🌌 Dark Sky Quality Planner

Turn a Sky Quality Meter reading, a Bortle class or a naked-eye limiting magnitude into the others, and see what each sky condition is worth for deep-sky work. Useful for choosing a site before you drive to it.

Enter a reading to compare sites.
The scale and the maths

SQM reads magnitudes per square arcsecond; higher is darker. A pristine zenith sky is about 21.99, a good rural site 21.3–21.7, and a city centre can be under 18. Naked-eye limiting magnitude relates to SQM roughly as NELM ≈ 7.9 − 0.36·(21.99 − SQM)² for dark skies. Sky brightness doubles with every 0.75 mag of SQM, so a site one magnitude darker is not slightly better — it is 2.5× darker, and faint nebulosity that is invisible at Bortle 6 becomes obvious at Bortle 3.