Bortle Scale Calculator: How Dark Is Your Sky?

The Bortle scale runs from Class 1 for the darkest possible sky to Class 9 for an inner-city sky. Rather than guess, answer a few questions about what you can actually see overhead and this estimates your class, your naked-eye limiting magnitude, and what is realistically within reach. The single best test is the Milky Way: if you cannot see it at all, you are in suburban skies or worse.

The Bortle scale was devised by amateur astronomer John Bortle and published in Sky & Telescope in 2001. It is a judgement based on what the unaided eye can see, so two people can rate the same sky a class apart; treat the result as a sensible estimate, not a measurement. For an objective reading, a sky quality meter gives a number in magnitudes per square arcsecond, and many UK dark-sky reserves publish their typical figures.