Bortle scale: how dark is your sky?

John Bortle's nine-step scale rates a site by what you can actually see, from the Milky Way casting shadows (class 1) to a sky too bright for most constellations (class 9). Pick a class, or estimate yours from the faintest star you can see or a sky-meter reading.

Bortle class

Illustration only: a made-up patch of sky showing just the stars brighter than this class's naked-eye limit, with the Milky Way and town glow suggested.

Class 5: Suburban sky

Naked-eye limit5.6–6.0about 5.6 to 6.0

12.5-inch scope14.5–15.0about 14.5 to 15

SQM equivalent20.2–20.8mag/arcsec²

Only hints of the zodiacal light, on the best spring and autumn nights.
The Milky Way is very weak or invisible near the horizon and looks washed out overhead.
Lights are visible in most, if not all, directions.
Over most of the sky, clouds are noticeably brighter than the sky itself.

Faintest star you can see

magnitude

Enter the naked-eye limiting magnitude you measured (for example with a star chart of a known area) to find the matching class.

Sky Quality Meter reading

mag/arcsec²

Enter a zenith reading from an SQM or similar meter to see the naked-eye limit Schaefer's formula predicts for a typical observer.

All nine classes at a glance

Naked-eye and 12.5-inch (32 cm) telescope limits are Bortle's; SQM equivalents are computed with Schaefer's formula.

Class	Naked eye	12.5-inch scope	SQM equivalent	Key signs
1. Excellent dark-sky site	7.6–8.0	17.5	—	Zodiacal light, gegenschein and M33 obvious; Milky Way casts shadows
2. Typical truly dark site	7.1–7.5	16.0–17.0	—	Milky Way richly structured; clouds are dark holes; M33 easy
3. Rural sky	6.6–7.0	16.0	21.95+	Some glow on the horizon; Milky Way still complex; M33 with averted vision
4. Rural/suburban transition	6.1–6.5	15.5	21.0–21.7	Light domes over towns; Milky Way impressive overhead only; M33 hard
5. Suburban sky	5.6–6.0	14.5–15.0	20.2–20.8	Milky Way washed out; clouds brighter than the sky
6. Bright suburban sky	about 5.5	14.0–14.5	≈20.0	Grey-white glow up to 35°; Milky Way only near the zenith
7. Suburban/urban transition	about 5.0	14.0	≈19.3	Whole sky grey-white; Milky Way gone; clouds brilliantly lit
8. City sky	about 4.5	13.0	≈18.7	Sky whitish grey or orange; constellations lose stars
9. Inner-city sky	4.0 or less	—	≈18.0	Bright even at the zenith; only the Moon, planets and bright clusters

Using the scale

Bortle published the scale in Sky & Telescope in 2001 because a naked-eye limiting magnitude alone depends too much on eyesight and effort. So judge your sky by several signs at once: the zodiacal light, the Milky Way's structure, how clouds look at night and whether the Triangulum Galaxy (M33) shows.

His magnitudes assume an experienced observer making an effort. Schaefer's formula, used for the SQM figures, predicts only about magnitude 6.6 for a typical observer under a pristine 22.0 mag/arcsec² sky, and Andrew Crumey's 2014 study of human contrast thresholds questions some of the scale's dark-sky claims. That is why classes 1 and 2 have no SQM equivalent here.

Darker skies from a backyard

Set up where a fence, shed or hedge hides nearby lamps.
Give your eyes time in the dark and read charts by dim red light; the red-light mode turns every page red on black.
Observe high in the part of the sky farthest from the town's glow.
Fully shielded outdoor lights that point downwards keep the glow out of the sky.
Bortle notes that light pollution dims galaxies, nebulae and comets far more than stars, so for those a darker site helps most.

Questions

What Bortle class is a typical suburban backyard?

Bortle's class 5 is the suburban sky: only hints of the zodiacal light, a washed-out Milky Way overhead and clouds brighter than the sky. Brighter suburbs are class 6, where the Milky Way shows only near the zenith, and the edge of a city is class 7.

How do I measure my naked-eye limiting magnitude?

After your eyes have adapted to the dark, find the faintest star you can hold steadily high in the sky and look up its magnitude on a chart. Repeat on several nights, since haze and moonlight change it.

Why are there no SQM numbers in Bortle's article?

Bortle described the classes by what you see, not by meter readings. The SQM equivalents on this page are worked out from his naked-eye limits with Schaefer's 1990 formula as given by Birriel and colleagues in 2014, so they are approximate, and the formula cannot reach the limits Bortle gives for classes 1 and 2.

Sources

John E. Bortle, Introducing the Bortle Dark-Sky Scale, Sky & Telescope, February 2001, pp. 126–129: class names, naked-eye limits, 32-cm reflector limits and the signs of each class, which we paraphrase.
J. J. Birriel, C. E. Walker, C. R. Thornsberry, Analysis of Seven Years of Globe at Night Data, JAAVSO 42 (2014) 219, equation 1: NELM = 7.93 − 5 log₁₀(10^{4.316 − B/5} + 1), from B. E. Schaefer, Publications of the Astronomical Society of the Pacific 102 (1990) 212. It assumes two eyes, natural pupils and no atmospheric absorption.
US National Park Service, Metrics Guide to Night Skies Report: about 22.0 mag/arcsec² for the darkest part of a pristine sky.
A. Crumey, Human contrast threshold and astronomical visibility, Monthly Notices of the Royal Astronomical Society 442 (2014) 2600, arXiv:1405.4209, on the reliability of the darkest classes' limits.
Classes from a magnitude: Bortle gives ranges for classes 1 to 5 and single figures for 6 to 9 (about 5.5, 5.0, 4.5 and 4.0), so the estimator splits the gaps halfway between his figures.