Bortle Scale Calculator
Estimate your sky darkness on the Bortle scale based on naked-eye limiting magnitude. Enter values for instant results with step-by-step formulas.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Bortle Class = f(NELM) adjusted for elevation, humidity, and Moon phase
The Bortle class is determined primarily by the Naked-Eye Limiting Magnitude (NELM) — the faintest star visible without optical aid. Higher NELM values indicate darker skies. Elevation improves seeing by ~0.1 mag per 1000m, high humidity reduces it, and moonlight can reduce effective NELM by up to 3 magnitudes.
Worked Examples
Example 1: Suburban Backyard Assessment
Problem:From your suburban backyard, the faintest star you can see is magnitude 5.2. Elevation is 200m, humidity 60%, and the Moon is at 30% illumination. What is your Bortle class and what can you observe?
Solution:Base NELM: 5.2 -> Bortle Class 6 (Bright Suburban)\nElevation adjustment: +200/1000 x 0.1 = +0.02\nHumidity (60%): -0.1\nMoon (30%): -0.9\nEffective NELM: 5.2 + 0.02 - 0.1 - 0.9 = 4.22\nEffective Bortle: Class 8 (City Sky)\nSQM: ~19.50 mag/arcsec2 (base)\nStars visible: ~830
Result:Base: Bortle 6 | Effective: Bortle 8 (with Moon) | Can see M31, M44 with difficulty
Example 2: Dark Sky Site Evaluation
Problem:At a mountain observatory at 2,500m elevation, you see stars to magnitude 7.2. Humidity is 30% and it is a new Moon (0%). Evaluate the site.
Solution:Base NELM: 7.2 -> Bortle Class 2 (Typical Dark Site)\nElevation: +2500/1000 x 0.1 = +0.25\nHumidity (30%): no penalty\nMoon (0%): no penalty\nEffective NELM: 7.2 + 0.25 = 7.45\nEffective Bortle: Class 2\nSQM: ~21.89 mag/arcsec2\nStars visible: ~7,600\nAstrophotography: Excellent
Result:Bortle 2 | Excellent for deep sky | M33 easy naked eye | Zodiacal light vivid
Frequently Asked Questions
What is the Bortle scale and how is it used in astronomy?
The Bortle Dark-Sky Scale is a nine-level numeric scale that measures the night sky brightness at an observing location. It was created by amateur astronomer John E. Bortle and published in Sky and Telescope magazine in February 2001. Class 1 represents the darkest possible sky visible from Earth, found only in the most remote locations far from any artificial lighting. Class 9 represents inner-city skies where only the Moon, planets, and a few of the brightest stars are visible. Astronomers and stargazers use the Bortle scale to rate potential observing sites, plan what objects they can observe, determine appropriate telescope equipment, and evaluate conditions for astrophotography. The scale correlates with measurable quantities like sky brightness in magnitudes per square arcsecond.
What equipment works best for different Bortle classes?
Equipment choices should match your sky conditions for optimal results. At Bortle Class 1 to 3, any telescope performs well, and wide-field refractors excel for sweeping the rich Milky Way fields. Visual observers benefit from large Dobsonian reflectors of 10 to 16 inches that can reach magnitude 15+ targets. At Bortle Class 4 to 5, narrowband filters like OIII and UHC become important for nebula observation, boosting contrast against the brighter background. Telescope choice matters less than filter choice here. At Bortle Class 6 to 7, narrowband and light pollution filters are essential. Astrophotographers should use narrowband imaging with Ha, OIII, and SII filters. At Bortle Class 8 to 9, focus on planets, the Moon, and double stars. Solar observation is equally effective from any Bortle class since it occurs during daylight.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy