Memory usage of String java

Question

I want to calculate size of my Message objects in my project. I use the protobuf protocol for serializing and receiving records. In calculation I get a problem. I want to calculate size of Strings after deserializing. protobuf uses this method to read String

/**
 * Read a {@code string} field value from the stream.
 * If the stream contains malformed UTF-8,
 * replace the offending bytes with the standard UTF-8 replacement character.
 */
public String readString() throws IOException {
  final int size = readRawVarint32();
  if (size <= (bufferSize - bufferPos) && size > 0) {
    // Fast path:  We already have the bytes in a contiguous buffer, so
    //   just copy directly from it.
    final String result = new String(buffer, bufferPos, size, "UTF-8");
    bufferPos += size;
    return result;
  } else if (size == 0) {
    return "";
  } else {
    // Slow path:  Build a byte array first then copy it.
    return new String(readRawBytesSlowPath(size), "UTF-8");
  }
}

After reading about UTF-8, character and String in Java, I get confused. My contents is just ASCII for example numbers and standard latin characters. According to this how many bytes each character of my Strings consumes memory?