Calculate sky darkness

Map a sky-quality-meter reading to a representative Bortle class and naked-eye limiting magnitude.

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ToolSky Darkness Calculator
Input
Output
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Preview

How it works

The magnitude-per-square-arcsecond reading is matched against descending thresholds in the stored nine-class table. The selected class returns representative limiting magnitude and typical visible features.

  • The thresholds and descriptions are fixed table values.
  • Higher readings represent darker skies; limiting magnitude is representative, not measured from the input.

Worked example

A dark rural sky
A sky quality meter reading of 21.7 mag/arcsec2
Input
											21.7
Output
												Sky brightness mag arcsec2: 21.7
Bortle class: 3
Class name: Rural sky
Naked eye limiting magnitude: 6.6
What is visible: Some light pollution shows near the horizon, the Milky Way still shows real structure overhead, and M15, M4 and M5 are naked eye objects.

When to use this

Observing reports record SQM readings, astronomy clubs compare sites, and planning guides choose targets for rural or urban skies.

Edge cases

  • A reading near a threshold can switch class with small meter or sky changes.
  • Moonlight, haze, and observer vision change visibility without changing the table class.
  • Bortle is qualitative and does not predict every observer or instrument’s limit.

References