Equivalent Width Calculator

Upload a spectrum of a star, brown dwarf, or exoplanet, define the line and continuum windows, and measure its equivalent width.

1. Load a spectrum

Spectrum file (.txt, .dat, .csv)

Wavelength unit

Two numeric columns: wavelength and flux. Extra columns are ignored. Whitespace- or comma-separated files are supported; comment lines starting with # are ignored. Wavelengths must be in the selected unit.

No spectrum loaded.

Important Notes

Your spectrum is processed locally in your browser and is not uploaded to any server.

2. Inspect Spectrum & Define Regions

Inspect the spectrum, drag a rectangle to zoom into both wavelength and flux ranges, and enter the wavelength ranges you want to test. Click Show Regions to display your choices on the plot. No equivalent width is calculated in this section.

Line regionBlue continuumRed continuumFitted continuum (after calculation)

Line Name (optional)

Line startLine end

Blue continuum startBlue continuum end

Red continuum startRed continuum end

3. Calculate Equivalent Width

Before calculating: Please check that the wavelength regions you selected and displayed in Section 2 are the ones you intended to use.

Equivalent width

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Load a spectrum, show your regions, and calculate.

4. How Is Equivalent Width Calculated?

The equivalent width (EW) measures the strength of a spectral line relative to its surrounding continuum. We fit a straight line (linear least squares) to the flux samples in the selected blue and red continuum windows, then integrate the continuum-normalized line profile over the selected line region using the trapezoidal rule.

\[\boxed{\mathrm{EW}=\int_{\lambda_1}^{\lambda_2}\left[1-\frac{F_\lambda}{F_c(\lambda)}\right]\,d\lambda}\]

Here, \(F_\lambda\) is the observed spectral flux, \(F_c(\lambda)\) is the fitted continuum flux, and \(\lambda_1\) and \(\lambda_2\) mark the start and end of the line region.

Schematic absorption line. The shaded region illustrates the missing flux relative to the continuum.

EW is reported in the selected wavelength unit (Å or µm). Whenever possible, choose continuum windows free of strong spectral features.