Light Pollution Sky Quality Calculator
Estimate the faintest star visible through your telescope by combining sky brightness, aperture, altitude, observer age, and moon phase. Useful for planning deep-sky sessions at any site.
Last updated: September 2026
Limiting Magnitude
6.53 mag
Formula below · 2 sources (nasa.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Naked-eye limiting magnitude (NELM) is the faintest star you can see without optics. The calculator converts the sky brightness measured by a Sky Quality Meter (mag/arcsec²; darker skies give higher readings) with the standard relation NELM = 7.93 − 5·log₁₀(10^(4.316 − SQM/5) + 1): 22.0 gives about 6.6, 20.0 about 5.9, 18.0 about 4.0. If you leave the SQM reading blank, a typical value for the selected Bortle class is used instead (class 1: 22.0, 3: 21.8, 5: 20.0, 7: 18.7, 9: 17.8). Two rough site adjustments are then applied: +0.1 mag per 1,000 m of elevation (thinner air above you) and −0.5 mag × relative humidity (haze); these are rules of thumb, not measured relations. Individual eyesight varies by ±0.5 mag or more.
How to use
Suppose your SQM reads 19.5 mag/arcsec² at a 1,200 m site with 45% humidity. Step 1 — 10^(4.316 − 19.5/5) = 10^0.416 ≈ 2.61; NELM = 7.93 − 5 × log₁₀(3.61) ≈ 7.93 − 2.79 = 5.14. Step 2 — elevation: +1.2 × 0.1 = +0.12. Step 3 — humidity: −0.45 × 0.5 = −0.23. Result ≈ 5.04: stars down to about magnitude 5 are visible, a suburban-to-rural sky. Without a meter, leave SQM blank and pick your Bortle class.
Frequently asked questions
How does sky brightness in mag/arcsec² relate to the Bortle scale?
Sky brightness in magnitudes per square arcsecond is a quantitative measure that maps closely onto the 9-level Bortle scale. A pristine Bortle 1 site measures roughly 22.0–22.5 mag/arcsec², while a typical suburban Bortle 6 sky sits around 19.0–20.0 mag/arcsec². Urban skies (Bortle 8–9) can fall below 18 mag/arcsec². You can measure your sky's value with a Sky Quality Meter (SQM), or use published Bortle-to-SQM conversion tables. Entering a measured SQM reading is more reliable than relying on the Bortle class alone.
Why does my limiting magnitude differ from the calculator's estimate?
The SQM-to-limiting-magnitude relation describes an average observer. Age matters (the maximum pupil shrinks from about 8 mm in youth to around 5 mm by age 60 and the lens yellows), as do dark adaptation (20-30 minutes), averted vision (worth up to about 1 magnitude), and experience. Expect your own limit to differ by half a magnitude or more either way.
When is the best time to observe faint deep-sky objects to maximise limiting magnitude?
The ideal conditions combine a new Moon, a high-altitude dark-sky site with a sky brightness above 21.5 mag/arcsec², low humidity, and the target object near the zenith to minimise atmospheric path length. New Moon windows last about 5–7 days around each new moon, so planning sessions in those windows is the single biggest improvement most observers can make. Combining a large aperture telescope with a moonless, transparent night at altitude can push limiting magnitude past 15–16 for visual observing and considerably deeper for astrophotography.