Changing time to decimal

Can anyone please help with this - I've looked elsewhere but can't find anything suitable.

Cell A1 has a start time (e.g, 09:00). Cell B1 has finish time (e.g, 12:30).

Cell C1 is to calculate the total time between the two using B1 - C1.

In the above example, this gives 02:30 (two hours thirty minutes)

What I want is for this to read as a decimal (so 2.5 - two and a half hours), but how do I go about with formula to do this (not VBA).

Sorry if the above explanation seems patronising, but a lot of other posts I've seen contain posts by people who getting embroiled into a debate as to how time can be expressed as a decimal and why it would be needed.

Thanks.