Grayscale Image

Convert an image to grayscale with the formula you choose

Turning a colour picture into shades of grey sounds simple, but there is more than one right answer. Our eyes are far more sensitive to green than to blue, so a plain average of red, green and blue makes blue skies too light and green leaves too dark. The grayscale image converter offers the standard luma formulas from Rec. 709 and Rec. 601, true linear-light luminance, average, desaturate and single-channel conversions — plus a pure black-and-white mode with automatic thresholding.

Use it for black-and-white photography, print-ready documents, cleaner scans or images for e-ink screens. Results appear in a before/after split with a live histogram, are processed at full resolution on your own device, and fully grey PNGs are saved as single-channel grayscale files, which are smaller than RGB.

How to use the Grayscale Image

1. Add your images by dropping them, choosing them or pasting.
2. Choose a Conversion formula. Rec. 709 suits most photos; try the red channel for dramatic skies.
3. Lower Strength for a partly desaturated look, or tick Pure black and white for two-tone output.
4. In black-and-white mode, keep Automatic threshold on, or untick it and set the threshold yourself.
5. Download the converted image, or every image at once as a ZIP.

Grayscale Image features

Grayscale formulas compared

How pure colours turn grey with each method (0 = black, 255 = white):

Which formula should you use?

• Rec. 709 luma — 0.2126 R + 0.7152 G + 0.0722 B, the weights of the sRGB and HDTV primaries. It is the default here and the formula behind CSS grayscale().
• Linear luminance — the same weights applied after decoding sRGB to linear light, then re-encoded. This is physically correct relative luminance, the quantity WCAG contrast ratios use, and it keeps saturated colours lighter than luma does.
• Rec. 601 luma — 0.299 R + 0.587 G + 0.114 B, from standard-definition television and used for JPEG’s YCbCr. Reds come out lighter than with Rec. 709.
• Average and Desaturate ignore how we see colour: every pure primary turns the same grey, which can flatten a picture.
• Single channel imitates the coloured filters used with black-and-white film. The red channel darkens blue sky and smooths skin; the blue channel exaggerates haze and freckles.

Pure black and white with Otsu’s method

Black-and-white mode computes the grey value with your chosen formula, then turns every pixel white if it is at or above the threshold and black otherwise. With Automatic threshold on, the threshold comes from Otsu’s method (Nobuyuki Otsu, 1979): it tries every split of the histogram into a dark and a light class and keeps the one that maximises the between-class variance w₀ × w₁ × (μ₀ − μ₁)². For scanned text, signatures and line art with a clear gap between ink and paper, it usually lands in exactly the right place; the value used is shown under the controls.

Tips

• Black-and-white photos often need more punch; follow up with the contrast tool.
• For an antique brown tone try Sepia Image; for a negative, Invert Image.
• For document scans, a manual threshold of 150–190 removes paper texture and the shadows of folds.
• Grayscale cannot be undone in the saved file — the colour information is discarded, so keep the original.