Bortle Scale & Light Pollution Calculator

Enter an SQM reading or a naked-eye limiting magnitude to get your Bortle class, how much brighter your sky is than natural darkness, what you can see, and what the skyglow means for wildlife.

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What the Bortle scale measures

John E. Bortle, an amateur astronomer, published his nine-class Dark-Sky Scale in Sky & Telescope in February 2001. He described what an observer sees at each level: from Class 1, where the zodiacal light is bright enough to cast a faint glow and the Milky Way throws shadows, to Class 9, an inner-city sky where only the Moon, planets and a few dozen stars survive. Because the scale is built on descriptions, it is easy to use with no equipment, and later work attached numbers to each class.

Two numbers are commonly paired with each class. The first is sky surface brightness from a Sky Quality Meter, in magnitudes per square arcsecond (mag/arcsec2), where bigger means darker. The second is naked-eye limiting magnitude (NELM), the faintest star you can see, where bigger also means darker. A pristine sky is around 22 mag/arcsec2 and NELM 7.6 or more; a city centre may read 17 to 18 and NELM 4 or less.

  • Classes 1 to 3: truly dark to rural skies where the Milky Way dominates.
  • Classes 4 and 5: the rural-to-suburban transition, where light domes appear on the horizon.
  • Classes 6 and 7: bright suburban skies where the Milky Way is lost.
  • Classes 8 and 9: city skies, where most of what you see is the glow itself.

Sources: Bortle (2001), Sky & Telescope, for the class descriptions; H. Spoelstra's Sky Brightness Nomogram (Dark Skies Awareness), as tabulated on Wikipedia, for the approximate SQM and NELM ranges; Falchi et al. (2016), Science Advances, for the 22.0 mag/arcsec2 natural baseline.

How the skyglow multiple is calculated

Astronomers define magnitudes so that a difference of 5 magnitudes is exactly a factor of 100 in brightness (Pogson ratio), which makes one magnitude a factor of about 2.512. For a sky reading S in mag/arcsec2 and the natural baseline of 22.0 used by Falchi et al. (2016), total sky brightness relative to natural is 10^(0.4 x (22.0 - S)). The same reading converts to luminance as L = 10.8 x 10^4 x 10^(-0.4 x S) cd/m2, so 22.0 is about 0.17 mcd/m2 and 18.0 about 6.8 mcd/m2. Subtract 1 for the natural part and you have artificial skyglow as a multiple of the natural sky.

  • SQM 22.0: ratio 1.0, no measurable artificial light.
  • SQM 20.3 (class 4/5 edge): total about 4.8 times natural, artificial about 3.8 times.
  • SQM 19.5: total 10 times natural, artificial 9 times.
  • SQM 18.0: total about 40 times natural.
  • SQM 17.0: total 100 times natural.

If you enter star counts instead of an SQM reading, the tool uses the typical SQM for the class that your limiting magnitude falls in, so the multiple is an estimate and is labelled that way. Measure with an SQM when you can: the logarithmic scale means a small reading error becomes a large brightness error.

Why light pollution is an environmental problem, not just an astronomy problem

Falchi et al. (2016, Science Advances) estimated that more than 80 percent of the world's population, and about 99 percent of people in the United States and Europe, live under light-polluted skies, and that the Milky Way is hidden from over a third of humanity. Kyba et al. (2017, Science Advances) found that Earth's artificially lit outdoor area grew about 2.2 percent a year from 2012 to 2016, with no sign of falling in the countries that were already bright.

Ecologists treat artificial light at night as a pollutant because many species use darkness as a cue (Gaston et al., 2013, Biological Reviews). The documented effects include:

  • Migrating birds drawn into lit cities, raising collision risk and exhausting them.
  • Insects circling lamps instead of feeding, mating or pollinating, and fireflies unable to see each other's flashes.
  • Sea turtle hatchlings crawling toward streetlights instead of the sea.
  • People whose sleep and melatonin timing shift under bright, blue-rich night light.

The read-out in the calculator pairs your class with the level of effect to expect, but actual impact depends on the species nearby and how close the lights are. It is a prompt to look at your own lighting, not a biological survey.

Measuring your sky and cutting skyglow

For a good reading, wait until astronomical twilight has ended (roughly 90 minutes after sunset at mid-latitudes) with no Moon above the horizon, point the meter at the zenith, and take several readings, discarding any caught by passing headlights. No SQM? Use the limiting-magnitude method: after at least 20 minutes of dark adaptation, find the faintest star you can hold in the Little Dipper bowl or in the Great Square of Pegasus and enter its magnitude. Clouds, haze, moonlight and a low horizon all push readings brighter, so repeat on a clear, moonless night before drawing conclusions.

To reduce your own contribution, follow the DarkSky (International Dark-Sky Association) principles: shield every light so it points down, use warm colour temperatures of 3000 K or less, use only the brightness the task needs, and switch lights off with timers or motion sensors. Lower-wattage, shielded lighting also saves money, so run your numbers in the Home Energy Audit Calculator.

Related tools on this site: use the Carbon Footprint Calculator to see where lighting sits in your household emissions, the Electricity vs Tree Offset Calculator to compare saved electricity with tree planting, and the Plant Hardiness Zone Finder if you are choosing night-friendly native plantings for a darker, wildlife-friendly yard.

Frequently Asked Questions

Common questions about the Bortle Scale & Light Pollution Calculator

Choose SQM reading, type the number from your Sky Quality Meter, and the tool matches it to the approximate ranges in H. Spoelstra's sky brightness nomogram (the table Wikipedia uses): 21.76 or darker is Bortle 1, 21.60 to 21.75 is class 2, 21.30 to 21.60 is class 3, 20.30 to 21.30 is class 4 (some tables split off 20.30 to 20.80 as a class 4.5), 19.25 to 20.30 is class 5, 18.50 to 19.25 is class 6, 18.00 to 18.50 is class 7, and anything brighter than 18.00 is class 8 or 9. No published SQM value separates 8 from 9, so the tool treats readings below about 17.5 as class 9. Take several readings pointing at the zenith and average them for the most reliable class.

Sources & References

The new world atlas of artificial night sky brightness

Falchi et al., Science Advances 2(6), e1600377.
- Natural sky background of 22.0 mag/arcsec2 (174 ucd/m2) used as the baseline for the skyglow multiple.
- Artificial brightness expressed as a ratio to that natural background.
→Source: Science Advances(2016)

Introducing the Bortle Dark-Sky Scale

John E. Bortle, Sky & Telescope, February 2001.
- The nine class names and the description of what is visible in each class.
→Source: Sky & Telescope(2001)

Bortle scale: approximate SQM and NELM per class

Class table citing H. Spoelstra's Sky Brightness Nomogram (Dark Skies Awareness).
- SQM floors 21.76, 21.60, 21.30, 20.30 (classes 4 and 4.5 merged), 19.25, 18.50, 18.00.
- NELM floors 7.6, 7.1, 6.6, 6.1, 5.6, 5.1, 4.6, 4.1.
→Source: Wikipedia(2026)

Sky Quality Meter: mag/arcsec2 to cd/m2

Conversion used for the luminance read-out.
- L [cd/m2] = 10.8 x 10^4 x 10^(-0.4 x SQM).
→Source: Unihedron(2026)

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