Bortle Scale to SQM Converter
Convert between the Bortle Dark Sky Scale and Sky Quality Meter (SQM) readings to understand light pollution levels and optimal conditions for astronomy and astrophotography.
Sky Quality Measurement
Sky Quality Results
Observing Conditions
Select a Bortle class or enter SQM reading to see conditions
Bortle Dark Sky Scale Reference
Class 1 - Excellent Dark Sky
SQM: 21.7-22.0+ mag/arcsec²
Limiting Magnitude: 7.6-8.0+
Zodiacal light visible, Milky Way casts shadows, airglow visible on horizon
Class 2 - Typical Dark Sky
SQM: 21.5-21.7 mag/arcsec²
Limiting Magnitude: 7.1-7.5
Milky Way structure visible, zodiacal light evident, M31 visible to naked eye
Class 3 - Rural Sky
SQM: 21.3-21.5 mag/arcsec²
Limiting Magnitude: 6.6-7.0
Milky Way still prominent, some light pollution on horizon
Class 4 - Rural/Suburban Transition
SQM: 20.4-21.3 mag/arcsec²
Limiting Magnitude: 6.1-6.5
Milky Way visible overhead, light domes on horizon
Class 5 - Suburban Sky
SQM: 19.1-20.4 mag/arcsec²
Limiting Magnitude: 5.6-6.0
Milky Way washed out except overhead, clouds brighter than sky
Class 6 - Bright Suburban Sky
SQM: 18.0-19.1 mag/arcsec²
Limiting Magnitude: 5.1-5.5
Milky Way invisible, M31 requires effort, sky grayish
Class 7 - Suburban/Urban Transition
SQM: 16.8-18.0 mag/arcsec²
Limiting Magnitude: 4.6-5.0
Sky noticeably bright, entire sky has grayish cast
Class 8 - City Sky
SQM: 15.5-16.8 mag/arcsec²
Limiting Magnitude: 4.1-4.5
Sky bright gray/orange, only brightest stars visible
Class 9 - Inner City Sky
SQM: <15.5 mag/arcsec²
Limiting Magnitude: <4.0
Sky brilliantly lit, only planets and brightest stars visible
Astrophotography Recommendations by Sky Quality
| Bortle Class | Best Targets | Techniques | Limitations |
|---|---|---|---|
| 1-2 | All deep sky objects, Milky Way panoramas | Shorter exposures possible, natural sky gradients | None - ideal conditions |
| 3-4 | Galaxies, nebulae, star clusters | Standard techniques work well | Some gradient correction needed |
| 5-6 | Bright nebulae, planets, moon | Narrowband filters helpful, gradient removal essential | Milky Way photography difficult |
| 7-8 | Planets, moon, bright double stars | Heavy filtering required, very precise processing | Most deep sky photography impractical |
| 9 | Planets, moon only | Specialized filters essential | Deep sky photography nearly impossible |
