Limiting Magnitude Calculator

About this calculator

This calculator estimates a telescope's limiting magnitude — the faintest star it can detect — using the approximation 2.7 + 5·log₁₀(D), where D is the aperture diameter in millimeters, reflecting the fact that a larger light-collecting surface gathers more photons from a dim, distant source. Because the magnitude scale is logarithmic and inverted (lower numbers mean brighter objects), each roughly 2.5x increase in aperture area adds about one magnitude of reach.

Amateur astronomers deciding which deep-sky objects (faint galaxies, nebulae, globular clusters) a given telescope can actually show, and shoppers comparing telescopes before buying use this to set realistic expectations. Enter the aperture diameter to get the estimated limiting magnitude.