JPEG quality estimate · image
What quality was this JPEG saved at?
Every JPEG stores the quantization tables it was compressed with. Because the standard encoder builds those tables from a single quality setting, we can read them back and estimate the quality the file was last saved at, from about 1 to 100. Drop in a JPEG to see it.
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How the estimate works
JPEG throws away detail by dividing the image's frequency coefficients by a quantization table and rounding. The standard encoder derives that table from one quality number by scaling a fixed base table, so the table left behind in the file is a fingerprint of the quality used. This tool reverses that scaling to recover the number. When the table exactly matches a standard one it reports the quality with confidence; when the table is custom, from an unusual encoder, it reports the closest estimate and says so.
Why it is an estimate, and re-saves
The quality reflects the last save, not the whole history. Every time a JPEG is opened and re-saved it is quantized again, and quality only ever falls, so a file that began at 95 and was re-exported at 80 reads as 80. A high number does not guarantee an untouched original, and a low number does not prove tampering, only heavy or repeated compression. To look for signs of editing rather than just compression, use the photo edit check and JPEG ghost.
The other facts
Alongside quality the tool reports the chroma subsampling, how much colour detail was discarded
relative to brightness, where 4:4:4 keeps all of it and 4:2:0 keeps a
quarter, and whether the file is baseline or progressive. Those, with the estimated quality, are a
quick signature of the encoder and settings that produced the file.