Convert an SQM reading to the Bortle scale
A sky quality meter gives a number in magnitudes per square arcsecond; observers talk in Bortle classes; star-counters quote the faintest star they can see. This SQM to Bortle converter links all three. Type a meter reading, a naked-eye limiting magnitude (NELM) or pick a Bortle class, and it returns the matching class, the limiting magnitude, the sky luminance, how much brighter the sky is than a pristine one, and a plain description of what you can expect to see.
Below the converter is a full Bortle scale chart with the approximate SQM range, Bortle’s own limiting magnitudes, the formula limiting magnitude and good targets for each class. The arithmetic runs in your browser.
How to use the Bortle Scale & SQM Converter
- Choose what you know: SQM reading, Limiting magnitude or Bortle class.
- Type the value, for example 20.5 mag/arcsec², or a limiting magnitude of 6.0.
- Read the class, the coloured bar, the limiting magnitude, the luminance and the brightness ratio.
- Read “What you can see” for the class, and compare neighbouring classes in the chart.
- Press Copy result to paste the line into an observing log.
Bortle Scale & SQM Converter features
Three-way conversion
SQM ↔ Bortle class ↔ naked-eye limiting magnitude, using one published formula in both directions.
Honest bands
SQM ranges are labelled approximate, and Bortle’s observer-based NELM values sit beside the formula values.
Physical units
Luminance in mcd/m² and the ratio to a natural 22.0 sky for light-pollution reports.
What you can see
A description of the Milky Way, the zodiacal light and test objects for every class, in our own words.
Sanity checks
Warnings for readings darker than the natural sky and for limiting magnitudes the formula cannot handle.
The formulas
The conversion between sky brightness B (mag/arcsec²) and the naked-eye limiting magnitude uses the relation published by Unihedron, the maker of the SQM, based on Bradley Schaefer’s 1990 paper on limiting magnitudes: NELM = 7.93 − 5·log10(104.316 − B/5 + 1), and in reverse B = 21.58 − 5·log10(101.586 − NELM/5 − 1). Worked example: B = 20.5 gives 100.216 = 1.644, so NELM = 7.93 − 5 × log10(2.644) = 7.93 − 2.11 = 5.82. Luminance follows L = 10.8 × 104 × 10−0.4B cd/m², so 20.0 mag/arcsec² is 1.08 mcd/m².
Every 1.0 mag/arcsec² is a factor of 2.5 in brightness, so the difference between a 22.0 desert sky and an 18.0 suburb is 40 times. The formula describes an average observer; practised observers with dark-adapted eyes routinely beat it by half a magnitude or more, which is why Bortle’s own NELM values, written for experienced observers, are higher.
Why the SQM bands are approximate
John Bortle introduced his nine-step scale in Sky & Telescope in February 2001. He defined each class by what can be seen — the structure of the Milky Way, the zodiacal light, test galaxies such as M33 — and by limiting magnitude, not by meter readings. The SQM ranges used here come from the Dark Skies Awareness nomogram of the International Year of Astronomy 2009, which also adds a class 4.5. Other charts shift the edges by 0.1–0.3. Meter readings vary with the Moon, twilight, cloud, airglow, season and where the meter points, so treat a single reading as a guide and take several on different nights.
Using the result
A darker sky helps galaxies and nebulae most; clusters, double stars and planets survive light pollution well. The Messier objects visible tonight list lets you pick your sky darkness, the telescope magnification calculator shows how aperture changes the limiting magnitude, and the magnitude calculator converts magnitude differences into brightness ratios.
Bortle Scale & SQM Converter FAQ
What SQM reading is Bortle 4?
Roughly 20.8 to 21.3 mag/arcsec² on the nomogram bands, with 20.3–20.8 as an intermediate 4.5. Bortle defined class 4 by a limiting magnitude of 6.1–6.5 for an experienced observer.
What does the Bortle scale mean?
It is a nine-step scale of night-sky darkness, from 1 (pristine, the Milky Way casts shadows) to 9 (inner city, only the brightest stars). Each class describes what an observer can see.
How do I convert SQM to Bortle?
Enter the reading in mag/arcsec² and read the class. Higher numbers are darker: about 22 is class 1, 19.25–20.3 is class 5, and below 18 is class 8–9.
What is a good SQM reading for astronomy?
Above 21.3 is a rural dark sky where faint galaxies and the Milky Way’s structure are easy; above 21.7 is excellent. Below about 19 most galaxies fade and the Milky Way disappears.
Why does my limiting magnitude not match the chart?
Limiting magnitude depends on eyesight, dark adaptation, experience and the part of the sky you test. The formula is for an average observer; Bortle’s column is for an experienced one.
Is the Bortle class the same as light pollution maps?
Maps estimate sky brightness from satellite data and models, so they give a typical value; a meter reading on the night is more direct. Both are converted to Bortle classes with approximate bands like these.