What is the astronomical magnitude scale?

The magnitude scale is a logarithmic measure of brightness. Each step of 1 magnitude corresponds to a factor of ~2.512 in flux. Lower (and negative) magnitudes indicate brighter objects. The Sun is −26.7 mag; the faintest naked-eye stars are ~+6 mag.

What is a zero-point flux?

The zero-point flux F₀ is the flux density that corresponds to magnitude 0 in a given photometric band. For the V band, F₀ ≈ 3.63 × 10⁻⁹ W/m²/μm (or 3631 Jy in the AB system). Different band systems (Vega, AB) use different zero-points.

What is a Jansky (Jy)?

A Jansky is a unit of spectral flux density: 1 Jy = 10⁻²⁶ W/m²/Hz. It is common in radio and infrared astronomy. Many catalog fluxes are given in mJy (millijansky) or μJy (microjansky).

Which zero-point system does this tool use?

This tool uses Vega-system zero-points for broad photometric bands (B, V, R, I, J) and the AB system zero-point (3631 Jy) for the generic Jansky mode. Zero-points are standard literature values from Bessell (1998) and Fukugita et al. (1995).

Can I use my own zero-point?

Yes. Select "Custom" in the Band selector and enter your own zero-point flux. This lets you work with any photometric system or filter.

Flux to Magnitude Converter

Name: Flux to Magnitude Converter
Availability: InStock
Author: Nham Vu

Convert stellar flux (W/m² or Jansky) to apparent magnitude and back, with standard zero-points for V, B, R, I, and J photometric bands.

Conversion Settings

Conversion Direction

Photometric Band / Zero-Point

Flux Unit

Flux

Result

Enter a value and click Convert

Formula Reference

m = −2.5 × log₁₀(F / F₀)

F = F₀ × 10^{−m / 2.5}

F = source flux, F₀ = band zero-point flux, m = apparent magnitude.

Vega Zero-Points (Bessell 1998)

Band	F₀ (Jy)	λ_eff (μm)
B	4260	0.44
V	3631	0.55
R	3064	0.64
I	2416	0.80
J	1589	1.22

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Summary

Convert stellar flux (W/m² or Jansky) to apparent magnitude and back, with standard zero-points for V, B, R, I, and J photometric bands.

How it works

Select the conversion direction: flux to magnitude or magnitude to flux.
Choose a photometric band (V, B, R, I, J) or enter a custom zero-point flux.
Enter the flux value (in W/m² or Jy) or the apparent magnitude.
The tool applies the standard formula: m = -2.5 × log₁₀(F / F₀).
Read the converted result and reference zero-point used for the calculation.

Use cases

Convert catalog flux densities to apparent magnitudes for observation planning.
Cross-check photometry pipeline output against published star catalogs.
Estimate source brightness in different photometric bands.
Verify magnitude-flux relationships during telescope data reduction.
Teach the magnitude scale in introductory astronomy courses.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu