overcoats: size up or true size?

im in the market for overcoats, and while i dont know my regular size in them, i learned that i could just grab one of my suit jackets and look at their sizing (36R).

given that i plan to wear sweaters and other layers underneath the overcoat, should i size up to make room (38R)? but i also read online that overcoats already account for this by measuring more than the tagged size:

The label size of an overcoat should correspond to the size of your chest, just like a suit coat would. So if you have a 42 inch chest, you wear a size 42 jacket and a size 42 overcoat. Generally speaking, the chest of an overcoat will measure about four inches more than its tagged size – so a size 42 will be about 46", giving you room for your suit or sport coat underneath.

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Aaron writes to ask: When it comes to sizing, do overcoat sizes match suit sizing, or do you buy commensurately...
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