Enter the observed irradiance and the reference flux into the Calculator. The calculator will evaluate the Apparent Magnitude. 

Apparent Magnitude Formula

M = -5*log(10) (Fx/Fx0)


  • MI is the Apparent Magnitude (())
  • Fx is the observed irradiance
  • Fx0 is the reference flux

To calculate Apparent Magnitude, divide the observed irradiance by the reference flux, take the log of the result, then multiply by -5.

How to Calculate Apparent Magnitude?

The following steps outline how to calculate the Apparent Magnitude.

  1. First, determine the observed irradiance. 
  2. Next, determine the reference flux. 
  3. Next, gather the formula from above = M = -5*log(10) (Fx/Fx0).
  4. Finally, calculate the Apparent Magnitude.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

observed irradiance = 101

reference flux = 6


What is the difference between Apparent Magnitude and Absolute Magnitude?

Apparent Magnitude measures how bright a celestial object appears from Earth, while Absolute Magnitude measures the intrinsic brightness of an object, assuming it is placed at a standard distance of 10 parsecs from the observer.

Why is the logarithmic scale used in the Apparent Magnitude formula?

The logarithmic scale is used because the range of brightness in the universe is vast. A logarithmic scale allows us to compress this range into a more manageable scale where a difference of 5 magnitudes corresponds to a brightness factor of 100.

Can the Apparent Magnitude be a negative value?

Yes, the Apparent Magnitude can be negative. Objects brighter than the defined zero point of the scale, such as the Sun, Venus, and Sirius, have negative magnitude values.

How does the observed irradiance affect the calculation of Apparent Magnitude?

The observed irradiance directly influences the Apparent Magnitude calculation. A higher irradiance indicates a brighter object, leading to a lower (potentially negative) magnitude, while a lower irradiance indicates a dimmer object, resulting in a higher magnitude value.