Bortle Scale to SQM Converter
Turn a Bortle class into an SQM reading in magnitudes per square arcsecond, or the other way round, and see how dark the sky really is. Includes typical places in Pakistan for each class.
Bortle scale and SQM
Convert between a Bortle class and a Sky Quality Meter reading.
Higher is darker. A pristine sky reads about 22, a city centre 17 or lower.
Bortle class for SQM 21.00
Class 4
Rural to suburban transition
- SQM range
- 20.49 to 21.69 mag/arcsec²
- Naked-eye limit, typical observer
- magnitude 6.1
- Naked-eye limit, Bortle's figure
- 6.1 to 6.5
- Sky brightness
- 0.43 mcd/m²
- Compared with a pristine sky
- 2.51x brighter
- Milky Way
- Clear high in the sky but loses detail towards the horizon.
- Zodiacal light
- Visible but faint, and it does not reach halfway up the sky.
- Triangulum galaxy (M33)
- Difficult, needs averted vision when high.
- Typical place in Pakistan
- Countryside within reach of a town, such as the edges of Skardu or Chitral, or rural Punjab.
| Class | SQM | Naked-eye limit |
|---|---|---|
| 1 | 21.99 or darker | 7.6 to 8.0 |
| 2 | 21.89 to 21.99 | 7.1 to 7.5 |
| 3 | 21.69 to 21.89 | 6.6 to 7.0 |
| 4 | 20.49 to 21.69 | 6.1 to 6.5 |
| 5 | 19.50 to 20.49 | 5.6 to 6.0 |
| 6 | 18.94 to 19.50 | 5.1 to 5.5 |
| 7 | 18.38 to 18.94 | 4.6 to 5.0 |
| 8 | 17.80 to 18.38 | 4.1 to 4.5 |
| 9 | below 17.80 | 4.0 or less |
Bortle's limiting magnitudes assume a keen, fully dark-adapted observer. The typical observer figure from an SQM reading is usually lower, by up to a magnitude at the darkest sites.
How this is calculated
- 1
Match the SQM range
Each Bortle class covers a band of sky brightness. For example, class 4 runs from 20.49 to 21.69 mag/arcsec² and class 8 from 17.80 to 18.38. Higher SQM numbers mean a darker sky.
- 2
Naked-eye limiting magnitude
For a reading, the typical observer's faintest visible star is estimated as NELM = 7.93 − 5 log10(10^(4.316 − SQM/5) + 1). Bortle's own figures, for an experienced observer, are also shown.
- 3
Sky brightness in physical units
Luminance in candela per square metre is L = 108,000 × 10^(−0.4 × SQM), shown here in millicandela (mcd/m²). A pristine sky at 22 is about 0.17 mcd/m².
Frequently asked questions
What is the Bortle scale?
A nine-step scale published by John Bortle in 2001 to rate how dark a night sky is, from class 1 (an excellent dark site) to class 9 (an inner-city sky). It is based on what an observer can see, such as the Milky Way and faint galaxies.
What is a good SQM reading?
Above 21.5 is a dark rural sky and 21.8 or more is excellent. Suburbs usually read 19 to 20.5, and city centres 18 or lower.
What Bortle class is Lahore or Karachi?
Most of urban Lahore and Karachi is class 8, and the brightest commercial areas are class 9. You need to travel well clear of the city glow, often 100 km or more, to reach class 4 skies.
Where are the darkest skies in Pakistan?
The high valleys of Gilgit-Baltistan, such as the Deosai Plains, Shimshal and upper Hunza, reach class 1 to 2. Parts of the Cholistan and Thar deserts and inland Balochistan are also very dark.