Use a Disto, the laser ones are more accurate, but more expensive.
If you are going to use it outside, make sure you get one with a bigger range as they are more powerful and work better in sunlight.
Ultra-sonic ones are far cheaper, have a lower distance range, however they work in all brightness conditions, so sunlight doesn't effect them.
If you need mm perfect, go laser, if you need to the nearest few inches, go with a ultra-sonic one.