Frequently Asked Questions
Everything you need to know about the Bortle Scale Estimator.
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.