Wow. I run it at 12 in my truck only because the large tires introduce a lot of road noise to overcome. In my office I feel intrusive if I have it any higher than 5 - I normally keep it at 4.
If I have all the windows and sunroof open in my truck, on the highway, even 15 won't do. If I were going to make a habit of doing so, I'd need an amplified speaker.
Walking down a city street, you should be able to get away with 7 to 10. Remember, as others said, not everyone around you wants to hear what you're listening to.
If you have issues hearing it, look for a bargain pair of earbuds of the type that will fit any smartphone or music player.
If you want a cheap (free) way of making it sound louder in a vehicle, do what, again, smartphone or music player people do, and stick it in the cupholder. The speaker being at the bottom of the radio, the sound will reverberate around the enclosed area of the cupholder and give the impression that it's amplified. I know plenty of people who put their phone in the (empty!) sink or a plastic tub, to focus the sound from its downward-facing speaker out into the room.