. Charles's law says that at constant pressure volume rises in step with absolute temperature, which is why a balloon swells when warmed. Gay-Lussac's law says that at constant volume pressure climbs with absolute temperature, the reason a sealed can can burst in a fire. Each is a special case of a single deeper relationship.
Folding them into one relation
If pressure times volume divided by temperature is constant in every one of those special cases, it must be constant in general for a fixed amount of gas. That gives the combined gas law, P1V1/T1 = P2V2/T2, connecting any initial state to any final state. Rearranging it solves for whichever quantity you are missing, which is exactly what this calculator does. Set any two matching variables equal, such as T1 = T2, and the combined law collapses neatly back into Boyle's law.
Why the Kelvin scale is non-negotiable
The temperature terms sit as ratios, so the scale has to start at true zero for those ratios to mean anything. On the Celsius scale, 20 degrees is not twice as hot as 10 degrees in any physical sense, but 200 K really is twice the thermal energy of 100 K. That is why the calculator converts Celsius to Kelvin with the 273.15 offset before doing any arithmetic and converts back only for display. Feeding raw Celsius into the ratio is one of the most common mistakes in gas-law homework.
When the combined gas law breaks down
The law assumes an ideal gas: point-like molecules with no attraction between them and a fixed amount of substance. Those assumptions hold well for dry air and light gases near room conditions. They fail as pressure climbs high enough that molecules crowd together, or as temperature drops toward the point where the gas condenses into a liquid. It also does not apply if gas leaks out, dissolves, or reacts, since then the amount of gas is no longer constant and you need the full ideal gas law or a real-gas model.