Körbchengrösse?

This cannot be converted, because 164 is a garment size for body size of normal weighted children or teenagers (child size).

The teen sizes start with the dress size 158 from about 13 years. For 14-year-olds Size 164 for 15 years the 170 and for 16 years the 176.

Where the size of the body does not have to be taken exactly, there is a clearance of approximately plus/minus 5cm for all garment sizes.

For a real bra you have to measure the upper and lower range.

For the upper width, the measuring tape is wound and measured around the widest point of the breast (at the level of the nipples). It should be loose and not squeeze the breasts together. The measuring tape must be both at the same chest and back height (so not sloping).

For the bottom, the measuring tape is wound and measured directly under the chest. It should be tight.

Often you hear this recommendation:

Underbust measuring for bra band is done snugly and tight while measuring underbust for determining bra cups is done loosely.

At the bottom, the measuring tape is tightly applied and somewhat looser at the top.

https://en.wikipedia.org/wiki/Bra_size#Consumer_measurement_methods

There is no real standard for measuring:

When measuring the breast circumference, the recommendations range from measuring in the lying, measuring in hanging breasts to measuring in raised breasts or measuring in a well-fitting bra.

https://de.m.wikipedia.org/wiki/B%C3%BCstenhalter#BH-Gr%C3%B6%C3%9Fen

That’s why some measure forward. And/or exhaled. Or something else…

One of many online calculators:

https://www.blitzcomputer.de/bh-groesse/