Multiply its internal Length by Width by Height.
But you need to include any storage space in the door, and exclude any that is lost to the compressor and other parts.
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I'd look first on its label or in its manual, for the specifications given by its manufacturer!
If they are unavailable, measure the useable interior in inches or cm. You needn't measure it to the last detail of shelf-supports, internal lamp housing, etc, but don't ignore major features like an internal freezer cabinet or a moulded shelf forming the casing over the "works".
Then use the normal Volume = L X B X H.
Divide by 1728 or 1 000 000 to give the result in cubic feet or cubic metres respectively.
(I know the centimetre is not an ISO "preferred unit" but I'm not going to add extra noughts to a simple domestic calculation for the sake of bureaucracy!)
My fridge has an internal shelf over the motor - I'd subtract its volume from the total for the whole cabinet measured as if it was not there.
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If you see a specification on a refrigerator as 18 cubic feet, for example. This represents the capacity (how much volume of stuff that you can fit into the fridge).
That is not possible. A triangle is an area. Cubic meters belong to volume or capacity.
Capacity or volume of a cone = 1/3*pi*radius2*height measured in cubic units
You have to eat some hot-cheetos then poop them out and eat them again.
If you mean the units, capacity, or volume, is expressed in cubic meters, cubic decimeters (= liters), cubic centimeters (= milliliters), etc.