Volvo, IMO, denotes what size of the wheel is required for fitting a particular Rotor inside of it. If a Rotor is described as 18", it does not mean it is an 18" diameter rotor, but rather, you will require an 18" wheel (minimum) to clear the Rotor and Caliper assembly.

320 mm Rotor - T5
345 mm Rotor - T6
366 mm Rotor - T8.

I might be mistaken, so please confirm with some official sources.

 

This is correct. Rear rotors are t5/ 302mm t6/t8 is 320

 

Just an FYI... If you pop the wheel of the size is on the brake caliper

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Thanks to everyone but this is weird
Usually the size of the brake discs depends on the power and weight of the vehicle
Example: T5 or D4 with sunroof, towing hitch, 21" wheels..... is 140kg-180kg heavier than the T6 /D5 without this equipment
T5 has 320mm is heavier than T6, T6 has 345mm

 

I don't know that they change the disc size with most of the options, so wouldn't they be sized for the power difference between the T6 and T5? I could see them bumping up for the T8 for both power and weight.

 

This is actually pretty common in the auto industry, denoting brake sizes by the minimum wheel size required. It doesn't say anything about why the brakes are spec'd that way (diameter is only on component of rotor heat capacity), or what size wheels the vehicle comes with (which can vary) it's just shorthand for brake sizing as there is only one 17", 18" and 19" front rotor system on the SPA, so you know which rotor it is. But it's an industry term that does not particularly make sense to the consumer.

I just happened into this post ha, but I can tell you the models don't correspond, at least on the 60 series SPA cars, every one I have seen has the 345 (18") front brakes.

Best just to take a tape measure and confirm, there is a whole inch of difference between each, you can hardly mess it up even if you eyeball it with a tape measure with the wheel on.

BTW a small detail, but the caliper bracket, not the caliper has the size of the rotor on it. The caliper is actually the same between the rotors, or at least the 345 and 366, the bracket is just changed to position the caliper for the respective rotor.

And that's probably more than you wanted to know ;-)

 

t5/ 320mm rotor has a different caliper/ pad then the 345/366

 

It seems that size of the wheel, reflected in inches of its diameter, and the size of brake disc, reflected in millimeters of its diameter, are confused in this tread. Statement like " For what it is worth my 2017 XC90 T8 has 19 and 18 sized rotors that measure 371mm and 345mm." is simply ridiculous. 19" equals 482.6 mm and 18" equals 457.2 mm. Installed wheel size on a vehicle must have a minimum clearance between the wheel and the brake system (disc and caliper size combination). Smallest Volvo front brake disc of 322 mm will accommodate a wheel of certain minimum size (reflected in inches) but there is no limit of its maximum, except of rim availability.