Measuring the darkness of the sky
A sky quality meter (SQM) measures the brightness of the night sky in magnitudes per square arcsecond. The scale runs backward, like star magnitudes: a higher number means a darker sky. A pristine site reads about 22.0; a suburban backyard around 19 to 20; a city center 17 to 18 or less.
From SQM to what you see
The calculator converts the reading into:
- Naked-eye limiting magnitude (NELM), the faintest star visible straight overhead, with the widely used formula NELM ≈ 7.93 − 5 log10(10^(4.316 − SQM ÷ 5) + 1).
- Approximate Bortle class, the 1-to-9 scale of sky darkness created by John Bortle in 2001.
- Luminance in millicandelas per square meter.
- Brightness relative to a natural sky of 22.0, and the share coming from artificial light.
Reference values
| SQM | Bortle class | NELM | Typical place |
|---|---|---|---|
| 22.0 | 1 | 6.6 | Remote desert or mountain |
| 21.5 | 4 | 6.4 | Rural area far from towns |
| 21.0 | 4 | 6.1 | Countryside |
| 20.0 | 5 | 5.5 | Suburbs |
| 19.0 | 6 | 4.8 | Bright suburbs |
| 18.0 | 8 | 4.0 | City |
The SQM-to-Bortle mapping is approximate. The Bortle scale is defined by what you see (the Milky Way, zodiacal light, airglow), not by a single number.
Example
A reading of 20.0 means the sky is about 6.3 times brighter than a natural sky, so roughly 84% of its glow is light pollution. Stars to magnitude 5.5 are visible and the Milky Way is faint.
Getting good readings
- Measure at the zenith, on a moonless night, at least 90 minutes after sunset.
- Take several readings and average them; clouds and haze change the result.
- The Moon brightens the sky by several magnitudes near full.
- Dark sky parks and reserves certified by DarkSky International often read 21.5 or better.
Why it matters
Darker skies show fainter stars, more meteors, and better views of galaxies and nebulae. Each step of one magnitude per square arcsecond is a factor of 2.5 in sky brightness, so moving from a 19 to a 21 sky makes faint objects stand out more than six times better against the background.