If you use an app like “light pollution map” or a Sky Quality Meter, it often reports this value (higher = darker).
NELM is the faintest star you can see with the naked eye under good dark adaptation.
1) Can you clearly see the Milky Way structure (not just a faint haze)?
2) How many stars can you see in the Little Dipper (Ursa Minor) on a clear night?
3) How bright is the sky near the horizon / city glow?
4) After 20 minutes of dark adaptation, how “black” does the sky look overhead?
This survey is an estimate. For best accuracy, use SQM/mag/arcsec² when possible.
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Notes: Bortle is a descriptive class system (1 = darkest, 9 = brightest). This tool maps common brightness ranges into a practical planning estimate.
