BYTETOOLS

JPEG Quality Checker

Estimate the 1-100 quality a JPEG was saved at by reading its quantization tables, plus chroma subsampling, encoding mode and bits per pixel.

Drop a JPEG to estimate the quality it was saved atReads the DQT tables in your browser — the file is never uploaded

Choose a JPEG to recover the 1–100 quality slider value it was exported with, along with its chroma subsampling and encoding mode.

What is the JPEG Quality Checker?

A JPEG never records the quality slider it was exported with, but it does carry the quantization tables that slider produced.

  • Quality recovered from the DQT tables using the IJG/libjpeg formula
  • Per-table scale factor and an exact-fit flag for luminance and chrominance
  • 8x8 luminance quantization matrix drawn as a heat map
  • Chroma subsampling, baseline versus progressive and sample precision reported
  • Bits per pixel and megapixels to show whether a file is over-compressed
  • Plain-language verdict on what the quality level means for visible artifacts

How to use the JPEG Quality Checker

  1. 1

    Drop a JPEG file into the upload area, or click it to browse.

  2. 2

    Read the estimated quality tile and the plain-language verdict banner underneath it.

  3. 3

    Compare the luminance and chrominance rows, including each table's scale factor and how exactly it fits a standard table.

  4. 4

    Study the 8x8 luminance heat map to see which frequencies were quantized away.

  5. 5

    Click Copy report to save the numbers as plain text.

About the JPEG Quality Checker

A JPEG never records the quality slider it was exported with, but it does carry the quantization tables that slider produced. The ByteTools JPEG Quality Checker reads those DQT tables and compares them against the standard IJG luminance and chrominance tables, working backwards to the 1-100 setting that would have generated them.

Alongside the quality number you get the chroma subsampling read from the component sampling factors, whether the file is baseline or progressive, the sample precision, the bits per pixel and a list of the embedded metadata. The 8x8 luminance table is drawn as a heat map so you can see which spatial frequencies the encoder threw away hardest.

The whole analysis happens 100% locally in your browser — the JPEG is read as an array buffer and scanned marker by marker in JavaScript on your device. Nothing is uploaded to a server and no copy is kept, so you can audit client photos, stock library downloads or your own export settings in complete privacy.

Frequently asked questions

Can you tell what quality a JPEG was saved at?

Yes, to a close approximation. The quantization tables inside the file are a direct function of the quality setting, so comparing them against the standard IJG tables recovers the number. It is an estimate rather than a stored value, because the setting itself is never written into the file.

Does re-saving a JPEG at quality 100 restore lost detail?

No. JPEG compression is lossy and permanently discards the high-frequency detail that the quantization tables divided away. Saving again at a higher quality only produces a bigger file that faithfully preserves the damage already present.

What is a good JPEG quality setting for the web?

Most photographs look visually identical to the original somewhere between quality 75 and 85, which is where file size drops sharply without obvious blocking. Below about 60 the 8x8 blocks and color banding usually become visible on smooth gradients and skin tones.

What is chroma subsampling in a JPEG?

Chroma subsampling stores color information at a lower resolution than brightness, because human vision is far more sensitive to luminance detail. 4:2:0 halves the color resolution in both directions and saves a lot of space, while 4:4:4 keeps full color and suits screenshots and text.

Why does the tool say my quantization tables are not an exact match?

Some encoders ship their own tuned quantization tables instead of the standard IJG ones — Photoshop, MozJPEG and Guetzli all do this. In that case the number shown is the closest equivalent IJG quality rather than a literal reading of the slider that was used.

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