How to mesure radiator support in case

[Mihir_1337]

for example Corsair H100i that is 240mm radiator but on the website Corsair says it's 276mm long
and just for namesake a cooler master case that supports 240mm radiator max won't support that exact radiator will it ?
so one needs to buy the case that supports more than 276 mm ( 360mm ) case for that radiator am I correct ?

English isn't my first language correct me if I'm wrong and explain this whole radiator support thing to me

 

[Gam3r01]

They are called 240mm because they have 2x120mm fans, its just a measure of the size of the fans, not the cooler.
If a case says it supports a 240mm water cooler, it will most likely have the extra room.