๐ญBortle Scale Estimator
Estimate your sky's Bortle class via questionnaire, SQM reading, or the reference table.
This is an approximate, subjective estimate โ not a precise photometric measurement. For precision, use a real Sky Quality Meter or a satellite light-pollution map. Nothing here is stored beyond this session.
Free Bortle Scale Estimator
How dark is your sky, really? Our free Bortle scale estimator helps you figure out where your stargazing spot falls on the 9-level Bortle scale โ from pristine class 1 dark-sky sites where the Milky Way casts shadows, to washed-out class 9 inner-city skies where only the brightest stars are visible. Answer a few simple questions about what you can see overhead, and get an instant estimate of your class.
Already know your Sky Quality Meter (SQM) reading? Enter it directly for a more precise match. Prefer to browse instead? A full reference table describes every Bortle class, its typical limiting magnitude, and what you should expect to see at each level.
No sign-up, no downloads, no limits โ just a clearer picture of your night sky. See the About page or the FAQ for more.
About the Bortle Scale Estimator
Features
Guided questionnaire
Plain-language questions map to a best-estimate class.
Reference table
All 9 classes with name, limiting magnitude, and description.
SQM input
Maps a magnitude-per-arcsecond reading to the nearest class.
Honest framing
Explicitly noted as approximate, not a precise measurement.
Frequently Asked Questions
What is the Bortle scale?
A 9-level scale rating night-sky darkness, from class 1 (pristine, no light pollution) to class 9 (inner-city sky).
How does this estimator work?
Answer a short questionnaire about what you can see in the night sky, and it maps your answers to the most likely Bortle class.
Is it free?
Completely free, no sign-up, downloads, or limits.
Is this an exact measurement?
No, it's an approximate estimate based on visual cues; a Sky Quality Meter or satellite light-pollution map gives more precise results.
Can I enter an SQM reading directly?
Yes, if you have a Sky Quality Meter value, enter it for a closer match to a Bortle class.
What can I see at a Bortle class 1 site?
The Milky Way is bright enough to cast faint shadows, along with zodiacal light and airglow.
What does Bortle class 9 mean?
An inner-city sky where only the Moon, planets, and a handful of the brightest stars are visible.
Is my questionnaire data stored?
No; everything runs in your browser for the current session only and isn't sent anywhere.
Can I just look up the scale without answering questions?
Yes, a full reference table of all 9 classes is available to browse directly.
Does it work on mobile?
Yes, fully responsive on all screen sizes โ handy for checking in the field.