What it does
Optical astronomers quote magnitudes, radio and infrared astronomers quote Jansky, and theory papers and spectra use erg s⁻¹ cm⁻² Å⁻¹. Turning one into another needs the band’s zero point and its wavelength, which are easy to get wrong. Type a brightness in any of these units, pick the band, and this converter shows it in all the others at once: Vega magnitude, AB magnitude, Jy, mJy, µJy, f_ν in cgs and SI, f_λ in cgs and SI, and photons per second per cm² per Å.
How to use it
- Type the value and choose its unit.
- Choose the band: Johnson-Cousins U, B, V, R, I; 2MASS J, H, Ks; WISE W1 to W4; SDSS u, g, r, i, z; or Any wavelength for a single wavelength you type in micrometres.
- The table shows the same brightness in every unit. The row you typed is in bold. Tap a value to copy it.
Where the numbers come from
- Vega zero points (the flux of a magnitude 0 star): Johnson-Cousins from Bessell, Castelli and Plez (1998); 2MASS from Cohen, Wheaton and Megeath (2003); WISE from Jarrett et al. (2011).
- AB magnitudes use 3631 Jy in every band (Oke and Gunn 1983): m_AB = −2.5 log₁₀(f_ν / 3631 Jy). SDSS magnitudes are close to AB (u is about 0.04 mag brighter in AB terms, z about 0.02 fainter), so SDSS bands offer AB only.
- f_λ = f_ν c / λ², and the photon flux is f_λ / (hc/λ), both at the band’s effective wavelength. For broad bands this is a close guide rather than exact, because the true conversion depends on the shape of the spectrum across the band.
- The difference AB − Vega for each band is shown under the band: about 0 in V, 0.89 mag in 2MASS J, 1.37 in H and 1.84 in Ks.
Worked example
A star of V = 12.000 (Vega) has a flux density of 0.05763 Jy (57.63 mJy), which is 5.763 × 10⁻²⁵ erg s⁻¹ cm⁻² Hz⁻¹. At 0.545 µm that is 5.816 × 10⁻¹⁴ erg s⁻¹ cm⁻² Å⁻¹, or 0.0160 photons per second per cm² per Å. Its AB magnitude is 11.999. A star of Ks = 10.000 (Vega) has Ks = 11.840 in AB magnitudes.
Frequently asked questions
Why do AB and Vega magnitudes differ? Vega’s spectrum is not flat in f_ν, so a magnitude 0 star on the Vega system has a different flux in each band, while the AB system uses the same flux everywhere. The gap is small in the visible and grows in the infrared.
Can I use it for radio flux? Yes. Choose “Any wavelength” and type the wavelength in micrometres (21 cm is 210000 µm); Jy, f_ν and f_λ all convert.
Does it send anything? No. It runs in your browser and works offline.
Last updated 2026-11-08.