Sky Quality Meters and light-pollution maps report sky brightness in magnitudes per square arcsecond. This converter turns a reading into the Bortle class photographers use and the faintest star you could see with the naked eye.
Higher SQM numbers mean darker skies; 22 is about as dark as Earth gets.
How it works
NELM ≈ 7.93 − 5·log₁₀(10^(4.316 − SQM/5) + 1). Bortle classes use common SQM thresholds.
FAQ
What SQM do I need for the Milky Way?
Above about 20.5 the Milky Way is clear; above 21.5 it shows rich detail.
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