⭐ Star Magnitude Brightness Calculator

Enter two apparent magnitudes to see how many times brighter (or dimmer) one object is compared to the other. Remember: on the magnitude scale, lower numbers are brighter.

Magnitude of Object A

Magnitude of Object B

Turning a magnitude gap into a brightness ratio

The magnitude scale is logarithmic and runs backwards: smaller numbers are brighter, and five magnitudes is defined as exactly one hundred times the light.

ratio = 100^((m₂ − m₁) ÷ 5) = 2.512^(m₂ − m₁)

The page opens on magnitude 1.0 against 6.0 and reports a ratio of 100 — the gap between a first-magnitude star and the faintest thing a good dark-sky eye can reach. One magnitude alone is 2.51×. Sirius at −1.46 against Vega at 0.03 gives 3.94×. Ratios below ten print to two decimals, larger ones as whole numbers, and the tool always names the brighter object rather than making you decide which way round the fraction went.

Using it properly

Negative magnitudes are fine. The Sun at −26.74 against the full Moon at −12.74 is a 14-magnitude gap, which the tool returns as 398,107×.
These are apparent magnitudes. The result compares how bright two objects look from Earth, not how luminous they truly are. A nearby dim star can outshine a distant supergiant; for intrinsic output you need absolute magnitude, measured at a standard 10 parsecs.
Aperture buys magnitudes. Light gathered scales with the square of the aperture, so a 200 mm telescope against a 7 mm dark-adapted pupil collects (200 ÷ 7)² = 816 times more light, worth 7.3 magnitudes of extra depth.

Extended objects such as galaxies do not follow this comfortably. A magnitude 8 galaxy spreads its light over many arcminutes, so it can be far harder to see than a magnitude 8 star, which is a point.

Frequently asked questions

How much brighter is a magnitude 1 star than a magnitude 6 star?

Exactly 100 times. That is how the modern scale was defined, to fit the six classes the naked-eye catalogues had used for centuries.

Why are brighter objects given negative numbers?

The scale was fixed on the old first-to-sixth ranking before anyone measured the Sun, Venus or Sirius against it. Keeping the arithmetic meant letting the brightest objects run off the bottom end.

What magnitude can I see through my telescope?

Roughly 7.5 plus five times the base-ten log of the aperture in millimetres, which puts a 200 mm scope near magnitude 19 in theory. Light pollution, altitude and your own eye usually cost several magnitudes of that.