Convert JPG to Grayscale — Black & White, Free
Convert any JPG to grayscale using luminance, average, or desaturation. Browser-only — no upload, no signup.
About Grayscale JPG
Converting a JPG to grayscale strips colour information so each pixel becomes a shade of grey. There are several formulas: luminance (0.299R + 0.587G + 0.114B — matches human perception, recommended), average ((R+G+B)/3 — naive), desaturation (max+min/2 — like Photoshop's Desaturate). The ZTools Grayscale JPG tool offers all three with a side-by-side preview, plus an optional contrast boost since grayscale images often look flat without one.
Use cases
- Black & white print version. Newspaper / receipt / B&W laser printer can't reproduce colour. Pre-convert to grayscale to control exactly how the conversion looks.
- Aesthetic monochrome mood. Hero image in greyscale reads as serious / nostalgic. Better to convert deliberately than accept whatever the browser does in CSS.
- Reduce visual noise in a layout. A page with many colour images can feel chaotic. Convert auxiliary images to grey so the colour ones pop.
- Test colour-blind accessibility. View design as grayscale to confirm contrast still works without hue cues.
How it works
- Load JPG. Drag-drop. Decoded to canvas pixels.
- Pick formula. Luminance (recommended), average, or desaturation. Live preview updates.
- Optional contrast boost. Slider 0-100 to brighten / deepen blacks and whites. Greys without contrast often look flat.
- Save as JPG. Re-encoded with the chosen quality. Filename suffix `-grayscale`.
Examples
Input: Colourful sunset photo, luminance grayscale
Output: Bright sky stays bright; deep blues become medium-grey; reds become darker grey. Matches what your eye expects.
Input: Same photo, average grayscale
Output: Reds appear too bright (R weighted equally with G and B). Less natural-looking but mathematically simpler.
Input: Desaturation
Output: Faithful to Photoshop's default. Slightly less "perceptually correct" than luminance but more familiar to designers.
Frequently asked questions
Which formula should I use?
Luminance for photographic content (matches human perception, used by Rec.601/Rec.709). Average for diagrams. Desaturation when matching Photoshop's look.
Why does my grayscale image look flat?
Tonal range got compressed. Add 10-20 contrast boost to bring back the punch.
Will my JPG file size change?
Slightly smaller — JPEG's chroma channels carry less information after grayscale. Saving in JPEG's grayscale mode (one channel) saves more — toggle "true grayscale JPEG" for that.
Does it work on PNG / WebP?
This tool handles JPG. For PNG/WebP, the same algorithms apply — use the corresponding tool.
Privacy?
Browser-only.
Pro tips
- Default to luminance — closest match to how humans perceive brightness.
- After converting, check the histogram (if your viewer shows one) — clipped highlights / shadows mean the formula is wrong for this image.
- For B&W print prep, also pre-sharpen — photo paper accepts more detail than backlit screens.
- For designers used to Photoshop, "desaturation" matches that workflow exactly.
Reviewed by Ahsan Mahmood · Last updated 2026-05-06 · Part of ZTools.
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