Memory usage of String java

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?